Doctoral Dissertations

Date of Award

12-1996

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Geology

Major Professor

Stephen G. Driese

Committee Members

C. Mora, K. Walker, T. Ammons

Abstract

A Late Mississippian paleosol, in central Tennessee, satisfying all of the morphological criteria set forth in Soil Taxonomy for Holocene Vertisols provides quantitative paleoclimate information, in addition to the now commonplace interpretation of precipitation seasonality based on the presence of vertic features. Paleo-precipitation was estimated using the empirical relationship between depth to pedogenic carbonate horizon in Quaternary soils. Burial compaction, erosional truncation, and high paleoatmospheric CO2 concentration, all factors which complicate paleo-precipitation estimates, are unusually well-constrained for this paleosol. Allowing for 10% compaction, the paleosol had a pre-burial depth of 100 cm for the pedogenic carbonate horizon, yielding a mean annual paleo-precipitation estimate of 648 mm ±141. This is comparable to the mean annual precipitation for Brownsville, Texas, where similar soils are found today. A dolomicrite crust, developed in gilgai micro-lows, is well-preserved in the paleo-Vertisol. Higher Late Mississippian paleotemperatures and rates of evapotranspiration associated with a lower latitude paleogeography for central Termessee during the Late Mississippian may explain in part why Holocene coastal Vertisols in the Brownsville region lack surficial crusts. The Late Mississippian climate of central Tennessee is qualitatively defined as semi-arid. Burial compaction is one of several major obstacles to estimating paleo-precipitation from depth to pedogenic carbonate in favorably preserved paleosols. Paleosols must be decompacted and pre-burial depth to pedogenic carbonate obtained. Vertic paleosols may be particularly good candidates for paleoprecipitation estimates. because of an increased likelihood of preserving clastic dikes, one of the best features for estimating burial compaction. Compaction estimates from clastic dikes and literature - based depth of burial estimates suggest that vertic paleosols undergo significantly less burial compaction than may be commonly assumed. Late Carboniferous vertic paleosols, buried to 2.5 to 3.0 km, compacted to 93% of their original thickness. In contrast, clastic dikes in a non-pedogenic shale directly underlying one of the Late Carboniferous paleosols records compaction to 70% of original thickness. Similarly obtained burial compaction and burial depth estimates for Early Carboniferous, Ordovician, and Proterozoic vertic paleosols are used to test a burial compaction curve and equation specific for vertic paleosols. Results suggest that this "vertic-calibrated" curve and equation can be used to estimate burial compaction for vertic paleosols lacking clastic dikes, but additional testing is needed. Naturally high bulk densities may have limited the compactability of vertic paleosols. Likewise, high initial bulk density and an abundance of swelling-clays may have severely limited the transmissivity of vertic paleosols as they passed from pedogenic to burial environments. Thus, upon burial vertic paleosols may have behaved as closed systems, which has implications for understanding their diagenetic modification. Additional efforts to understand burial compaction of vertic paleosols also promise to improve our understanding of aquifer/aquiclude and hydrocarbon/reservoir relationships in sedimentary basins containing intercalated paleosols. The Harlem coal (Upper Pennsylvanian) is genetically related to the underlying, vertic, sub-Ames paleosol complex. Color patterns in the paleosol complex appear to have survived burial diagenesis and provide sensitive data on soil drainage and redox conditions. Catenary relationships preserved by the paleosol complex, indicate that soil drainage setting, largely a reflection of subtle topographic differences on the paleo-landscape, controlled initiation of the Harlem coal swamp. In the study area, paleotopography reflected the presence of an abandoned fluvial channel tract, or perhaps a marginal-marine bar complex (distributary mouth bar?) stranded on a broad, newly exposed surface created by withdrawal of the pre-Ames sea from the Appalachian basin. The Harlem peat accumulated in isolated topographic lows following local establishment of saturated zones in the top of the paleosol complex. Development of perched water tables was facilitated by formation of clay-enriched subsoil intervals. The clay-enriched intervals record the onset of long-wet/short-dry seasonality in precipitation and a corresponding decrease in the intensity of churning in the soil body. As the new, wetter climate became established, perched zones of saturation grew upwards to intersect the soil surface and initiated coal swamps. During the subsequent "Ames" transgression, the paleolandscape influenced deposition such that marine limestones were deposited on local topographic highs. The casual, routine invocation of a transgression-driven, broad, ubiquitous, uniformly rising regional water table to explain the initiation of coals swamps is not compatible with the gross color patterns preserved in the paleosol complex. Rather, the incipient Harlem coal swamp, feed by saturated zones in the top of the paleosol complex, records the onset of a wetter climate nourished by a new source of surface moisture, made available by expansion of the "Ames" sea.

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