Low-amplitude, high-frequency climatically-driven sea-level changes in the Lower Wilcox

Understanding how climatically-driven changes can produce significant eustatic fluctuations, especially during periods of Earth’s history that lacked continental-scale ice sheets has been a long-time question for geologists. In this paper we use an example from Lower Wilcox greenhouse time to explain how climatically- driven short term eustasy controlled sedimentary deposition. We base our interpretations on a geochemical proxy model for orbital-scale greenhouse humid–arid climatic shifts and on palynological analysis.

During the humid greenhouse phase, intense chemical weathering, more precipitation, and therefore higher sediment supply advanced the deltas closer to the shelf edge. The regressive half-cycle is characterized by the presence of Deltoidospora which indicates coastal marshy conditions. During the arid greenhouse phase, less precipitation and therefore, less sediment supply decreased deltaic progradation and favored short term marine transgressions. We see a corresponding increase in the palynological indicator Classopollis classoides, a thermophilic and thus a reliable proxy for hot climatic conditions.

Low amplitudes and high frequencies excursions of sea level changes produced modest facies belt shifts recorded in 86 m of core from 2 wells in Karnes county (Moczygemba #11 and Olinick #16). Subsurface sandstone mapping based on about 800 well logs reflects short excursions of the shoreline during repeated cross shelf transits amid an overall backstepping succession.