Implications of the sedimentology and isotope geochemistry of calcic horizons in the Pliocene-Early Pleistocene St. David Fm. , Arizona

Conference · Wed Jan 01 00:00:00 EST 1992 · Geological Society of America, Abstracts with Programs; (United States)

OSTI ID:5588761

Smith, G A ^[1]; Wang, Y; Cerling, T E ^[2]

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1. Univ. of New Mexico, Albuquerque, NM (United States). Dept. of Earth and Planetary Science
2. Univ. of Utah, Salt Lake City, UT (United States)

Two types of calcic horizons occur in pedogenically modified alluvial mudstones of the Plio-Pleistocene St. David Fm.: (1) nodular-carbonate zones (stage 2 morphology) with downward decreasing carbonate abundance, located below leached red muds; (2) massive to nodular zones (stage 3) with sharp bases, upward decrease in carbonate abundance, associated with variably leached muds typically exhibiting green mottles. Type 1 horizons are traceable for hundreds of meters; type 2 horizons vary in thickness and carbonate content over distances as short at 15 m. and are laterally contiguous with pond deposits. Type 1 horizons are interpreted as vadose aridosol calcic horizons, whereas type 2 horizons are partly gleyed hydromorphic soils with calcite derived from groundwater as well as from leaching within the soil. O and C isotopic compositions of age-equivalent 1 and 2 carbonates are similar suggesting that type 2 calcite formed in the unsaturated zone, probably in the capillary fringe above the water table. These latter horizons define a limited time of elevated water table, which corresponds to a cooler/wetter climate and higher sedimentation rates. If the distinction between these two soil types was not made, the greater morphological maturity'' of type 2 horizons would lead to misinterpretation of more arid climatic conditions or lower sedimentation rates. Type 1 horizons occur within parallel piedmont-facies belts representing channel/channel margin and floodplain deposition. Paleosols are morphologically similar, although floodplain soils are more clay rich. C-isotope analyses suggest, however, different vegetational histories for the two depositional tracts that reflect different responses of channel-margin and floodplain settings to increasing seasonality of precipitation during the late Pliocene.

• OSTI ID: 5588761
• Report Number(s): CONF-921058-; CODEN: GAAPBC
• Journal Information: Geological Society of America, Abstracts with Programs; (United States), Vol. 24:7; Conference: 1992 annual meeting of the Geological Society of America (GSA), Cincinnati, OH (United States), 26-29 Oct 1992; ISSN 0016-7592
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
ARIZONA
GEOLOGIC FORMATIONS
CALCITE
GEOCHEMISTRY
STRATIGRAPHY
GROUND WATER
ISOTOPE RATIO
LITHOLOGY
MINERALOGY
PALEOCLIMATOLOGY
SOILS
WATER TABLES
CARBONATE MINERALS
CHEMISTRY
DEVELOPED COUNTRIES
GEOLOGY
HYDROGEN COMPOUNDS
MINERALS
NORTH AMERICA
OXYGEN COMPOUNDS
PALEONTOLOGY
PETROLOGY
USA
WATER
580000* - Geosciences