ECHINOID FAUNA FROM THE ANAHUAC (LATE OLIGOCENE) REEF AT DAMON MOUND, BRAZORIA COUNTY, TEXAS

Damon Mound is the name of a large (about 35 km²) topographic high rising 25 meters above the flat coastal plain in northwestern Brazoria County, Texas, about 50 kilometers southwest of Houston. The mound is the surface expression of a piercement salt dome, and the remnants of an Anahuac (Late Oligocene) coral reef are preserved in a fault-delineated block embedded in the caprock of the salt dome and exposed by caprock quarrying operations. Salt dome tectonics has resulted in the uplift of the reef limestone on the order of 2000 meters above the equivalent zone in the subsurface. A relatively small section of reef, covering slightly less than a hectare in area, is exposed. Echinoid remains are present, and although whole-body fossils are rare the amount of diversity (at least eight species) represented by this small sample size is significant. The reef can be divided into four zones, each with characteristic echinoid species. The reef drape, composed primarily of tests of the foraminifer Heterostegina texana, represents the deepest water environment, and includes Clypeaster marinanus, Agassizia mossomi, and Lovenia alabamensis. The overlying zone consists of interbedded calcilutites and Porites coral thickets, and includes the species Echinometra prisca, Clypeaster marinanus, Clypeaster sp. cf. oxybaphon, Brissus exiguus, and Schizobrissus dubius. The reef core of massive corals includes Prionocidaris cojimarensis and Echinometra prisca. The shallow-water back reef deposits include only Clypeaster marinanus. The Damon Mound fauna is more closely related to tropical Late Oligocene and Early Miocene faunas of Mexico and the Antilles than to Gulf and Atlantic Coast faunas of the United States at the same latitude. During the Oligo-Miocene Damon Mound and similar salt dome reef buildups in the northwestern Gulf of Mexico may have been havens for tropical species that could not have otherwise survived at that latitude. Comparisons can be made with the modern-day coral reefs on the Flower Garden Banks in the northwestern Gulf of Mexico.