ABSTRACT

Damon Mound is the surface expression of an underlying shallow salt dome. Quarrying operations have exposed a fault-bounded block which had been diapirically uplifted about 1000 m to the surface, and which is composed of Upper Oligocene Anahuac Formation shale and limestone, a unit previously known only from the subsurface. The limestone facies of the Heterostegina Zone, the middle of three Anahuac zones, is present in outcrop. Detailed investigations have demonstrated the former presence of a major Oligocene coral reef which surmounted the dome and which exhibited characteristic Oligocene reef community succession. At the base of the exposed Anahuac Formation sequence lies the well-known dark-gray Heterostegina larger foraminiferal limestone, a unit which, as a calcareous shale, is found regionally in the subsurface of much of the Gulf Coast and represents a transgressive marine shelf deposit. This unit contains the abundant tests of the larger foraminifers Heterostegina, Lepidocyclina, Miogypsina, and Operculinoides. The Heterostegina limestone rests on an early Oligocene black marine clay, which in turn lies on the caprock limestone. The sequence above the foraminiferal limestone represents an example of the Heterostegina coralline limestone, a facies found only on a number of locally developed former topographic highs (salt domes and faulted shelf margins) in the subsurface Anahuac Formation of Texas and Louisiana.

At Damon Mound, the exposures reveal shallowing-upward sequence of reef development. Porites douvillei, an efficient sediment-rejecting branching coral, first colonized the muddy-lime sea floor, and Porites coral wackestones alternate with grayish-green terrigenous mudstone as colonizing coral beds were episodically engulfed by increased clay sedimentation. Deposition continued and the carbonate content of the sediments increased as carbonate-secreting organisms became more abundant and clay influx decreased. Eventually coral growth dominated and massive beds of Porites lime packstone formed, to be succeeded in turn by a classic Oligocene reef core community consisting of Antiguastrea, Montastrea, Agathiphyllia, Stylophora, Stylocoenia, Goniopora, Favites, Porites, Siderastrea, and Stephanocoenia. This unit, a coralgal reef boundstone, shoals upward to a fossiliferous lime grainstone containing the bivalves Kuphus and Lucina, and then to an unfossiliferous massive lime-grainstone back-reef deposit; both grainstones are composed of the eroded sand-sized debris of organic reef-tract constituents. The uppermost unit in outcrop consists of an eroded coral- and caprock regolith which unconformably overlies the reef-tract sequence.

The study sheds new light on the paleoecology of Oligocene hermatypic corals which thrived during the period of maximum Paleogene reef construction and diversity, because the Damon Mound reef contains the best example of such a biota living at the northern limit of the Oligocene reef belt. There are a number of important and interesting contrasts and comparisons which can be made with the Damon Mound Oligocene reef and with both more tropical Caribbean Oligocene reefs and Holocene reefs, such as the modern Gulf of Mexico Flower Garden reef.