Bottom Current Deposition and Sediment Reworking in the Boquillas/Eagle Ford Fm: Val Verde County, West Texas, USA
Frebourg, Gregory; Ruppel, Stephen C.; Loucks, Robert G.
Roadcuts along Highway 90 west of Del Rio, Texas, provide numerous, high quality exposures of the Boquillas Formation, an equivalent of the Eagle Ford Fm of South Texas. The Boquillas can be subdivided into four units (A to D). Unit B consists of organic matter rich, calcareous mudrocks and interbedded calcarenite/calcsiltite beds and nodules, and volcanic ash deposits. The mudrocks comprise coccolith hash, clay minerals and, in lesser and variable amounts, tests of globigerinid foraminifera. These rocks display large, outcrop and smaller scale erosion surfaces, low-angle foresets, low-angle current ripples, scour-and-fill features, shell lags and plane-bedding. The organic matter-poor calcarenite/calcsiltite beds and nodules are composed of dominantly of globigerinid tests and fragments, and show foresets, trough-stratification, and plane-bedding. Where undeformed by compaction, isolated nodules show barchanoid morphologies. The sedimentary features encountered in both fine grained mudrocks and coarse grained calcarenite/calcsiltites testify to deposition and reworking under bottom currents, the difference in grain sizes and types being a function of changes in sediment supply. Volcanic ash deposits are closely associated with each calcarenite/calcsiltite horizon. This relationship suggests that iron input linked to volcanic ash falls provides nutrients that drive planktonic foraminifera blooms. The resulting increases in globigerinid test sediment flux results in the accumulation of carbonate-rich, organic matter poor deposits. Conversely, the organic matter rich deposits are linked to periods of lesser primary productivity and lower accumulation rates, condensing the organic matter. The coarser, organic matter-poor accumulations form laterally continuous blanket like deposits (beds) near the sources of these blooms but become less continuous (elongate nodular bedforms) away from the source areas due to bottom current transport. Knowledge of the paleocurrent patterns and production zone(s) may help predict productive versus less productive intervals, as the organic matter rich-accumulations are expected down-current where the planktonic input becomes starved.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013