Coral Reefs of the Southern Gulf of Mexico

Chapter 3. Origin and Geology

Origin and Geology W. David Liddell Gulf of Mexico Basin The Gulf of Mexico is a roughly circular basin encompassing parts of the southeastern United States and eastern Mexico. It is some 1,500 km in diameter, up to 3,700 m deep, and has been filled with 10–15 km of Mesozoic and Cenozoic sediments (Salvador 1991a). Its boundaries are the Florida Escarpment and Platform to the east, the Campeche Escarpment and Platform to the south, the Sierra Madre Orientál to the west and the Gulf Coastal Plain to the north and northwest (Fig. 3.1). These boundaries reflect Mesozoic-Cenozoic carbonate platform growth (Florida and Yucatán platforms ), Laramide compression (Sierra Madre Orientál), and progradation (Gulf Coastal Plain) (Ewing 1991). While the transition between the basin and the Florida and Yucatán platforms is abrupt, the transition to the coastal plain to the north and west is gentle. The coastal plain is much broader (500 km) to the north and narrower (50 km) to the west. The deep, central part of the basin is underlain by oceanic crust, while the Gulf Coastal Plain and the Florida and Yucatán platforms are underlain by continental or transitional (rifted continental ) crust. As the emphasis of this volume is on the southern Gulf of Mexico, this discussion will largely be restricted to two areas, the Yucatán Platform and the western Gulf Coastal Plain. Yucatán Platform The Yucatán Platform includes the subaerially exposed Yucatán Peninsula and associated submerged shelf (dominated by the Campeche Bank). The platform is composed of flat-lying, predominantly carbonate sediments dating back to the Late Cretaceous (Table 3.1) and reaching some 3–4 km in thickness. Lesser amounts of evaporitic sediments also occur. Average rainfall on the northern peninsula is 70 cm/yr (Ferré-D’Amaré 1985), considerably less than that of the Tampico-Veracruz area along the western Gulf. Relief on the peninsula is low, on the order of 100 m or less, and the landscape is dominated by karst topography. Thus, there is little surface drainage and limited terrigenous sediment is transported to the waters adjacent to the peninsula. There is the possibility of freshwater discharge into the coastal areas of the Gulf through the karst system, as suggested by Ferré-D’Amaré (1985) for the eastern coast of the Yucatán Peninsula. The shelf is broad to the north and west, reaching some 240 km in width, and narrow to the east along the boundary with the Caribbean. The broad northern and western portion is known as the Campeche Bank, which is the site of numerous bank reefs (sensu Logan 1969b) and at least one atoll or “atoll-like” structure (see Figs. 2.1, 2.6–2.9; Plate 12). It is bounded to the west by the steep Yucatán Escarpment, a product of Cretaceous and later platform growth, to the north by the Yucatán Terrace and to the east by the Yucatán Channel (Bryant et al. 1991). The shelf-slope break occurs between 180 and 270 m. A submarine 3 Overview A4339.indb 23 A4339.indb 23 7/6/07 11:18:48 AM 7/6/07 11:18:48 AM 24 liddell terrace between 90 and 135 m corresponds to the late Wisconsinian sea-level lowstand, and additional terraces at 50–64 m and 33–36 m likely represent stillstands , or pauses, in sea-level change (Logan 1969a). Western Gulf Coastal Plain This section of the Gulf coast extends southward from Tampico to Veracruz and the Isthmus of Tehuantepec (Fig. 3.1). The Sierra Madre Orientál orogenic belt is a Laramide feature (latest Cretaceous to early Tertiary) and forms the western boundary of the plain. The Isthmus of Tehuantepec is bordered on the east by the Yucatán Platform. This portion of the western Gulf Coastal Plain can be subdivided into three physiographic “embayments ” (the Tampico, Veracruz, and Isthmus of Tehuantepec) that are separated by highlands, such as the Trans-Mexico Neovolcanic Belt (Plate 13). These embayments correspond to structural basins that have received thick fills of terrigenous sediment. Rainfall is high on the coastal plain, 100–150 cm/yr, and several major rivers , namely (from north to south) the Moctezuma-Panuco, Tuxpan, Tecolutla, and Papaloapan, cross the plain. These have been responsible for the delivery of considerable quantities of fine-grained terrigenous sediment to the Gulf. Subsidence in the Gulf since at least the Late Jurassic has resulted in a gentle...