The Role of Source-Area Climate and Weathering on Sedimentation Along the Paleogene Gulf Coast: Onshore Wilcox Group, Texas, USA
Source-area weathering, largely controlled by climate, can strongly influence the composition of sediment supplied to basins. We tested the idea that chemical weathering, in light of wide climate swings through the Paleogene, may have contributed to the observed shifts in both sediment supply and reservoir quality of onshore Wilcox–Frio deposits in Texas. More broadly, the relationship between weathering and basin sediments can help reservoir predictions at stratigraphic boundaries connected to climate, and may be extended to deepwater fields where the critical risk is reservoir quality. We sampled shale and sandstone pairs from fifteen 4th order stratigraphic cycles documented in cores from seven Wilcox Group penetrations in the Houston embayment. This approach allowed a look at weathering shifts recorded in both mudstone and sandstone, and over multiple time scales, from ~100 kyr (4th order cycles) to several million years (Paleocene–Eocene transition) within a single depocenter. Auxiliary sampling of the Wilcox Group in the Rio Grande embayment and the Oligocene Frio Formation tested the idea that any weathering control should be traceable outside the Houston embayment and upsection into the colder Oligocene. The “degree of weathering” represented by each sample was defined through combined geochemical analyses (XRF, ICP-MS) and petrographic point counts. We tracked the relative loss of mobile versus immobile major elements (i.e., the Chemical Index of Alteration) and labile versus stable minerals along with subsurface depth and depositional factors like environment, progradation, and shoreline migration. Rare-earth and trace element patterns in the shales confirmed that geochemical and petrographic shifts were not a function of bedrock composition. Results indicate that the intensity of weathering of the source terrane increased with global temperature into the Eocene to generate more aluminous muds and quartz-rich sands, corresponding with higher aggradation rates in the Upper Wilcox (Houston embayment), and impacting reservoir quality in terms of the balance of quartz, feldspar, and rock fragments supplied to the basin. This trend strongly reverses with decreasing global temperatures into the Oligocene Frio Formation. The weathering index preserved in the onshore Wilcox Group shales and sandstones tracks with stratigraphic patterns on time scales of several million to <1 million years and is a sensitive signal of climate change.
AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017