Abstract

The Yucatan Shelf (Campeche Bank) is a deep, inclined shelf which projects into the southern Gulf of Mexico from the Peninsula de Yucatan. The shelf is approximately 22,000 square miles in area and extends 70-180 miles from the shoreline to a shelf-slope break in depths of 600 to 900 feet. The Peninsula do Yucatan and much of the seaward shelf are underlain by karsted limestones of Tertiary and Pleistocene age. There are no major rivers due to the karst topography of the hinterland. The effect of the small run-off influx is a lack of terrigenous detritus on the Yucatan Shelf. The late Quaternary sediments are pure carbonate types composed of skeletal and non-skeletal constituents. The skeletal component of shelf sediments is dominated by benthonic mollusca, corals, foraminifera and carcareous algae; non-skeletal grains include pellets, ooids, lithic (limestone) fragments and some aggregate grains. Emergent coral reefs, submerged reef banks and biostromes form a series of prominences above the shelf level around the outer margins.

The stratigraphy of late Quaternary marine sediments is relatively simple. In the area bounded by the 300 foot depth contour there is a thin veneer, 6 inches to 4 feet thick, of unconsolidated carbonate sediments overlying cemented limestones. The unconsolidated carbonates range in age from about 16,000 years B.P. to present; this sequence accumulated under the transgressive conditions of postglacial time. The underlying limestones may be any age ranging from Wisconsin to Tertiary. In special areas the postglacial sediment veneer may be thickened and consolidated; these thickenings are the coral reefs and related structures. The essential point about the relationship between the postglacial sediment mantle and the cemented limestones is that the contact is an erosional unconformity representing the period of sub-aerial exposure during the glacial low sea level and the passage of the re-advancing Holocene sea.

In the area seaward of the 300 foot isobath there is a more complex stratigraphic relationship between the postglacial veneer and the underlying sediments. In a broad zone between the 300 and 450 foot depth contours the postglacial veneer overlies an erosion surface on semiconsolidated sediments of Wisconsin age. The contact is interpreted as an erosional unconformity continuous with the unconformity above 300 feet depth but milder in degree. The unconformity appears to grade through a submarine disconformity on the shelf margin into normal sedimentation on the continental slope where postglacial sediments overlie Wisconsin sediments with no apparent break in sedimentation.

Within the late Quaternary deposits of the Yucatan Shelf there are a number of areally circumscribed and lithologically distinct bodies of sediment which may be designated as sedimentary (rock) units. There are two types of formation; (1) Sedimentary blankets which cover wide areas of shelf as thin strata up to a few feet thick and (2) Local thickenings of the late Quaternary veneer, reefs, reef banks and biostromes which are grouped into a single sedimentary unit and range from 20 to 120 feet in thickness.

The sediment types and sediment distribution on the Yucatan Shelf can be related to the history of sea level during the glacial and postglacial periods. The sedimentary patterns are developed in terms of Wisconsin and Holocene paleogeography.