Reynolds, J.H., Ramos, V. A., Jordan, T. E., and Hernández, R. M., 1997, Diachronism of regional Neogene stratigraphy in the Andean foreland, NW Argentina: arguments for retiring the Calchaquense and Araucanense terminology, Geological Society of America Abstracts with Programs (Salt Lake City, UT), v. 29 (6), p. A-228.

DIACHRONISM OF REGIONAL NEOGENE STRATIGRAPHY IN THE ANDEAN FORELAND, NW ARGENTINA: ARGUMENTS FOR RETIRING THE CALCHAQUENSE AND ARAUCANENSE TERMINOLOGY

REYNOLDS, J. H., JR Magstrat Ltd., LLC, P.O. Box 300, Webster, NC 28788 , magstrat1@wcu.campus.mci.net, RAMOS, V. A., Depto. de Cs. Geológicas, Universidad de Buenos Aires, Buenos Aires, Argentina, JORDAN, T. E., Dept. of Geological Sciences, Snee Hall, Cornell University, Ithaca, NY 14853-1504, and HERNANDEZ, R. M., XR Exploracionistas y Servicios, s.r.l., Alsina 181, Salta 4400, Argentina

Prior to the mid-1980ís a simple, two-phase, Tertiary tectonic history of the Andes in NW Argentina was inferred on the basis of regional lithostratigraphic facies correlations. The older phase (Calchaquense) was variously thought to represent Eocene, Oligocene, or Miocene events while the younger phase (Araucanense) was assigned Miocene or Pliocene ages by different workers. Lithostratigraphic units within the Calchaquense are generally distal basin-filling facies while Araucanense units are proximal facies. Chronostratigraphic work using magnetostratigraphy and isotopic ages since the mid-1980's demonstrates extreme diachronism in the Calchaquense and Araucanense "phases". Abundant age information is now available from three fold-thrust belts: the Precordillera (flat subduction area), the Santa Barbara System (Transition Zone), and the Sierras Subandinas (normal subduction area).

Lithostratigraphic units in the Precordillera show contact ages that differ by millions of years. The top of the Calchaquense at Vinchina is younger than the top of the Araucanense at Las Juntas 70 km away. In the Santa Barbara System, the Calchaquense top has an upper contact age that varies by 4 m. y. in sections 50 km apart. Contact ages in the Sierras Subandinas exhibit similar patterns across the normal subduction region.

Chronostratigraphy demonstrates that the two-phase model for the Andean Orogeny in NW Argentina is overly simplistic. The proximal/distal transition in distinct stratigraphic sections represents local tectonic conditions in a generally eastward-migrating orogeny that began in northwest Argentina in the early Miocene and has been continuous to the present. Continued use of the Calchaquense and Araucanense terms serves only to obscure the tectonic information brought forth by the chronostratigraphic data. These terms have outlived their usefulness and should be retired.

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Reynolds, J. H., Idleman, B. D., Hernández, R. M., and Naeser C. W., 1994, Preliminary chronostratigraphic constraints on Neogene tectonic activity in the Eastern Cordillera and Santa Bárbara System, Salta Province, NW Argentina: Geological Society of America Abstracts with Programs (Seattle, WA), v. 26(7), p. A-503.

PRELIMINARY CHRONOSTRATIGRAPHIC CONSTRAINTS ON NEOGENE TECTONIC ACTIVITY IN THE EASTERN CORDILLERA AND SANTA BARBARA SYSTEM, SALTA PROVINCE, NW ARGENTINA

REYNOLDS, J. H., IDLEMAN, B. D., Dept. of Geosciences & Anthropology, Western Carolina University, Cullowhee, NC 28723; HERNANDEZ, R. M., XR Exploracionistas Regionales s.r.l., Grand Bourg (4000) Salta, Argentina; NAESER C. W., U. S. Geological Survey, Mail Stop 908, National Center, Reston, VA 22092

We present formation ages for the Neogene Oran Group to constrain the uplift history of the Andes in NW Argentina. Three magnetic polarity stratigraphic sections, supported by zircon fission track ages and an 40Ar/39Ar age from intercalated airfall tuffs, were studied in the Eastern Cordillera and Santa Barbara System. Sections from Rio Metan/Rio de las Piedras, Rio Gonzalez, and in Alemania contain formations deposited in a broad range of environments: the Rio Seco (ephemeral stream), Anta (mud flat/lacustrine), Quebrada de Jesus Maria (ephemeral stream), Rio Guanaco (distal fan), and Rio Piquete (medial fan).

Isotopic ages come from Anta Formation airfall units. Zircons from Alemania yielded a fission track age of 14.4 ￿ 1.3 m.y. Hornblende from Rio de las Piedras gave a total gas 40Ar/39Ar age of 14.1 ￿ 0.7 m.y. Contact ages were compiled from the paleomagnetic sections. No basal Rio Seco age could be determined.

The Rio Seco-Anta boundary is 14.9-15.0 Ma at Rio Gonzalez and Rio Metan. The Anta-Jesus Maria contact is ~13 Ma in all areas. A regional unconformity marks the base of the Rio Guanaco Formation at ~10 Ma. Rio Piquete strata at Rio Metan were deposited beginning ~5 Ma.

Rio Seco strata probably represent distal fluvial facies derived from uplift of the Argentine Puna to the west. Anta strata are thickest in the east at Rio González, in the densely vegetated Santa Bárbara System, suggesting that the broad Anta Basin was present from 15-13 Ma. This probably resulted from continued uplift of the Puna and initiation of uplift in the Santa Bárbara System in the east.

Initiation of uplift in the Eastern Cordillera, around 13 Ma, spread Quebrada de Jesus Maria strata eastward. Continued uplift of the Eastern Cordillera and the Santa Barbara System were responsible for Rio Guanaco deposition starting ~10 Ma. A second orogenic pulse, starting ~5 Ma and lasting until at least 2 Ma caused deposition of the coarse Rio Piquete strata.

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Butz, D. J., Foldesi, C. P., and Reynolds, J. H., 1995, Provenance of the Payogastilla Group: a preliminary uplift history of the Central Andes, Salta Province, NW Argentina: Geological Society of America Abstracts with Programs (Knoxville, TN), v. 27, p. 39.

PROVENANCE OF THE PAYOGASTILLA GROUP: A PRELIMINARY UPLIFT HISTORY OF THE CENTRAL ANDES, SALTA PROVINCE, NW ARGENTINA

The Neogene Payogastilla Group, situated in an intermontane basin between the Sierra de Quilmes, to the west, and Sierra de Leon Muerto, to the east. These ranges are situated within the western part of the Eastern Cordillera just east of the Puna. The stata provide a good record for

Neogene Andean deformation. Preliminary paleomagnetic data indicate an age for the Payogastilla Group at 13-6(?) Ma. The Neogene Payogastilla Group has three formations, the Angastaco Formation, the Palo Pintado Formation, and the San Felipe Formation. The Angastaco Formation, ~ 3,750 meters thick, consists of tan conglomerate, sandstone, and minor siltstone in a repetitive fining upward sequence intrepreted as a series of braided stream and alluvial fan deposits. The Palo Pintado Formation, ~ 1,250 meter thick, overlies the Angastaco Formation conformably in a series of repetitive fining upward cycles of sandstone, siltstone, and mudstone beds interpreted as an alluvial plain deposits with minor braided stream deposits in upper third of the section. The San Felipe Formation, ~ 650 meters, is a series of red conglomeritic fining upward sequences interpreted as braided stream and distal fan deposits.

There were 26 medium-coarse grain samples collected at intervals of ~ 150 meters. Point-counting at 300 points per slide was used in the determination of sediment source area based on lithic fragment composition. Fragments of volcanic, sedimentary, metamorphic, and plutonic origin were used in provenance determinations .

A trend through the Payogastilla Group indicates a change in the types of lithic fragments throughout the section. This trend is seen as a change from large amounts of volcanics to few sedimentary fragments to a large increase in the metamorphic fragments and a final change into a plutonic source. A change is also seen at the top of the San Felipe were there is a source direction change from the west to the east.

Sediment source studies indicate an inital flux of volcanic material prior to ~ 11 Ma. This has been interpreted as being sourced from the late Miocene uplift of the Puna. Decrease of volcanic lithics followed by an initial influx of sedimentary lithics at ~ 9.4 Ma is correlated with initial foreland uplifts segragating the basin from Puna sedimentation. Uncovering of metamorphic units beneath sedimentary beds involved in basement uplifts is thought to be responsible for a rapid increase of metamorphic lithics at ~ 9.5 Ma within the section. Plutonic clasts relatedto the uncovering of granites in the frontal ranges appears at ~ 7.7 Ma. After which sedimentation is thought to have changed source direction from west to east.

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Kotila, J. M., Hilliard, R., Foldesi, C. P., Butz, D. J., and Reynolds, J. H., 1996, Neogene tectonic history of the Transition Zone, central Andes, NW Argentina, Geological Society of America Abstracts with Programs (Denver, CO), v. 28 (7), p. A-442.

NEOGENE TECTONIC HISTORY OF THE TRANSITION ZONE, CENTRAL ANDES, NW ARGENTINA

KOTILA, John M., HILLIARD, Ronald, FOLDESI, Christopher P., BUTZ, Daniel J., and REYNOLDS, James H., Dept. of Geosciences & Anthropology, Western Carolina University, Cullowhee, NC 28723-9047

Sandstone provenance and magnetostratigraphy from the Angastaco, Río Yacones, and Arroyo González sections allow preliminary regional synthesis of the tectonic evolution of the Central Andes in the Transition Zone (24°-28°S) in Salta Province, NW Argentina. Three tectonic provinces comprise the Argentine portion of the Transition Zone: 1) the Puna/Altiplano on the west; 2) the Cordillera Oriental, an eastward-verging thrust belt; and 3) the Santa Bárbara System in the east, a predominantly westward-verging thrust belt.

Data across the region indicate that Puna-derived sedimentation initiated 16-15 Ma. Sandstones from the eastern Santa Bárbara System at Arroyo González suggest that westward-verging thrusting in the western Santa Bárbara ranges began 14-13 Ma. Piggyback thrusting migrated eastward, uplifting the ranges of the province throughout late Neogene time.

Angastaco sandstones in the western Cordillera Oriental suggest that western Cordillera Oriental ranges began to rise 13-12 Ma and also migrated eastward. Sandstones from Río Yacones, located at the boundary between the Santa Bárbara System and Cordillera Oriental, suggest that the eastern Cordillera Oriental ranges were still starting to rise 6 Ma as uplift migration collided with the older Santa Bárbara ranges.

Crustal shortening is accommodated differently in the Cordillera Oriental and Santa Bárbara System. Restriction of eastward thrust migration in the Cordillera Oriental resulted in vertical uplift elevating peaks to over 5,000 m. Westward-verging, but eastward-migrating piggyback thrusting remained unrestricted throughout the Neogene in the Santa Bárbara System. Most summits attain elevations no higher 2500 m.

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Hilliard, R., Kotila, J. M., and Reynolds, J. H., 1996, Sandstone provenance from Arroyo González, Salta Province, NW Argentina: a preliminary uplift history for the Santa Bárbara System, Geological Society of America Abstracts with Programs (Denver, CO), v. 28 (7), p. A-442.

SANDSTONE PROVENANCE FROM ARROYO GONZALEZ, SALTA PROVINCE, NW ARGENTINA: A PRELIMINARY UPLIFT HISTORY FOR THE SANTA BARBARA SYSTEM

HILLIARD, Ronald, KOTILA, John M., and REYNOLDS, James H., Dept. of Geosciences & Anthropology, Western Carolina University, Cullowhee, NC 28723-9047

Sandstone provenance and magnetostratigraphy from Arroyo González allow insight into the Neogene tectonic development of the predominantly westward-verging Santa Bárbara System in the eastern part of the Transition Zone in Salta Province, NW Argentina. Eighteen samples were collected from ~2100 m of strata containing the Río Seco, Anta, and Quebrada Jesús María formations of the Orán Subgroup. The lower Río Guanaco Formation caps the section. Thin sections were analyzed using a modified Gazzi-Dickinson method with 300 points/slide.

Metamorphic and plutonic clasts are abundant at the base of the section (Río Seco and basal Anta). Fine grain sizes in the Anta Formation precluded sample collection between 435-1238 m. Sedimentary clasts increase in the lower third of Jesús María (1300 m level) corresponding with decreasing metamorphic and plutonic trends. These trends reverse at ~1450 m (Quebrada Jesús María) as sedimentary clasts decrease and crystalline clasts increase. At the top of the Quebrada Jesús María and into the Río Guanaco (>1800 m) sedimentary fragments increase and the metamorphic/plutonic component diminishes.

Using magnetostratigraphy, we interpret that Río Seco metamorphic clasts originated on the Puna >15 Ma. Uplift of the Sierra de Santa Bárbara, 13-12 Ma, washed sedimentary clasts eastward. The unroofed metamorphic core renewed the abundance of metamorphic clasts 12-11.25 Ma. Uplift of the Sierra de Metán caused the second pulse of sedimentary fragments 10.75-10.0 Ma, exposing the metamorphic core at ~10.0 Ma. The Sierra de González uplifted well after 10 Ma folding of ~2.5 km of strata deposited above our paleomagnetic section.

These data suggest that uplift in the Santa Bárbara System migrated eastward between 13-<10 Ma and probably throughout the Neogene. Westward thrusting formed a series of piggyback basins unlike the foreland thrust belts present to the north and south.

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Reynolds, J. H., Hernández, R. M., Idleman, B. D., Naeser, C. W., and Guerstein, P. G., 1996, Río Iruya revisited: The Neogene tectonic development of the western Sierras Subandinas, Salta Province, NW Argentina, Geological Society of America Abstracts with Programs (Denver, CO), v. 28 (7), p. A-59.

RIO IRUYA REVISITED: THE NEOGENE TECTONIC DEVELOPMENT OF THE WESTERN SIERRAS SUBANDINAS, SALTA PROVINCE, NW ARGENTINA

REYNOLDS, James H., Western Carolina University, Cullowhee, NC 28723, HERNANDEZ, Roberto M., XR Exploracionistas s. r. l., Salta 4400, Argentina, IDLEMAN, Bruce D., Lehigh University, Bethlehem, PA, NAESER, Charles W., USGS, Reston, VA, and GUERSTEIN, Pablo, Carleton University, Ottawa, Ont., Canada

The Río Iruya exposes strata between the Sierra del Pescado and Sierra de Pintascayo in northern Salta Province. Magnetostratigraphy, 40Ar/39Ar and fission track ages, and sandstone petrology, from the 7.5 km pile of Neogene, continental, foreland basin-filling strata, allow synthesis of the tectonic development of the western Sierras Subandinas in NW Argentina. Arroyo Solazuti and Quebrada La Porcelana data extend the model eastward.

Regional Neogene sedimentation commenced ~16 Ma in basins east of the rising Puna. At ~13 Ma, sediment accumulation rates diminished, suggesting that initial uplift in the Cordillera Oriental interrupted sediment flow from the Puna. As the Cordillera Oriental rose, it provided sediment for the basin. Eastward migrating uplift caused high sedimentation rates.

A regional depositional hiatus began at ~8.3 Ma, probably representing initiation of west-verging uplift of the Sierra de Cinco Picachos, the westernmost range in the Sierras Subandinas.

Reinitiation of sedimentation around 6.9 Ma probably correlates with uplift of the Sierra del Pescado along east-verging thrust faults. A sigmoidal-shaped sedimentation curve between 7-4 Ma suggests that the Sierra de Pintascayo began to rise at this time, impounding sediment flowing from the west and causing very high sedimentation rates of conglomeratic strata seen in the upper part of the Río Iruya section. Sometime after 4 Ma the Sierra Baja de Orán began to rise as evidenced by a fault truncating the top of the Solazuti section east of the Río Iruya section. Similar structural truncations at La Porcelana suggest the Sierra de Ramos arose after 2 Ma.

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Jordan, T. E., Reynolds, J. H., and Erikson, J., 1995, Foreland Basin Evolution: Clues to Late Oligocene-Miocene Andean Growth, Bolivia and Argentina, IUGG XXI General Assembly (Boulder, CO), p. A-446.

FORELAND BASIN EVOLUTION: CLUES TO LATE OLIGOCENE -MIOCENE ANDEAN GROWTH, BOLIVIA AND ARGENTINA

T.E. Jordan (Dept. Geological Sciences and INSTOC, Cornell University, Ithaca, NY 14853-1504, USA; 607-255-3596; e-mail: jordan@geology.cornell.edu); James H. Reynolds III (Dept. of Geosciences,West Carolina University, Webster, NC 28788-0399); Johan Erikson (INSTOC, Cornell University, Ithaca, NY 14853-1504, USA)

The foreland basins of the eastern flank of the Andes (18-33° S) provide insight to the early deformation of the mountain system. Whereas it is difficult to extract late Oligocene - early Miocene structural characteristics from fault data due to erosion and overprinting, the foreland basins contain detailed records of those time intervals. Recent magnetostratigraphic and isotopic dating demonstrates the history of those basins. The foreland basin of the Aconcagua-Las Ramadas thrust belt (Principal Cordillera; 32° 30'-33° 30'S) began accumulation at approximately 19 Ma, thus significant shortening probably began at about 19 to 20 Ma. The first of two stages of subsidence of the foreland basin of the Precordillera thrust belt (29-33°S) began ~20 Ma and was due to thrusting in the Western Precordillera. In the foreland basin of the Eastern Cordillera of Argentina (18-25°S), isotopic ages from the lacustrine Anta Fm indicate deposition between 15-14 Ma; magnetostratigraphy shows that the base of the Anta is as old as 15.2 Ma.

These beds overlie up to 300 m of the Río Seco Fm, suggesting that accumulation probably began ?16-15.2 Ma. The foreland basin of the Eastern Cordillera at 18-20°S (Bolivia) includes a transgressive sequence (fluvial Petaca Fm and marine Yecua Fm) widely considered correlative with the RíoSeco - Anta. However, at least locally, these units have pre-Miocene ages, as indicated by a Deseadan-age fossil in the lower unit and an unpublished fission-track date from a bentonite above both units. Obviously, lithostratigraphic units are not age equivalent within the Subandean basin.

Thus, over nearly 1500 km, the foreland basins did not simultaneously begin to subside. We interpret that, whereas the Aconcagua-Las Ramadas andPrecordillera thrust belts began major shortening ~20 Ma, the Eastern Cordillera near 20°S was active prior to 24 Ma.

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Reynolds, J. H. and Algar, S. T., 1995, Magnetic Polarity Stratigraphy of the Upper Gros Morne and Mayaro Bay formations, SE Trinidad, Geological Society of America Abstracts with Programs (New Orleans, LA), v. 27(7), p. Añ443.

MAGNETIC POLARITY STRATIGRAPHY OF THE UPPER GROS MORNE AND MAYARO BAY FORMATIONS, SE TRINIDAD

REYNOLDS, J. H., Dept. of Geosciences & Anthropology, Western Carolina University, Cullowhee, NC 28723-9047; Algar, S. T., Dept. of Earth Sciences, Dartmouth College, Hanover, NH 03755-3571

Magnetic polarity stratigraphy was used to date about 2,500 m of surface exposures of Neogene marginal marine reservoir rocks in the Upper Gros Morne and Mayaro Bay formations in southeastern Trinidad. Samples from sea cliff exposures at Guayaguayare Bay, Galeota Point, and Mayaro Bay refine the temporal framework of sedimentation and tectonism in the region while generally agreeing with age assignments derived through other techniques. Such results suggest the potential utility of magnetic polarity stratigraphy in hydrocarbon exploration.

The Upper Gros Morne Formation is composed of ripple-laminated siltstones and fine-grained sandstones. The overlying Mayaro Bay strata are characterized by fine- to very fine-grained sandstones, often exhibiting trough cross-bedding. Fine siltstones and claystones are interbedded with the sandstones, usually on a decimeter to meter scale but some fine-grained units are as thick as 40 m. The strata represent Orinoco Delta delta-front facies. All strata were subjected to gentle N-S compression prior to being cut by Pleistocene right-lateral strike-slip faulting.

An early Pliocene age is assigned to the Upper Gros Morne Formation and the Mayaro Bay Formation is late Pliocene to Pleistocene. We place the contact between the Mayaro Bay and Upper Gros Morne formations at ~3.55 Ma. The highest Mayaro Bay rocks we sampled were deposited ~0.88 Ma, younger than the late Pliocene age previously assigned to these strata. A linear sediment accumulation rate of 0.67 mm/a is proposed for these beds. Uplift, folding, and faulting occurred in rocks as young as 0.88 m.y. that were deposited in a relatively deep water environment, suggesting a very rapid uplift rate. Trapping structures in the Gros Morne Formation and secondary migration of hydrocarbons, at least in this part of Trinidad, developed since ~0.88 Ma during deformation of the strata. The data also suggest a Plio-Pleistocene clockwise rotation of ~13° north of the Marcel Fault System.

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Reynolds, J. H. , Peterson, V. L., and Burr, J. L., 1995, Teaching library research, technical writing, map reading and Piece on Earth in general education geology: Geological Society of America Abstracts with Programs (Knoxville, TN), v. 27, p. 83.

TEACHING LIBRARY RESEARCH, TECHNICAL WRITING, MAP READING AND PIECE ON EARTH IN GENERAL EDUCATION GEOLOGY

REYNOLDS, J. H. , PETERSON, V. L., and BURR, J. L. Dept. of Geosciences & Anthropology, Western Carolina University, Cullowhee, NC 28723-9047

Few students enter college with skills or appreciation for technical writing. Few technical writing opportunities are provided at the introductory level. To meet student needs in our large-enrollment freshman general education courses we attempt to provide challenging writing opportunities that maximize learning potential, stimulate student interest, and require development of library research skills. To meet this challenge, we modified an approach used at Syracuse University (Siegel and others, 1993 - "Pieces of the Earth"). Our "Piece on Earth" project is a semester writing project that allows the student to develop better technical writing, library research, and map-reading skills while applying geological concepts learned in class.

During the first class we randomly assign each student a 2° USGS topographical quadrangle within the 48 coterminous United States. This becomes their "Piece On Earth" (POE). A fictitious consulting firm employs the students and the "boss" needs information about each POE. During the semester, they write 4-5 one-page papers, including maps, that summarize geographical and geological features of their POE. Assignments require students to use WCU library and map library resources. Grading is based on content, style, grammar, referencing, format, spelling, and presentation. Liberal resubmission policies allow students to improve scores until they achieve a grade with which they are content. Students experience feedback crucial to good writing.

POE topics generally parallel course material. The first two assignments familiarize students with the geographical aspects of thier POE. Students first locate their POE and describe its cultural features. Prompt grading allows rapid assessment of individual writing ability. The second paper focusses on the POE physiography. Students compile a physiographic map, gaining further appreciation of topographic maps. The remaining papers let students compile maps and report on the POE geological features. These include description of the distribution and age of lithologic units, tectonic features, tectonic activity, and other topics determined by the instructor or student. Geological journal articles are usually needed for these papers. A final paper includes all shorter papers, discussion of an individually chosen additional topic, and a synthesis of the material presented.

Many students exhibit rapid and dramatic improvement in technical writing ability. They also gain useful expertise with map reading and use of library resources.

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Reynolds, James H., 1993, Software development as a class project: a Hypercard-based guide to the dinosaurs: Geological Society of America Abstracts with Programs (Burlington, VT), v. 25(2), p. 74.

SOFTWARE DEVELOPMENT AS A CLASS PROJECT: A HYPERCARDô-BASED GUIDE TO THE DINOSAURS

REYNOLDS, James H., Dept. of Geosciences & Anthropology, Western Carolina University, Cullowhee, NC 28723

A college-level dinosaur course undertook a stack-development project over a two-year period. The objective was to involve students in building an illustrated, electronic study guide to complement textbook assignments. Project development used Hypercard and a Macintosh computer.

Work focused on the Ornithischian dinosaurs during the first year and on the Saurischians the second year. Individual student groups concentrated on different dinosaur families. Each group provided a drawing, published skeleton and skull diagrams, location in time and space, and general information about as many genera as possible within their chosen taxa. Students divided work into library research, drawing, and map making. The instructor designed the stack scripting and card format.

Students entered research onto genera-specific cards. Illustrations were scanned to bit-mapped images. Additional cards were created to include a bibliography, adapt an animated display of Mesozoic plate tectonic motions from Schmidt (1988) and to display the Mesozoic time scale.

The stack is indexed by a taxonomic hierarchy of cladograms. A but-ton designates each taxon. Selecting the button introduces the next level of the hierarchy. At the generic level, buttons call up individual dino-saur illustrations. From the illustration, additional information is accessed with icons. Open routing through the stack serves the specific needs of the user.

Students acquired a detailed knowledge, as well as an affinity, for their particular dinosaur families and for general dinosaur classification. Contributing to a large, long-term project generated pride and a sense of accomplishment.

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Reynolds, J. H., Damanti, J. F., and Jordan, T. E., 1991, Magnetostratigraphic Constraints on the Development of Paired Fold-Thrust Belts/Foreland Basins in the Argentine Andes (invited paper): American Association of Petroleum Geologists (Dallas, TX), v. 75, p. 660.

MAGNETOSTRATIGRAPHIC CONSTRAINTS ON THE DEVELOPMENT OF PAIRED FOLD-THRUST BELTS/FORELAND BASINS IN THE ARGENTINE ANDES.

REYNOLDS, JAMES H.*, Norwich University, Northfield, VT, DAMANTI, JOHN F., Miami University, Oxford, OH, and JORDAN, TERESA E., Cornell University, Ithaca, NY

Development of a paired fold-thrust belt/foreland basin is correlated to the flattening of the subducting Nazca Plate between 28-33°S. Magnetostratigraphic studies in Neogene basin-filling continental strata determine local basin subsidence rates and provide relatively precise chronostratigraphic correlation between different depositional environments. Temporal constraints on cross-cutting structures and synorogenic sediment compositions reveal the pattern of Neogene paleogeographic evolution in the Andean foreland. The western side is dominated by the Precordillera fold-thrust belt which impinges on block uplifts of the Sierras Pampeanas to the east.

The data demonstrate that most existing lithostratigraphic units are diachronous and require new tectonic interpretations. Increases in sediment accumulation rates closely correspond to changes in provenance and indicate that the Frontal Cordillera, on the Chile-Argentina border, was a positive topographic province by 18 Ma. The Precordillera evolved from ≈ 16 Ma to the present as thrusting migrated from west to east. Published ages from intercalated airfall tuffs constrain some sedimentary sections in the eastern Sierras Pampeanas where the earliest uplift occurred since 10 Ma. The youngest uplifts are on the west side close to continuing thrusting in the Precordillera.

Not all fold-thrust belt/foreland basin pairs are associated with flat subduction, suggesting that tectonic controls exceeding the scale of individual plate segments may be important. The hydrocarbon-producing Subandean fold-thrust belt/foreland basin, located in the area of "steep" subduction that underlies northern Argentina and Bolivia (18-24°S), is also believed to have evolved since middle Miocene time. Recently initiated magnetostratigraphic studies in the Subandean foreland basin will attempt to temporally constrain the Neogene tectonic evolution for comparison with the southern region. 

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Reynolds, J.H., Jordan, T.E., Tabbutt, K.D., Ré, G., Vilas, J.F., Bercowski, F., Milana, J.P., Beer, J.A., Damanti, J.F., and Ramos, V.A., 1990, Chronology of foreland deformation in the Precordillera-Sierras Pampeanas region of Argentina (28°-33° S): Symposium International "Géodynamique Andine": Paris: Editions de l'ORSTOM (Grenoble, France), p.145-146.

CHRONOLOGY OF FORELAND DEFORMATION IN THE PRECORDILLERA-SIERRAS PAMPEANAS REGION OF ARGENTINA (28°-33° S):

Se determinaron las edades de la deformación de la corteza en la zona de los Andes centrales por debajo de la cual existe la subducción de bajo ángulo. En estratos en las cuencas de antepaís se han medido edades por magnetoestratigrafía y por trazas de fisión en tobas. Deformación en la Precordillera comenzó hace 16 millones de años, y esta deformación y la deformación de las Sierras Pampeanas eran activos juntos en los últimos 8 millones de años.

Methods

Recent investigations result in a chronology of deformation in the Central Andean foreland above the zone of flat subduction of the Nazca Plate in northern Argentina. The chronology is based on a relatively precise chronostratigraphy in Neogene detrital strata in several parts of the Precordillera-Sierras Pampeanas region. The data allow minimum rates of uplift to be assigned to some ranges in the area. The new data reveal a rapid evolution of the Central Andes which was not discernible from traditional regional lithostratigraphy.

The foreland basin strata are dated using magnetic polarity stratigraphy and fission-track dating of zircons retrieved from interbedded volcanic airfall beds. Tectonic events are constrained either directly by dating cross-cutting relations or minimum ages are determined from changes in conglomerate and sandstone clast lithologies. Some events are dated, with less precision, using indirect methods such as dating fluctuations in sediment accumulation rates and/or changes in depositional environments. Additional chronologic information is available from the rare Neogene volcanic centers located in the study area.

Results

The data reveal that after the initial uplift of the Cordillera Frontal prior to ~18 Ma, the upper crustal locus of overthrusting migrated in a systematic west to east progression from ~16 Ma to the present in the Precordillera fold and thrust belt. At some time prior to ~8.2 Ma, crystalline basement blocks began to rise along moderately dipping reverse faults in the northwestern Sierras Pampeanas. Uplift of these ranges was less systematic; there is as yet no suggestion of a preferred direction for the migration of tectonic activity in the basement uplift region. Considerable young uplift (< 4.3 Ma) appears to have been focused on the western side of this tectonic province.

Minimum uplift rates (based on topographic relief) of several Pampean ranges are calculated to be between 0.5-0.8 mm/a, over the past 4-7 million years. These numbers indicate a rapid rise of the Laramide-style block uplifts contemporaneous with low-angle thrusting in the eastern part of the Precordillera. The data suggest that foreland deformation is closely synchronized with the initiation (~16 Ma) and propagation of flat subduction in the region.

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Reynolds, James H., Naeser, Charles W., and Hernández, Roberto, 1993, Chronostratigraphic calibration of Neogene sedimentation in the Argentine Subandes, Salta Province, northern Argentina: Geological Society of America Abstracts with Programs (Boston, MA), v. 25(6), p. A-473.

CHRONOSTRATIGRAPHIC CALIBRATION OF NEOGENE SEDIMENTATION IN THE ARGENTINE SUBANDES, SALTA PROVINCE, NORTHERN ARGENTINA

REYNOLDS, James H., Dept. of Geosciences, Western Carolina University, Cullowhee, NC 28723; NAESER, Charles W., USGS--Isotope Geology Branch, Box 25046, Federal Center, Denver, CO 80225; HERNANDEZ, Roberto, Los Eucaliptos 319, B° Tres Cerritos, Salta (4400), Argentina

The Río Iruya exposes a 7.5 km thick section of Neogene continental strata in Salta Province, northern Argentina. The river has incised a ~100 m deep canyon into the jungle since man-induced stream piracy occurred in 1893. Magnetic polarity stratigraphy, complemented by two fission track ages, was conducted to develop the first chronostratigraphic calibration of sedimentation in the Argentine Subandes fold-thrust belt. Exposure problems in the lowest 1200 m cause some uncertainty in the chronology but the upper 6300 m of strata allow a reasonable correlation. We apply the ages to four regionally recognized volcanic airfall marker horizons informally referred to by petroleum companies as Tuffs #1(3360 m level), #2 (4400 m), #3 (5400 m), and #4 (5565 m). Four closely spaced airfall beds comprise the distinctive Tuff #2.

Extrapolating the paleomagnetic data, we place the base of the section (0 m) at ≈14.7 Ma and the top at 1893. Fission track analyses on Tuff #1 resulted in a 5.28 ￿ 0.6 Ma date and an age of 4.3 ￿ 0.5 Ma on the basal bed of Tuff #2. Magnetic polarity data indicate a 3.8-3.9 Ma age for Tuff #3 and 3.7-3.8 Ma for Tuff #4. Section measurements across the region suggest that some strata are missing in the Río Iruya area. Our chronology indicates a depositional hiatus between ≈6.9 Ma and ≈5.7 Ma.

A rapid sediment accumulation rate (~1.0 mm/a) began at ~7 Ma and continued until ~2.5 Ma. From 2.5 Ma to 1893 the rate dropped dramatically to 0.2 mm/a. These results suggest that uplift of the western portion of the Argentine Subandes began at ~5.7 Ma. The basin depocenter migrated ~2.5 Ma as eastward thrusting continued.

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Reynolds, James H., 1989, Uplifting the Andes with the click of a mouse: Green Mountain Geologist (Northfield, VT), v. 15(4), p. 11-12.

UPLIFTING THE ANDES WITH THE CLICK OF A MOUSE

James H. Reynolds, Department of Earth Science, Norwich University, Northfield, VT 05663

Application of in-house, public domain, and commercially available Macintoshô softwear has streamlined chronostratigraphic analysis using magnetic polarity stratigraphy in the Central Andean foreland basin of north-central Argentina. Samples are collected at approximately 30 m intervals from sections of Neogene continental strata. A minimum of three samples is collected at each site. Paleomagnetic analyses of samples are done on a Schonstedt SSM-1ô flux gate spinner magnetometer connected to a Macintosh Plusô through a Taurus Oneô interface. The magnetometer is driven and paleomagnetic vectors are calculated by the program MacPMAGZ; data are stored in sample-specific files.

A series of file manipulation programs sorts the data and solves a number of paleomagnetic statistical calculations including: the Virtual Geomagnetic Pole Latitude (VGP Latitude) and Watson's (1956) "R" statistic to determine statitistical significance of the mean declination and inclination of each site. The accuracy of statistically significant data is tested with a reversal test using StereoNet (by Richard Allmendinger). The precision (a95) of the data is determined with Fisher5 (by David Douglass). Using MacStrat (by Richard Allmendinger) and MacDrawô to display the final results, the VGP latitude of each site is plotted against the stratigraphic level of the site and abstracted to the standard black and white bar diagrams used on paleomagnetic time scales. The local column is then correlated to the sea floor magnetic polarity time scale (Berggren and others, 1985). Stochastic modeling of the expected time intervals for the paleomagnetic sections is done using the program JohnsonñMcGee. Rates of sediment accumulation are then derived by plotting the stratigraphic level of all magnetic reversals vs. the position of each reversal on the paleomagnetic time scale using MacDrawô.

Increases in the rate of sediment accumulation are interpreted to reflect an increase in the rate of basin subsidence resulting from crustal loading due to overthrusting in the sediment source areas. Timing of the initiation of source area uplift is read directly from the plot. Comparison of paleomagnetic columns and their corresponding sedimentary facies from thick sections in widely separated parts of the basin allows an interpretation of the evolution of the paleogeography through time.

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Reynolds, James H., Malizzia, Daniel C., Tabbutt, Kenneth D., Johnson, Noye M., and Nickelsen, Jillian J., 1988, Chronology of Andean tectonic events affecting the Campo de Talampaya, La Rioja Province, Argentina: Geological Society of America Abstracts with Programs (Denver, CO), v. 20(7), p. A380.

CHRONOLOGY OF ANDEAN NEOGENE TECTONIC EVENTS AFFECTING THE CAMPO DE TALAMPAYA, LA RIOJA PROVINCE, ARGENTINA

REYNOLDS, J. H., Department of Earth Sciences, Dartmouth College, Hanover, NH 03755; MALIZZIA, D. C., Depto. de Cs. Geológicas, Universidad de Buenos Aires, Ciudad Universitária Pab. II, 1428 Ñuñez, Buenos Aires, Argentina; TABBUTT, K. D., JOHNSON, N. M., and NICKELSEN, J. J., Dept. of Earth Sciences, Dartmouth College, Hanover, NH 03755

The Campo de Talampaya is an intermontane basin, in La Rioja Province, Argentina, situated near the boundary between two tectonic provinces affected by subhorizontal subduction of the Nazca Plate. A 2190 m section of Neogene continental strata was dated at Río Mañero. The data constrain the timing of tectonic events that disturbed rocks exposed in the region. Magnetic polarity stratigraphy and fission track ages of zircons from interbedded airfall tuffs provided the chronologic information.

The Sierra de los Tarjados is located 3 km to the east of the top of the section and the Sierra Morada is at its western base. At the latitude of the study, these are the two westernmost blocks of the thick-skinned Sierras Pampeanas province. Forty-five kilometers west of the section, the easternmost range of the thin-skinned Precordillera rises above the intervening Bermejo Basin. The Neogene strata record the transition from an early phase, when deposition occurred in the foreland basin of the Precordillera to a later phase in which the basin was controlled by the basement uplifts of a broken foreland.

The lowest third of the section is dominated by lacustrine strata of the Río Mañero Formation. Airfall tuffs are absent below the 400 m level but are abundant between 400ñ650 m and correspond with a red to grey change in rock color. The remainder of the section is primarily low energy fluvial strata of the Desencuentro Formation. Airfall tuffs are abundant between 1450-1650 m.

A total of 32 magnetic reversals was encountered above the 400 m level. Below this level a post-depositional remanent magnetization obscures the true polarity of the strata. The paleomagnetic data indicate that the upper 1800 m of the section was deposited between 14.7ñ5.7 Ma. One fission track date from the lower volcanic unit (15.0 ￿ 1.2 Ma [￿ 2s]) and a second from the upper unit (7.4 ￿ 0.9 Ma) constrain the chronologic interpretation. A linear sediment accumulation rate of 0.18 mm/a in the Río Mañero Formation tapers off to 0.09 mm/a at the base of the Desencuentro Formation at 10.4 Ma, corresponding with the change in depositional environment. Above this level, the rate gradually increases to 0.31 mm/a. The rate increase corresponds well with the uplift in the eastern part of the Precordillera, suggesting that the increase may be due to an eastwardly migrating basin depocenter.

The section was tilted to the east by uplift of the Sierra Morada. The fault at the base of the Sierra de los Tarjados truncates the tilted strata. Fanglomerates derived from the Sierra de los Tarjados unconformably overlie the section. These observations indicate that uplift of the Sierra Morada and Sierra de los Tarjados, on the margins of the Campo de Talampaya, occurred after 5.7 Ma.

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Ramos, Victor, Reynolds, James H., Tabbutt, Kenneth D., and Jordan, Teresa E., 1988, Pliocene uplift of the Sierras de Umango and Espinal, La Rioja Province Argentina: Geological Society of America Abstracts with Programs (Denver, CO), v. 20(7), p. A61.

TIME CONSTRAINTS FOR THE UPLIFT OF THE SIERRAS DE TORO NEGRO, UMANGO, AND ESPINAL, WESTERN SIERRAS PAMPEANAS, ARGENTINA

RAMOS, V. A., Depto. de Cs. Geológicas, Univ. de Buenos Aires, Ciudad Universitária Pab. II, 1428 Ñuñez, Buenos Aires, Argentina, REYNOLDS, J. H., Dept. of Earth Science, Norwich University, Northfield, VT 05663, JORDAN, T. E., Dept. of Geological Sciences, Cornell University, Ithaca, NY 14853, and TABBUTT, K. D., Dept. of Earth Sciences, Dartmouth College, Hanover, NH 03755

New chronologic data were obtained from continental strata that are nested between uplifted ranges of the northwestern Sierras Pampeanas in the Quebrada de la Troya near Vinchina, La Rioja province, Argentina. Facies and cross-cutting relations permit the determination of the uplift history of the adjacent area.

The Neogene strata vary from 4000 to 10,000 m thick along strike and constitute the thickest exposed sequence of foreland basin strata in the flat-slab region of the Argentine Andes. The Sierra de los Colorados, in which the strata are exposed, is situated to the east of the Cordillera Frontal and to the west of the Sierra de Famatina, an uplifted basement block in the Sierras Pampeanas. Along strike, the strata are deformed by and coarsen toward other basement uplifts (the Sierra de Toro Negro to the north and the Sierras de Umango and Espinal to the south).

The Vinchina Formation is characterized by low energy fluvial facies that coarsen slightly to the north and south from a mud-rich axis. The overlying Toro Negro Formation is characterized by conglomeratic facies; the lower part is more uniformly conglomeratic.

Two interstratified airfall tuffs were sampled along the Quebrada de la Troya section, between the towns of Vinchina and
Jagüé. Fission track dating of zircons yields an age of 7.3 ￿ 1.2 Ma (￿2s) for a tuff located 1500 m above the base of the exposed section. A second age of 4.3 ￿ 1.0 Ma was obtained from a tuff located near the formational contact, 4700 m above the base. The limits set by these dates and a stratigraphic separation of 3200 m between the volcanic layers indicate a very rapid mean rate of sediment accumulation of 1.1 mm/a with a possible range between 0.62ñ2.67 mm/a.
These data, existing lithological studies, and lateral tracing of the strata reveal that the formational contact is time transgressive. It is youngest in the Quebrada de la Troya (4.3 ￿ 1.0 Ma) and older to the north (≈ 8.5 Ma near the Sierra de Toro Negro) and south (12.5 Ma south of the Sierra de Umango). Extremely coarse facies in the Río Las Chalas section in the north suggest significant uplift of the Sierra de Toro Negro shortly after 6.2 Ma. Uplift of the Sierras de Espinal and Umango commenced after 7.3 ￿ 1.2 Ma with significant uplift occurring after 4.3 ￿ 1.0 Ma. Relief of the two ranges is about 2600 m suggesting a minimum uplift rate between 0.49-0.79 mm/a with a mean of 0.60 mm/a.

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Reynolds, James H., Tabbutt, K.D., Johnson, N.M., and Jordan, T.E., 1987, Non-systematic uplift of the northwestern Sierras Pampeanas, Catamarca Province, Argentina: Interpretation of magnetic polarity stratigraphy data: Geological Society of America Abstracts with Programs (Phoenix, AZ), v. 19(7), p. 817.

NON-SYSTEMATIC UPLIFT OF THE NORTHWESTERN SIERRAS PAMPEANAS, CATAMARCA PROVINCE, ARGENTINA: INTERPRETATION OF MAGNETIC POLARITY STRATIGRAPHY DATA

REYNOLDS, James H., TABBUTT, Kenneth T., JOHNSON, Noye M., Dept. of Earth Sciences, Dartmouth College, Hanover, NH 03755; and JORDAN, Teresa E., Dept. of Geological Sciences, Cornell University, Ithaca, NY 14853

Deformation of the Central Andes, in western Argentina, proceeded systematically, from west to east, in the thin-skinned fold-thrust belt. The Sierra de Narváez, Sierra de Famatina, and Sierra de Fiambalá are thick-skinned uplifts in the northwestern portion of the Sierras Pampeanas, in Catamarca province, to the northeast of the fold-thrust belt. These ranges are located in the 'Transition Zone' between 'flat-slab' and 'normal' segments of the Central Andes. Uplift appears to be related to their proximity to the 'flat-slab' segment. The data presented here suggest that uplift commenced in the Late Miocene.

Magnetic polarity stratigraphy was done on two Neogene continental sedimentary sections, separated by approximately 20 km, exposed in the northeastern part of the Sierra de Famatina. Approximately 4 km of fluvial strata were measured and sampled along the Río Guanchín and in the Quebrada de la Troya. Four fission track ages from zircons, collected from interbedded tuffs, place temporal constraints on the paleomagnetic data. The strata were deposited during the 8.2-3.9 Ma interval. A 1.4 million year overlap (6.8-5.4 Ma) is present between the sections.

Fining of Quebrada de la Troya strata at 5.8 Ma may be related to elevation of base-level due to the initiation of uplift in the Sierra de Fiambalá, about 10 km to the east. The upper 700 m (5.4-3.9 Ma) of Río Guanchín strata coarsen upward and record the uplift of the Sierra de Narváez 10 km west of the area. Post-depositional folding and faulting at both localities appears to be related to uplift of the northern Sierra de Famatina.

These data are consistent with the findings of Tabbutt and others (1987) that the rise of the Sierra de Famatina was preceded by uplift to the east. This contrasts with the west to east deformational pattern seen in the thin-skinned area and suggests that the rise of the Sierras Pampeanas may have occurred in a non-systematic manner.

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Tabbutt, K.D., Reynolds, James H., Johnson, N.M., and Jordan, T.E., 1987, Fission track constraints on the uplift of the Sierra de Famatina, Argentina: A stratigraphic approach: Geological Society of America Abstracts with Programs (Phoenix, AZ), v. 19(7), p. 863.

FISSION TRACK CONSTRAINTS ON THE UPLIFT OF THE SIERRA DE FAMATINA, ARGENTINA: A STRATIGRAPHIC APPROACH

TABBUTT, Kenneth D., REYNOLDS, James H., JOHNSON, Noye M.,Dept. of Earth Sciences, Dartmouth College, Hanover, NH 03755; and JORDAN, Teresa E., Dept. of Geological Sciences, Cornell University, Ithica, NY 14853

The Sierra de Famatina is the highest range (5500 m) in the Sierras Pampeanas tectonic province of western Argentina. The Sierras Pampeanas are crystalline basement block ranges, structurally similar to the Laramide Ranges of the North American Cordillera. This thick-skinned deformation appears to have been induced by 'flat-slab' subduction of the Nazca plate beneath the South American continent. Neogene sedimentary basins flanking the range contain syntectonic clastic strata derived from the rising Pampean Ranges. Fission track dates were made from zircons extracted from 6 interbedded tuffs exposed along the Río Amarillo and Río Blanco on the east side of the Sierra de Famatina and one from the west side near Villa Unión. These dates enable time constraints to be placed on deposition. Correlation of lithofacies with deformational events at the sediment source elucidates the tectonic history of the Sierra de Famatina and adjacent ranges.

Prior to 7 Ma the Sierra de Famatina region was part of a foreland basin receiving fine-grained sediments from a distal thin-skinned fold and thrust belt located to the west. Prior to 5.8 Ma the Cadena de Paiman, 10 km to the east of the Sierra de Famatina, rose and deposited fanglomerates. Uplift of the Sierra de Famatina began around 6 Ma and coincided with silicious extrusions along major bounding faults. Further uplift around 4 Ma deposited cobble and boulder conglomerates derived from Sierra de Famatina. Subsequent to this event, basement was thrust over the upper conglomeratic sequence.

Given the relief of Sierra de Famatina (4500 meters), erosion of a minimum of 1100 m of overlying strata, and an initial uplift at 6 Ma, a minimum upthrow rate 0.93 mm/a is calculated. These data suggest rapid uplift of the western Sierras Pampeanas since latest Miocene time.

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Reynolds, James H., Nickelsen, J.J., Tabbutt, K.D., Jordan, T.E., and Johnson, N.M., 1986, Variation in sediment accumulation rate within the Central Andean foreland basin (16-10 Ma interval), San Juan and La Rioja provinces, Argentina: Geological Society of America Abstracts with Programs.(San Antonio, TX), v. 18(6), p. 729.

VARIATION IN SEDIMENT ACCUMULATION RATE WITHIN THE CENTRAL ANDEAN FORELAND BASIN (16-10 Ma INTERVAL), SAN JUAN AND LA RIOJA PROVINCES, ARGENTINA

REYNOLDS, James H.; NICKELSEN, Jillian J.; TABBUTT, Kenneth D., Dept. of Earth Sciences, Dartmouth College, Hanover, NH 03755; JORDAN, Teresa E., Dept. of Geological Sciences, Cornell University, Ithaca, NY 14853; JOHNSON, Noye M., Dept. of Earth Sciences, Dartmouth College, Hanover, NH 03755

Chronology of three, contemporaneous, continental Middle Miocene (16-10 Ma) sections is established in La Rioja and San Juan provinces, in the Central Andes of western Argentina. These sections lie above the flat-slab segment of the subducting Nazca Plate where the volcanic arc ceased activity in the Middle Miocene. The sections span the boundary between thin-skinned Precordillera thrust belt on the west and the thick-skinned Sierras Pampeanas on the east, constraining times of deformation in these two tectonic provinces. Ages are determined using magnetic polarity stratigraphy and fission track dates from zircons.

Gypsiferous lacustrine siltstones and fine grained sandstones in the Río Mañero section (16-10 Ma), within the Sierras Pampeanas, indicate a playa environment. Strata near Huaco (14-10 Ma), at the boundary between the tectonic provinces, were deposited in a low-relief, braided fluvial system. Despite differing sedimentary environments, preliminary sedimentation rates at Huaco and Río Mañero are 0.2 mm/year. In contrast sediments at Las Juntas in the Central Precordillera (16-10 Ma) accumulated rapidly (0.7 mm/year). In the 5000 m thick section, siltstones, interbedded with sandstones, represent a low-relief fluvial environment that coarsens upwards to braided stream deposits. Conglomeratic clasts include volcanic and granitic rocks and fine green sandstones indicating a source area in the high Andes west of the Precordillera and possibly the western Precordillera.

These stratigraphic sequences record intermediate and distal syntectonic sedimentation during mountain-building in the Andes. Upward coarsening in the western structural province suggests that deformation migrated eastward during the time interval when the magmatic arc waned and became inactive. Linearity of sediment accumulation rates suggests that fluvial systems maintain a delicate equilibrium even in an active orogenic environment.

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Reynolds, J. H., Galli, C. I., Hernández, R. M., Idleman, B. D., Kotila, J. M., Hilliard, R. V., and Naeser, C. W., in review, Middle Miocene tectonic development of the Transition Zone, Salta Province, NW Argentina: Magnetostratigraphic constraints from the Orán Group, Aº González, Geological Society of America Bulletin.

MIDDLE MIOCENE TECTONIC DEVELOPMENT OF THE TRANSITION ZONE, SALTA PROVINCE, NW ARGENTINA: MAGNETOSTRATIGRAPHIC CONSTRAINTS FROM THE ORÁN GROUP, Aº GONZÁLEZ

ABSTRACT

Magnetostratigraphy, isotopic dating and sandstone petrography constrain the depositional history of foreland basin strata in the Neogene Metán Subgroup of NW Argentina. The strata were deposited between ≈15.1 and ≈10.3 Ma in the eastern Sistema de Santa Bárbara. The region is positioned above the Cretaceous Salta Rift Basin in the Transition Zone between relatively steep- and flat-subducting segments of the Nazca Plate.

Strata from all formations within the Metán subgroup are shown to be diachronous with contact ages younger in the east. Changes in the lithic clast component of sandstones collected from the Aº González section suggests that fluvial strata from the basal Río Seco were derived from uplift of the Puna plateau which started to rise in Miocene time prior to 15.1 Ma. At ≈14.8 Ma basin accommodation began to increase dramatically in the fine-grained Anta Formation due to contemporaneous tectonism in the west, sea level rise, and location in a zone of weakness near the eastern boundary of the Salta Rift. Uplift in the western portion of the Cordillera Oriental (apparently) began by 13.7 Ma and rapidly migrated eastward to the eastern limit of the Cordillera Oriental at this latitude by ≈10 Ma between 25°-26° S. As thrusting migrated eastward, low energy depositional environments in the Anta Formation were overwhelmed by coarser-grained strata of the Jesús María Formation ≈13.7 Ma at Aº González. Above an erosional unconformanty that began ≈10.3 Ma, the basal Guanaco Formation from the overlying Jujuy Subgroup was deposited beginning sometime after 9 Ma.

Sandstones from Río Yacones suggest that initial Cordillera Oriental uplift continued for several million years longer between 24°-25° S. Uplift of the Sistema de Santa Bárbara, in the distal portion of the foreland, did not begin until after ≈9 Ma.

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Viramonte, J. G., Reynolds, J. H., Del Papa, C., and Disalvo, A., 1994, The Corte Blanco garnetiferous tuff: a distinctive late Miocene marker bed in northwestern Argentina applied to magnetic polarity stratigraphy in the Río Yacones, Salta Province: Earth and Planetary Science Letters, v. 121, p. 519-531.

THE CORTE BLANCO GARNETIFEROUS TUFF: A DISTINCTIVE LATE MIOCENE MARKER BED IN NORTHWESTERN ARGENTINA APPLIED TO MAGNETIC POLARITY STRATIGRAPHY IN THE RÍO YACONES, SALTA PROVINCE

ABSTRACT

We introduce the Corte Blanco Tuff, a white garnetiferous air fall unit, as a distinctive Neogene marker bed. Three whole rock K/Ar ages from rocks at the source of this unit indicate that it was erupted 8.73 ± 0.25 Ma from the La Pava-Ramadas Caldera on the Argentine Puna. Ash spread eastward across the foreland provinces of the Eastern Cordillera and Sierras Subandinas. Recognition of this dated marker unit in these provinces provides the first, easily identified, late Miocene time line in the vast, densely vegetated region to the east of the Puna. We encountered the unit in seven localities from all three morphostructural provinces in northwestern Argentina. A depositional gap in the air fall material is present between 20-150 km to the east of the caldera.

Recognition of the Corte Blanco Tuff in the Río Yacones, near Salta, allowed interpretation to be made of a magnetic polarity stratigraphy section erected in marginally suitable Neogene detrital strata of the Río Guanaco Formation. We interpret the strata we examined to have been deposited between 10.5-6.4 Ma and report a 10° clockwise rotation since the strata were deposited. This is the first numerically dated section in the Río Guanaco Formation of NW Argentina. These results reveal that uplift of the Eastern Cordillera was in progress at this time.

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Malizia, D. C., Reynolds, J. H., and Tabbutt, K. D., 1995, Chronology of Neogene sedimentation, stratigraphy, and tectonism in the Campo de Talampaya Region, La Rioja Province, Argentina: Sedimentary Geology, v. 96, p. 231-255.

CHRONOLOGY OF NEOGENE SEDIMENTATION, STRATIGRAPHY, AND TECTONISM IN THE CAMPO DE TALAMPAYA REGION, LA RIOJA PROVINCE, ARGENTINA

ABSTRACT

Chronostratigraphic, paleoenvironmental, and tectonic analysis of a 2,320 m thick Neogene continental section was conducted along the southern limit of the Campo de Talampaya. Magnetic polarity stratigraphy and fission track ages were used to develop a relatively precise geochronology. The strata record a period of mixed evaporitic and terrigenous clastic sedimentation (≈20-6.1 Ma) that persisted in the distal Bermejo Basin during eastward migration of the Precordillera fold-thrust belt. A critical transition of depositional environments and a change from western to eastern source areas occurred between 12.9-12.3 Ma. The saline mudflat environment of the Río Mañero Formation, derived from a western source and characterized by fine-grained sandstones, mudstones, and evaporites, yielded to a short period of playa lake deposition. This was soon overwhelmed by deposition of the Desencuentro Formation, composed of sandstones and mudstones with an eastern source area and deposited in a sand-flat/mud-flat environment. We propose that this stratigraphic transition resulted from initiation of uplift of the Sierra de Velasco, a range of the Sierras Pampeanas, located ≈65 km to the ENE of the section. Basement clasts, probably derived from the Sierra de Velasco, are found in conglomeratic lenses beginning in the lower part of the Desencuentro Formation.

The modern Campo de Talampaya basin formed after deposition of the top of the Alto de San Nicolás paleomagnetic section (6.1 Ma) when west-verging reverse faults uplifted the Sierra Morada and Sierra de los Tarjados. All strata in the section were tilted eastward during these culminating uplift events.

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Reynolds, J. H., Jordan, T.E., Johnson, N.M., Damanti, J. and Tabbutt, K. D., 1990, Neogene Deformation of the Flat-Subduction Segment of the Argentine-Chilean Andes: Magnetostratigraphic Constraints from Las Juntas, La Rioja, Argentina: Geological Society of America Bulletin, v. 102, p. 1607-1622.

NEOGENE DEFORMATION OF THE FLAT-SUBDUCTION SEGMENT OF THE ARGENTINE-CHILEAN ANDES: MAGNETOSTRATIGRAPHIC CONSTRAINTS FROM LAS JUNTAS, LA RIOJA, ARGENTINA

ABSTRACT

Substantial Neogene deformation occurred in the part of the Andes Mountains and the Sierras Pampeanas that overlie a sub-horizontal segment of the subducting Nazca plate in northern Argentina. The history of the volcanic arc suggests that the Nazca plate attained the present low-angle inclination between about 17 Ma and 10 Ma, with the region underlain by the flat Nazca plate widening eastward thereafter. We compare the history of deformation, provided by magnetostratigraphy of Miocene strata in foreland and intermontane basins, to the history of the subducting plate.

At Las Juntas, in the Precordillera thin-skinned thrust belt, 4780 m of Neogene nonmarine strata record the initiation and termination of sedimentation in a segment of a foreland basin. Magnetostratigraphic data, complemented by fission-track dating of an interbedded tuff, indicate that the strata span from 18 to ~9 Ma. The earliest strata correlate to the end of a time of volcanic activity in the magmatic arc and the beginning of reverse faulting in the Frontal Cordillera. Subsequently, the deformation front migrated eastward, generating the Precordillera thrust belt, and finally the Sierras Pampeanas basement uplifts. The rate of stratal accumulation varied systematically as new uplifts arose, with a maximum rate exceeding 1.0 mm/a between ~15 Ma and 12 Ma.

A correlation between the deformational history and the geometric history of the subducted slab suggests that thrusting migrated eastward as the angle of subduction shallowed. Although large-scale factors affecting the plate margin beyond the flat-slab segment may have partly induced the compressional deformation, this correlation suggests that processes related to flat subduction are of importance in deforming the continental plate.

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Reynolds, J. H., 1990, Magnetoestratigrafía de dos perfiles neógenos: Sierras Pampeanas del noroeste: XI Congreso Geológico Argentino, Actas 2, p. 271-274, San Juan.

MAGNETOESTRATIGRAFÍA DE DOS PERFILES NEÓGENOS: SIERRAS PAMPEANAS DEL NOROESTE

ABSTRACT

Dos perfiles magnetoestratigráficos revelan nuevos datos sobre la edad de los estratos, las estructuras y la historia tectónico-neógena de la parte noroeste de las Sierras Pampeanas (prov. de Catamarca). El perfil de la Quebrada de la Troya fue depositado durante los 8.2-5.7 Ma mientras que el del Río Guanchín tiene una edad entre los 6.5-3.9 Ma.

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Reynolds, J. H., 1987, Timing and sources of Neogene and Quaternary volcanism in south-central Guatemala. In Williams, S. N. and Carr, M. J. (eds.), Richard E. Stoiber 75th Birthday Volume. Journal of Volcanology and Geothermal Research, v. 33, p. 9-22.

TIMING AND SOURCES OF NEOGENE AND QUATERNARY VOLCANISM IN SOUTH-CENTRAL GUATEMALA

ABSTRACT

Five new and six existing radiometric age dates place constraints on the timing of volcanic episodes in a 1400 km2 area east of Guatemala City. The source of the voluminous Miocene rhyolitic welded tuffs was the newly discovered Santa Rosa de Lima caldera, in the northern part of the area, not fissure eruptions as was previously believed. Resurgence during the Pliocene included the eruption of more silicic tuffs, followed by post-collapse volcanism around the perimeter. Volcanism in the southern part of the area occurred along the Neogene volcanic front. The sources for these Late Miocene and Pliocene andesitic lavas were not fissure eruptions, as was once believed, but were four large volcanic centers, Cerro Pinula, Ixhuatán, Teanzul, and Cerro La Gabia. The Santa Rosa de Lima caldera structure deflects the Jalpatagua Fault forming tensional fractures along which eruptions in the Quaternary Cuilapa-Barbarena cinder cone field took place. Pleistocene ash flows were erupted from Ixhuatán and Tecuamburro volcanoes in the southern part of the area. Tephras from Ayarza, Amatitlán, and Atitlán blanket the northern and central portions. Present day activity is restricted to hot springs around the northern and eastern base of Tecuamburro volcano.

Based on the work in this area it is proposed that rocks of the Miocene Chalatenango Formation throughout northern Central America were erupted from calderas behind the Neogene volcanic front. Rocks of the Mio-Pliocene Bálsamo Formation in Guatemala and El Salvador were erupted from discrete volcanic centers along the Neogene volcanic front. Pliocene rocks of the Cuscatlán Formation probably represent post-collapse volcanism around earlier caldera structures.

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Last updated 5/10/01