INFLUENCE OF CLIMATE AND THE EXPANSION OF C4 GRASSES ON SEQUENCE-SCALE CYCLICITY AND LANDSCAPE DEVELOPMENT DURING THE LATE MIOCENE TO PLEISTOCENE OF WEST TEXAS

The Neogene records a period of global climate change and terrestrial ecosystem evolution; however, little is known about whether these changes influenced sedimentary and soil forming processes. The late Miocene to Pleistocene Fort Hancock and Camp Rice Formations of the northwest Eagle Flat Basin, an isolated rift basin formed during Basin and Range extensional tectonism, were analyzed to determine whether changing climate and concomitant C₄ grass expansion influenced fluvial depositional style.

Alluvial stacking-pattern analysis of a drill core through the Eagle Flat Basin strata (12.4 Ma to present) reveals sequence-scale trends in paleosol maturity, cycle thickness, and facies proportions. Higher-frequency cycles of fluvial sedimentation and associated pedogenesis are superimposed onto an overall fining-upward tectonic fill succession. Stable isotope geochemistry of bulk soil samples reveals an initial δ­­¹³C of -23.0 ± 0.6‰ and abrupt maximum, i.e., "spike" of -12.4‰ at 7.25 Ma, followed by an average δ­­¹³C of -21.1 ± 1.8‰. This change in δ­­¹³C of SOM, which we hypothesize records the advent of C₄ grasslands into this area, coincides with a decreasing sedimentation rate, less frequent channel avulsion events, and an increase in paleosol maturity. These may record an ecosystem induced increase in landscape stability and pedogenesis.

The δ­­¹⁸O and δ­­¹³C of pedogenic carbonate allows atmospheric pCO₂ reconstruction of the late Miocene to Pleistocene. The pCO₂ estimates suggest an overall decreasing trend from 12.4 Ma to present; however, periodic increases in pCO₂ occur at the third- to fourth-order scale. Stratigraphic stacking-patterns correlate well with Neogene pCO₂ reconstructions. Long-term decreases in pCO₂ correspond with reduced sedimentation and low pedogenesis rates, and rapid increases in pCO₂ correspond with increased sedimentation and rapid weathering rates. This suggests that sedimentation and pedogenesis are influenced by third- to forth-order global climate cycles.