Depositional and paleoclimatic evolution of the Cenozoic High Plains succession from core: Haskell Co., Kansas

Abstract

The Neogene Ogallala Formation and overlying, hydraulically connected Quaternary units form the High Plains Succession (HPS), includes one of the largest, most economically important, and agriculturally important freshwater aquifers in the world, the High Plains Aquifer. Acquiring quality sedimentologic and stratigraphic information from the HPS has been challenging because of a lack of surface exposures, difficulty in retrieving intact drill cores due to its unconsolidated and saturated nature, and highly heterogeneous, terrigenous nature. This study evaluates 98 meters of core taken from Haskell County, Kansas in order to reconstruct depositional and paleoclimatic evolution of HPS through bulk paleosol geochemical proxies, gamma ray petrophysics, high frequency fluvial stacking pattern analysis, and stable isotopic analysis. Sedimentologic analysis reveals a predominantly fluvial system that experienced systematic changes in depositional style ranging from bed-load to suspended-load with intermittent periods of landscape stability, resulting in soil formation expressed as paleosols. The section is dominated by sand, which is congruent with studies in Texas and Nebraska and lithofacies analysis supports a distributary fluvial system depositional model for the HPS. From vertical sequence stratigraphic analysis, 99 individual fluvial aggradational cycles (FACs), 10 FAC-sets, and three fluvial sequences (FS) are identified. The considerable distance from the Cenozoic paleoshoreline (well over 900 km), paleosol geochemical weathering indices, and mean annual precipitation (MAP) estimates, suggest fluvial sequence deposition in the HPS is likely controlled by tectonism, and the development of better age constraints could test the influence of paleoclimate; however, the interaction of these two allogenic processes are likely very closely related. Further paleoclimatic investigation show that the HPS exhibits systematic trends in delta13Ccarb, delta13Corg, delta18Ocarb, and through the use of transfer functions from geochemical data, C4 biomass percent, mean annual temperature (MAT), and mean annual precipitation (MAP) are estimated. The systematic, long-term upward increase expressed in delta13Corg C4 biomass estimations closely resembles those from published studies. Trends in C3 and C4 biomass also appear to track general trends in mean annual temperature (MAT). These findings are congruent with similar studies that infer that the evolution of the C4 photosynthetic pathway coincided with an increase in aridity on the High Plains. Although biomass trends parallel trends in MAT, mean annual precipitation (MAP) estimations from the CIA-K proxy also increase through time. Through paleo-temperature proxy analysis, the local paleoclimate appears to have warmed through time, but it also became wetter (average values over 800 mm/year). These conclusions do not support increased aridity during deposition of the HPS, and aridity could be due to other paleoclimatic factors including, but not limited to, increased precipitation during cold months, increased evapotranspiration rates, and/or an increase in wind gradient. Additionally, pedogenic delta13Ccarb and delta18Ocarb values exhibit a positive trend as expected with increasing aridity, indicating that the observed increase in C4 biomass in the HP1A core is probably associated with a long-term trend toward greater aridity. However, the increase in aridity is not associated with a lack of precipitation. Therefore, the conventional association of increasing aridity with decreasing MAP does not apply to the High Plains Succession of southwestern Kansas. In contrast it is likely that the observed increase in aridity occurred through a combination of seasonal paleoclimatic factors. From this study, it is not presently conclusive which allogenic process dominated depositional control on the HPS; however, plate tectonics appears to be the driving force behind the climatic transitions and, is therefore likely more influential. Improving the chronostratigraphic constraints on the HP1A core will further test these findings and allow for a more thorough comparison to other well-dated stratigraphic sections from published literature. This study is a first attempt to for develop a sequence stratigraphic framework and landscape reconstruction from a cored section of the HPS in Kansas, and it should serve as a template for developing improved regional stratigraphic correlations through future investigation of the three fluvial sequence boundaries from one-dimensional alluvial vertical sequence analysis.