Morphology and surface sediments, continental shelf off tabasco and campeche, mexico

摘要

At southwestern Gulf of Mexico, off the Ciudad del carmen Region, Campeche, in the continental shelf occurs a transition between the terrigenous sediments transported by the Grijalva and Usumacinta River System, and the calcareous biocalsts from marine origin. The surface of the shelf shows several ondulations and irregularities at 18, 36, 70 and 90m depth, interpreted as remnants of ancient coastal deposits and reefs some of them are pinnacles of 7m high, related with post glacio-eustatic sea level fluctuations. Erosional features are indicated by a small submarine valley off San Pedro-San Pablo river, from 30 to 100m depth, sumerged river beds at depths of 20-40m off La Machona and El Carmen lagoons, and sumerged terraces. Tabasco shelf is narrow, the upper edge of the continental slope is located between 125-130m depth, distant 45 to 65km from littoral; bottom slope gradually modified according with depth. General disposition of the surface is convex; bottom slope gradually modified according with depth. General disposition of the surface is convex; bottom's slope ranged from 6' off Punta Xicalango to 14' off Barra Santa Ana, in accordance with depth encreace. Toward Campeche State, the continental shelf is wide, 160km as mean width, and an approximate gradient of 1:580. Upper edge of the continental slope lies at 130m depth. Shelf's relief has similar characteristics to those from Tabasco shelf. Remanants of old coastal deposits were found at 36, 60-85, and 90m depth, together with death and living coral reefs and terraces. Shelf's sediments ranged from terrigenous on the Bay of Campeche to calcareous on the Bank of Campeche. The former are predominantly silts and clay with variable amount of gravel and sand. The latter are muds constituted by fragments of shells, corals and algae. Beach sediments ranges from coarse to fine grained terrigenous sands to calcareous materials ranging from fine grained gravels to very fine grained sands. By means of an scatter diagram of the graphic mean diameter (M_phi) versus sorting ( _phi) and skewness (Skphi) (Inman and Chamberlain, 1955), five textural groups were identified ranging from granules (fine grained pebbles) and very coarse grained sands to medium grained clay. Most sediments are polimodals and reflect the processes of mixture during the transport and deposit.