Abstract

Log-based facies mapping yields insights into the complex Wilcox Group stratigraphy of Central Texas (Bastrop-Lee-Burleson-Brazos) and its important groundwater and coal resources. On raster logs (primarily those with gamma-ray profiles), one can identify and aggregate thick channels (big rivers or amalgamated channels, over 100 ft [30 m] thick), other channels (20–100 ft [6–30 m] thick), generally thin and laminated coarsening-upward (CU) sandy units, and coals or coaly intervals. Color-based displays on sections clearly delineate the complexities of Wilcox sedimentation, and allow more reliable correlation.

The Hooper Formation (oldest, overlying Midway mudstone) consists of CU units and a few channels in its lower half and a mix of channels (some thick) and floodplain units (coals, thin sands, and some CU units) in its upper half. The unit is inferred to record deltaic and shoreline progradation followed by delta-plain conditions.

The overlying Simsboro Formation is a complex of sand-rich, dominantly channel deposits (both thick and thin) but also contains floodplain deposits (mud, thin sandstones and occasional CU units) and coals. At least three channel levels can be found in many wells; this has led to inaccurate correlations in past studies. The unit is inferred to record major fluvial aggradation but does not record a single episode of valley incision.

The overlying Calvert Bluff Formation consists of coal-rich floodplain units having channels in its lower section and a thick upper section dominated by thin CU units interspersed with channels (some thick) and some sporadic coals. This upper unit strongly suggests repetitive progradation into shallow standing water in either a lacustrine, floodplain or lagoonal setting.

The top unit, the Carrizo Formation, is a complex of sand-rich channel deposits, but few are thick or amalgamated. Some interspersed floodplain and CU deposits are found, but they lack coal. The unit represents a return to a sand-rich fluvial environment, but one lacking major river systems.

Net channel sand and percent channel sand maps characterize the principal targets for groundwater resources in each unit (CU units and other thin sands are volumetrically insignificant). They show generally consistent but complex patterns with individual channels rarely wider than a mile (1.6 km). Downdip penetration of fresh water probably depends on the degree of amalgamation of channels in each unit, a function of sand percentage and channel thickness. Flow across faults depends on individual occurrences of thick channels, and leakage along or across faults in the sand-rich Simsboro unit.