Shallow Water Euxinia and Density Stratification of the Cenomanian/Turonian Western Interior Seaway in Texas

Abstract:

The Eagle Ford Group was deposited at the south and east side of the Western Interior Seaway (WIS). These sediments contain evidence of anoxia and shallow-water deposition (between storm-wave and fair weather wave-base) in the proximal deposits at Lozier Canyon and Antonio Creek (LC/AC).

Shallow-water (above storm wave-base) depositional conditions are mostly based on the suite of primary sedimentary structures that includes: hummocky cross-stratification (HCS), swaley cross-stratification (SCS), symmetric ripples, gutter casts and shell lags. These occur in outcrop and support the interpretation that LC/AC is the most proximal location relative to other study locations. At Hot Springs in Brewster County, Texas, sedimentary structures are limited to shell lags, a few symmetric ripples and low-angle laminations. This is consistent with the distal expression of storm deposits. The primary sedimentary structures from the core from the subsurface in McMullen County, south Texas, are limited to those of low-energy currents, scouring, and reworking, which could indicate it is in the distally steepened portion of the carbonate ramp on which Eagle Ford Groups are deposited.

Pelagic foraminifera are the most microfossil and benthic foraminifera only occur in the Upper Eagle Formation at LC/AC. Molybdenum was also used as a proxy for occurrence and persistence of a sulfer-reducing zone. The geochemical data from the subsurface core suggests persistence of anoxia throughout its depositional history. At LC/AC, molybdenum is elevated (up to 5-6 times average shale value) and bioturbation in these zones is limited to the surfaces of symmetric ripples, suggesting that oxygenation events were brief and occurred only immediately after storm events. This is consistent with the hypothesis that the HCS, SCS, and symmetric ripples were caused by storms and temporarily mixed the ocean in a normally stratified water column. At LC/AC, the water column became persistently oxic from the latest Cenomanian through the end of the Turonian, suggesting either a relative drop in sea level or a deepening of the pycnocline.