Ambientes sedimentarios recientes en Laguna Madre, NE de México

César Estavillo González*, J. Eduardo Aguayo Camargo*

*Subdirección de Tecnología de Exploración, Instituto Mexicano del Petróleo.


The sedimentary province known as Laguna Madre at NE Mexico resulted of the fluvial and deltaic development of the Bravo River and also of the minor ones, such as the San Fernando and Soto La Marina rivers. These rivers supplied sediments to the of Mexico, then were distributed along the coastal zone by means of littoral currents and counter currents. By this way a Barrier Island was formed, separating the continental plain from the Gulf of Mexico and resulting by this process the littoral Laguna Madre.

Due to the Jeveloping of the Barrier Island wirt more than 210 km long and 60 km in widthness toward north and 30 rneters southern were formed four major sedimentary settings which are characterized by their organic and inorganic content, primary sedimentary structures and by the distribution of the sedimentary deposits.

The Fluvial Plain (1) is flat and it contents silt and clayey silts finely laminated with sorne buried channels filled with very fine sand. There are many coaly horizons interlayered with silty layers with sorne gasteropod and pelecypod shells from fresh and brackish water. The "Lagoon" (2) is a complex of subenvironments governed hydrodynamically by internal currents which run from north to south parallely to the Barrier ¡sland. Also the lagoon is controled by waves and tidal currents and by the discharge of rivers, such as the San Fernando and otber minors. There are sedimentary variations in this environments, since fine and medium grained sands mixed with mollusca shell debris at the interior bars; silts and clays with disseminated organic matter within them toward the middle portion of the lagoon; and fine grained sands dosed to the Barrier Island. The primary sedimentary structures vary according to the textural variations of the sediments; then, laminated silty-clay muds; crossed bedding and graded bedding sands and coarse silt. The Internal Bars(3) and back-barrier zone, are very well developed and by themselves form a whole sedimentary complex. Their sandy sediments are derived from the Barrier Island, and are transponed during cyclonic times, and deposited as washover fans into the lagoon setting; after so, are redistributed by means of internal currents within this environment. The sandy bars are composed by feldespat litharenites to subfeldsarenites, poorly to moderately sorted, with the same mineral composition of those ones from the Barrier Island, the sediments mix with the lagoonal silty day. Primary sedimentary srructures, such as: crossed bedding, graded bedding and cut and filled channels all them represent high energy environments. On surface exposure are faunal feet prints proper of this province, such as: birds reptiles and marnmals. The Barrier Island(4) is formed by severa! subenvironments: beach, internal portion formed by littoral dunes, and back-barrier closed to the lagoon environments. Sediments in each one of these subenvironments vary from feldespat litharenite to sublitharenite. These ones are coarse grain size, moderately well sorted in the beach; medium and fine grain size, well sorted in the dunes, and poorly sorted with variable grain size at back -barrier. Primary sedimentary structures in the beach subenvironment are formed in high energy and also at the interior portion of the barrier in which wind is the dominant agent; at back --barrier zone, energy is moderated and intermittent.

Under the economical poinr of view, the Laguna Madre province is an interesting area, because it is possible to see clayey sediments enriched in organic matter and interbedded sandy layers. The first ones, as hidrocarbons source rocks and the formers, as sedimentary traps, if the sedimentary and buria! conditions were as present.