Shelf and Lower-Shoreface Deposits in the Upper Midway Group and the Transition into Fluvial-Dominated Deltaic Deposits in the Hooper Formation (Lower Wilcox Group) in the Southeastern Texas Gulf Coast

Abstract

The Paleocene Upper Midway Group in the southeastern Texas Gulf Coast records a stratigraphic succession of shelf and lower-shoreface systems. Four (4) recognition criteria for these systems include (1) thin (<2 ft [<0.6 m]), erosion-based and lenticular, very fine-grained and fine-grained sandstone beds with swaley cross-stratification, (2) a diverse Cruziana trace-fossil assemblage consisting of Teichichnus, Palaeophycus, Planolites, and Schaubcylindrichnus, (3) net-sandstone trends consisting of strike-elongate ribbons and pods that merge updip (north and northwest) into broad, sheet-like patterns, and (4) an absence of northwestern and northern feeder systems inferred from net-sandstone maps. Net-sandstone trends in Upper Midway shelf deposits in southeastern Texas are irregular and narrow, strike-elongate pods and ribbons ranging in thickness from 20 to 35 ft (6 to 11 m). They grade southward and southeastward into extensive, muddy areas with <10 ft (<3 m) of net sandstone. In contrast, lower-shoreface deposits at the top of the Upper Midway Group contain wide (>10 mi [>16 km]) strike-elongate belts of >40 ft (>12 m) of net sandstone. These belts extend northeastward >50 mi (>80 km) along depositional strike. Sandstone-body continuity, inferred from wireline-log cross-sections, differs greatly among these shelf and lower-shoreface deposits. Sandstone-body continuity in Upper Midway lower-shoreface deposits at 10 mi (16 km) well spacing is approximately 60%, whereas it is only 20% at the same well spacing in shelf deposits. At 20 mi (32 km) well spacing, lower-shoreface sandstone-body continuity is 37% but only 4% in shelf deposits. Microscale heterogeneity in the Upper Midway Group is related to sedimentary structures and accessory features that disrupt vertical sandstone-bed continuity over scales of millimeters to centimeters. These include mudstone drapes, scour surfaces, shell debris, mudstone clasts, and trace fossils. Mesoscale heterogeneity, inferred from cores and wireline-log cross-sections, is defined by lateral sandstone-body pinchouts and vertical stacking patterns. In contrast, macroscale heterogeneities in the Upper Midway Group, inferred from net-sandstone maps, compose facies architecture at a regionally-mappable scale. They include paleogeographic elements such as shelf sand-shoal complexes, strike-fed shoreface systems, and abandoned-deltaic headlands. These paleogeographic elements extend for tens of miles or kilometers along depositional strike from southwest to northeast. The transition in depositional systems in the Upper Midway Group to the Lower Wilcox Group is abrupt, recording a change from wave-dominated shelf and shoreface systems to fluvial-dominated deltaic depositional systems. Dip-elongate, fluvial-dominated deltaic deposits at the base of the Wilcox Group (Hooper 1 depositional unit) have depositional axes oriented orthogonally with respect to those in the underlying Midway Group. Hooper 1 depositional axes are narrow (commonly <4 mi [<6.4 km] across. They are erosionally-based, net upward-fining, and composed of fine- to medium-grained sandstone with abundant siltstone and mudstone rip-up clasts. Distributary-channel trends in the Hooper 1 depositional unit are connected southward and southwestward to downdip-bifurcating trends representing upward-coarsening sub-delta deposits that pinch out into interdistributary mudstones. Microscale heterogeneity in fluvial-dominated deltaic deposits in the lower part of the Hooper Formation occurs at the scale of large (>2 in [>5.1 cm]) diameter siltstone and mudstone clasts in distributary-channel deposits. Mesoscale heterogeneity is controlled by lateral pinchouts of distributary-channel sandstone bodies into floodplain and interdistributary mudstones. Macroscale heterogeneity is represented by numerous dip-elongate distributary-channel depositional axes that segment lower-coastal-plain systems into discrete sandy depocenters. Six Hooper 1 deltaic depocenters occur along a 60 mi (96 km) span along the Angelina-Caldwell Flexure. Sandstone-body continuity along depositional strike (southwest to northeast) in the Hooper 1 depositional unit is poor, being only approximately 25% at 10 mi (16 km) well spacing. These sandstone-body continuity values record the presence of multiple, locally occurring deltaic depocenters within a fluvial-dominated deltaic system, in contrast to the wave-dominated shelf and lower-shoreface deposits in the underlying Upper Midway Group.