Variation in Detrital Mineralogy Caused by Provenance and Depositional Setting: Linking Tertiary Sandstones from the Upper Texas Coastal Plain to the Deepwater Gulf of Mexico

Study of Tertiary sandstones of the upper Texas coastal plain and shelf provides insight into reservoirs of the same age in the deepwater Gulf of Mexico. Onshore Wilcox sandstones were deposited in highstand, transgressive, and lowstand systems tracts. Wilcox sandstones are mostly lithic arkoses and feldspathic litharenites having an average composition of Q₅₉F₂₂R₁₉ today. Original detrital composition prior to diagenesis was Q₅₆F₂₇R₁₇. Provenance did not change significantly during Wilcox deposition in this area, nor does average sandstone composition vary among lower, middle, and upper Wilcox sandstones. However, lowstand deposits contain more rock fragments (mainly metamorphic and volcanic) than do deposits from highstand or transgressive systems tracts. The highstand sandstones were probably subjected to more reworking and winnowing, which reduced the volume of rock fragments. Slope-fan sandstones were deposited rapidly and not reworked in a high-energy environment, preserving more lithic grains. Wilcox sandstones deposited in deepwater environments in the Gulf of Mexico are likely to contain more rock fragments than do their linked highstand equivalents.

Yegua sandstones from the upper Texas coastal plain show similar partitioning of rock fragments preferentially into lowstand basin-floor fan deposits, whereas lowstand prograding-wedge deposits are more quartz rich. In contrast, composition of Miocene sandstones on the upper Texas and Louisiana shelf varies with geographic location and is controlled mainly by provenance. Miocene sandstones from the central Texas coast (Matagorda Bay) contain abundant volcanic rock fragments derived from volcanism in West Texas and Mexico. Miocene sandstones from offshore Louisiana are quartz rich, reflecting the greater distance from the volcanic source; Miocene sandstones from Galveston and High Island areas show intermediate composition.

In formations with minimal change in provenance, detrital mineral composition may vary subtly in different systems-tract settings. In formations with diverse provenance, however, mineralogical differences caused by depositional setting are likely to be overshadowed.