ABSTRACT

One of the most southwesterly occurrences of limestone in the Heterostegina zone of the Anahuac formation (Oligocene-Miocene) occurs on Damon Mound salt dome, in Brazoria County, Texas. Cutting and cores of this material were examined, and maps constructed showing its attitude and thickness. The maps demonstrate that the limestone is thin near the salt mass, thickens to its maximum within a couple of miles from the salt, and then thins outward and is absent regionally. The thin limestone high on the dome is believed to be the partly-eroded remains of an organic reef, which developed on all flanks but the north-northwest. The greatest thickness of limestone and of the associated calcareous shale occurs off the east and southwest flanks, where currents flowing past the reef and past the similar reef at Nash salt dome distributed calcareous detritus.

The Heterostegina limestone is overlain by a normal marine shale which was deposited after reef growth stopped during a rise in sea level. Subsequently this shale was removed from parts of the reef by erosion before it in turn was buried by basal Miocene sands. This erosion is shown by closed zero contours and irregular contours of the shale isopach. Erosion was greatest on the south flank where even the underlying Heterostegina unit was largely removed. This erosion was going on while the lowest Miocene sands were being deposited, locally incorporating so much lime in the sands that they look like limestone on the electrical logs (and have been called "Upper 'Het' limestone.").