An Outcrop and Core Study
*disclaimer* This was a project done in 2015, so the findings may be out of date as they are not updated with all the new drilling
The Boquillas formation is the outcrop or updip equivalent to the Eagle Ford section in the subsurface. The overall succession is similar, while the facies are slightly different \. From the Buda up to the overlying Austin, the facies in outcrop near Del Rio show much more evidence of being in a proximal location.
Most notably, there is bioturbation throughout the entire section. TOC is high and there are indeed mudrock intervals that were deposited in anoxic conditions. But, conditions must have been shallow enough to support a robust epifaunal community capable of bioturbation and shallow enough for reworking of sediment. The subsurface equivalent sections are much more dominated by anoxic strata, with very little evidence of bioturbation if any at all (it may be cryptic).The outcrop also allows the viewer to examine the lateral continuity of different structures, such as the cemented/replaced beds as well as the nodular beds. In core, these intervals seem to be unusual and perhaps a solitary feature, whereas in the outcrop, they are fairly continuous, indicating they beds represent some sort of regional environmental condition.
Comparison of the outcrop and core allows a better understanding of the sequence stratigraphy during this time period
The transition to Austin Chalk in core and outcrop is the same - alternations of white chalk with darker mudrock that can have a wispy appearance. Higher in the section, Austin Chalk in outcrop includes channels in the strata not viewed in core.
Upper Eagle Ford
Upper Eagle Ford in outcrop consists of bioturbated cycles of carbonate sediment and dark mudrock as well as nodular cycles of carbonate mud or grains alternating with mudrock. All outcrop facies are bioturbated. In the core, there is still no evidence of bioturbation, but there is an increase in grainy layers alternating with the mudrock and in some places, culminates as the “transitional facies” which is dominated by carbonate grain layers with evidence of reworking.
Middle Eagle Ford to Lower Eagle Ford
Lower Eagle Ford to Middle Eagle Ford Strata. Strata in outcrop changes to cycles of laminated mudrock alternating with cemented/replaced radiolarian and foraminifera packstone/grainstone intervals. In the core, the darker mudrock alternates with cemented zones as well, but these intervals resemble concretions as well. In all cases, these zones are fractured. Inoceramids are common throughout. Ashes start to appear, particularly in the outcrop.
Lower Eagle Ford
Lower Eagle Ford on top of the Buda. Contact with Buda looks erosional or disconformable in most places. Lower Eagle Ford consists of reworked clasts and limestone debris within a darker matrix host rock.
Lower Eagle Ford on top of Buda. Black mudrock with high TOC shows little evidence of reworking except immediately above the Buda. However, there are alternations of cemented zones that could represent the reworked sediment as seen in outcrop.