Masters Theses

Date of Award

5-2002

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Steven G. Driese

Committee Members

Claudia I. Mora, Michael E. Essington

Abstract

Claystone paleosols are common throughout the rock record (e.g., Pennington Fm., Upper Mississippian, TN) and formed in a variety of depositional environments. Studies of paleosols indicate that there is significant retention of primary soil features (i.e. slickensides, silgai and oriented clay fabric) that are indicative of the Vertisol soil order. These clay-rich soils form from shrink-swell processes associated with seasonal moisture deficit and increased evapotranspiration. The high bulk density, abundance of fine clays, and low topographic position aids in the formation and preservation of Vertisols in the rock record. Most soils form on stable surfaces where pedogenic processes, such as translocation of mobile constituents, structural development of the soil matrix, and accumulation of pedogenic carbonate and Fe-Me nodules, continue through time to alter the soil profile.

The study utilizes trends in macro- and micromorphology and bulk chemistry to: 1) address rates of Vertisol pedogensis and 2) identify the characteristics associated with time steps within the pedogenic process. Pedogenic processes are driven by interrelationships between soil forming factors: climate, topography, time, biota, and parent material. A chronosequence analyzes time-dependent variations in soil profile characteristics while constraining the other soil-forming factors. Soils formed on terraces of the Brazos River, on the Coastal Prairie of Texas represent various stages (~400 yrs to 35,000 yrs) of landscape stability and pedogensis. This chronosequence study illustrates predictable variations in profile thickness, lack of preservation of relict primary sedimentary bedding, depth of leaching, and degree of development of pedogenic carbonate, slickensides, and clay fabric. These properties are least developed in the Ships Series representing short-term pedogensis (~400 yrs). Conversely, maximum expression of these properties occurs in the Lake Charles Series, which represents ~35,000 years of pedogensis. The Pledger and Burleson Series, two intermediate-aged profiles, represent ~3,500 and ~18,000 years of soil formation, respectively.

Application of time-dependent morphologic and geochemical trends derived from the modern chronosequence has resulted in two significant conclusions. PaleoVertisols form on stable landscape surfaces occurring at sequence boundaries. The modern chronosequence aids in interpreting the frequency, extend and timing of marine and nonmarine cycles (~400 ka Milankovitch-scale sea-level fluctuations) during the late Mississippian in Tennessee. Based on the modern Vertisol analogs, paleoVertisols represent 2-3 orders of magnitude shorter time than initially estimated for the Pennington Fm. As such, there may be significantly more time not represented by rock than previously thought in cyclic marine/nonmarine intervals containing paleoVertisols. The chronosequence study also highlights an important problem regarding paleoclimate research utilizing paleosols. The results from this study suggest that time-dependent morphological and chemical features may mimic those that are climate-dependent. Soils formed in drier climates may display similar characteristics to those seen in younger profiles (Ships Series) whereas soils formed in wetter climates may exhibit features comparable to older profiles (Burleson Series), thus obscuring paleoclimate interpretations obtained from analysis of paleoVertisols.

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