Abstract: Synsedimentary Tectonic Control of Paleozoic Clastic Deposition in Maranhao Basin, Northeast Brazil

A. V. Carozzi, F. U. H. Falkenhein, R. G. Carneiro, F. R. Esteves, C. A. Contreiras

The Silurian to Eo-Carboniferous sequence of this basin was investigated by a detailed study of 547 cores belonging to 27 key stratigraphic wells. Paleoenvironmental interpretations for each chronostratigraphic interval were based on the associations of sedimentary structures and detailed petrography for the coarse clastic rocks and on distribution of clay minerals, on geochemistry of boron and trace metals, and on palynofacies for the shales.

Two sedimentary models were recognized. The first designated as "coalescent deltas" is valid for most of the investigated column and is divided into: fluvial, upper and lower deltaic-distributaries, delta-front, prodelta and offshore, shoreface (interdeltaic embayments) deposits. The second designated as "glacial disruption of coalescent deltas" is valid only during the Gondwana continental glaciation of Famennian to Strunian times. The corresponding diamictites, varvites, and shales with rafted pebbles are present throughout most of the deposits of the coalescent deltaic model.

The evolution through time of the depositional models reveals a predominant direction of supply of coarse clastics from southeast and east and a minor supply from north and west. The major opening of the basin was west-northwest toward the Middle Amazonas basin and minor ones toward the northeast (Upper Volta) and south (Parana). The intensity of the supply of coarse clastic materials decreased in time in three major oscillations. The deposition of the transgressive marine shales took place in the intervening time intervals and with increasing importance upward.

The variation through time of all sedimentologic and geochemical parameters for coarse clastic rocks and shales demonstrates that the entire sedimentation, and particularly the location of the deltaic reservoirs, was directed by the differential vertical movements of a mosaic of blocks. The latter are limited by three sets of reactivated tectonic lineaments originating from the underlying Precambrian basement and oriented northwest-southeast and northeast-southwest (eastward continuation of the Guianas and Guapore trends of the Middle Amazonas basin) and north-south (Sao Francisco trend).

The past and present activity of the lineaments is demonstrated independently by the coincidence of their trends with the basement-depth contours obtained by aeromagnetometry, with the fracture lines discovered by side-radar (Radam Project 1973), and with the many photogeology anomalies present throughout the 700,000-sq-km surface of the basin.

AAPG Search and Discovery Article #90972©1976 AAPG-SEPM Annual Convention and Exhibition, New Orleans, LA