Carbonate Depositional Motifs and Cycle Stacking Patterns in the Eagle Ford Formation, Texas

Forkner, Rob; Minisini, Daniel

The Eagle Ford formation, Texas, is predominantly a carbonate succession consisting of interbedded limestones and black marlstones. The Eagle Ford presents a number of distinct facies that exhibit distinct internally recurring motifs or facies associations. Using an example from outcrop near Del Rio, Texas, these motifs (from base to top) include: 1. Reworked chaotic with cross-stratified foram grainstones; 2. Homogeneous/laminated foram-rich packstones/grainstones interbedded with black marlstones; 3. bioturbated, pelleted wackestones and packstones interbedded with black marlstones; 4. bioturbated pelleted/skeletal wackestones and packstones interbedded with gray marlstones; 5. Nodular packstones and grainstones with interbedded gray marlstones; and 6. bioturbated chalky wackestones with interbedded marls. Depositional cycles within the Eagle Ford cannot be deciphered in the traditional sense of delineating shallowing-upwards facies successions because determining differences in actual water depth within individual successions is speculative at best. Nevertheless, Eagle Ford depositional cycles were described as those successions proceeding upward from black mudrock or marl upwards through a bed of carbonate sediment, which is a general trend that repeats throughout the formation.

Stacking pattern analysis of Eagle Ford indicates that the Eagle ford can be divided into 3 distinct intervals related directly to carbonate motifs.

These include: the classic lower Eagle Ford (predominantly current or storm reworked at the base, followed by pelagic sedimentation) where cycles tend to be comparably thin (characterized by motifs 1 and 2); the classic upper Eagle Ford (a combination of pelagic and in-situ carbonate sedimentation, with intensity of bioturbation and carbonate content increasing upward) where cycles thicken and thin upward into recognizable cycle sets (characterized by motifs 3 and 4); and a topmost member cycles again thin upwards into the overlying Austin chalk (characterized by motifs 5 and 6).

Similar depositional patterns linked to biostratigraphy are observed in outcrops both in West Texas and Central Texas and help unravel the depositional history of basin infill.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013