Understanding growth-faulted, intraslope subbasins by applying sequence-stratigraphic principles: Examples from the south Texas Oligocene Frio Formation: Discussion

Abstract

Brown et al. (2004) recently elaborated on previous sequence-stratigraphic models (e.g., Mitchum et al., 1990) to interpret large rollovers in the Frio Formation of Texas as depositional basins, in contrast to earlier interpretations of structural basins, filled primarily with shallow-water deposits (e.g., Ewing, 1986; Galloway and Morton, 1989). This discussion describes fundamental shortcomings in their observations and interpretations, specifically 1. Deep-water Frio facies are farther downdip and deeper than predicted by Brown et al. (2004). 2. Their model ignores the tenfold magnitude difference between the gradients of growth faults and clinoforms. 3. The comparatively thicker section on the downthrown block reflects greater subsidence rates associated with focused structural extension and not the development of steeply dipping fault scarps at the shelf margin. 4. Multiple methods of correlating well logs in growth-faulted regions are essential, including those disparaged as “traditional” by Brown et al. (2004), particularly in a “sequence-stratigraphic” framework. 5. Their sequence-stratigraphic model makes no reference to the past 15 yr of work on progradational components (i.e., highstand, lowstand prograding wedge, forced regressive wedge, and falling stage systems tracts); this especially hampers the stratigraphic interpretation of the progradational facies that dominate these growth-faulted zones. The disruption of stratigraphy and structure that occurs across growth faults has challenged geologists for many years. Which changes in thickness, age, depositional environments, and facies are related to growth faulting? Strata in growth-faulted settings are commonly characterized by shoreline environments and their resulting coarsening-upward regressive units or parasequences. In many situations, it is relatively straightforward to recognize parasequences in well logs, clearly bounded by marine flooding surfaces, expanding downdip across several growth faults, and it thus seems that an increase in subsidence rate was the cause of the thickening (e.g., figure 13 of Edwards, 1981). These parasequences display concomitant facies changes from proximal to distal (e.g., figures …