ABSTRACT

The Gulf of Mexico Basin extends about 1,100 miles east-west and 1,300 miles northsouth. The oldest sediments which are part of the basin fill are probably of Triassic age. They are confined to narrow grabens which are present along the basin margin and also farther into the basin. These sediments lie on Precambrian and Paleozoic igneous and metamorphic rocks, and also on Paleozoic sedimentary rocks.

It was during the Upper Jurassic that thick and extensive salt deposits were precipitated in the grabens which had limited connection to the sea. General subsidence brought a shallow sea over much of the basin shortly after the salt was formed, and Upper Jurassic limestone was deposited in all margins of the basin. Before the end of Jurassic, sediments from rising land masses reached the northern and western edge of the basin.

The sea expanded even more during Lower Cretaceous time, and deposition of carbonates took place to and beyond the borders of the Gulf basin on the west and east. Much land-derived sediment was also deposited along the northern margin of the basin.

The sea was restricted on the west but was expanded up the Mississippi Embayment and also across the western plains to connect with the Arctic Ocean during the Upper Cretaceous Period. Limestone, chalk and marl accumulated in this shallow epicontinental sea, and terrigenous clastics were, at times, deposited in coastal plain and shallow, near-shore environments along the northern and western margins of the basin.

The ancient Gulf began to be restricted to the north and west at the end of the Cretaceous. The Laramide orogeny elevated much of the North American continent, and sediments from these land areas were brought to the Gulf Basin throughout the Tertiary and Quaternary. There were many pulses of uplift of the hinterland; the basin was continuously subsiding. The shoreline position in this clastic province fluctuated greatly so that there were many major transgressions and regressions of the sea. However, the sediment supply was greater than subsidence, with the result that each formation was deposited farther seaward than the preceding ones. The eustatic rise of sea level at the end of the Pleistocene Period brought the northern Gulf over the continental shelf.

In the shallow marine platforms of the southeastern Gulf, there was continuous deposition of limestone.

Sedimentation is taking place today in many environments in the Gulf of Mexico Basin. Study of these Recent sediments and the associated faunas and floras has provided criteria for determining the depositional history of the Gulf Basin and many other basins.

The outcrop, thickness, generalized lithology and depositional environments are shown by maps and columnar sections for the Lower Cretaceous, Upper Cretaceous, Paleocene, Lower Eocene, Middle Eocene, Upper Eocene, Oligocene, and Miocene. The Upper Jurassic, Pliocene, Pleistocene and Recent sediments are illustrated with representive columnar sections.