Reconstrução paleoambiental de depósitos albianos na área de Coroatá, borda leste da Bacia do Grajaú, MA

Abstract

Cretaceous deposits outcropping along the ltapecuru River, municipality of Coroatá-MA, were analyzed in order to better determine their age and characterize their depositional paleoenvironments. Palynological data from six samples showed dominance of Araueariaeites, Perotriletes s.l. (Perotriletes, Crybelosporites and Paludites), as well as ephedroid polens (Equisetosporites, Gnetaeeaepollenites and Steevesipotlenites), suggesting the Comptieatisaeeus eearensis palynozone (P-280), of eo/mesoalbian age. Faciological and stratigraphic analysis led to the recognition of three genetically associated facies associations: 1) mouth bar, consisting of sandy bodies (1.0 to 2. 5m) amalgamated, with abundant plant remains, of lenticular geometry, and characterized internally by medium to large flute crossings, showing an ascending granocrescent pattern, and sometimes erosive base, with lagging clayey intraclasts; 2) distal/prodelta bar, characterized by lenses of fine, well-sorted sandstone, interdigitated with laminated clays, rich in organic plant matter, forming intermediate layers (wavy eflaser) organized in an ascending granodecrescent pattern; and, 3) medium to upper shorefaee, characterized by predominantly sandy facies, containing medium to large "seour and fit! ) of medium to large size, and cross stratification of channelized, elymbing, and swaley types. Sedimentary facies analysis points to the following depositional conditions: a) rapid deceleration of flow with high sediment load in a low-energy receiving basin; b) reworking by combined (oscillatory and unidirectional) flow due to normal and storm wave action; and, c) alternating deposition of sandy and suspended sediments. The faciologic associations characterize a deltaic-wave-dominated depositional environment. This delta would have developed near the coastal zone, as attested by the presence of marine fauna in related deposits located northwest of the study area.