[First Hit]

Datapages, Inc.Print this page

Abstract: Previous HitPalynofaciesNext Hit in a Sequence Stratigraphy Context: An Example of an Upper Aptian Section from Almada Basin, Brazil

Araujo, Carla; Valéria Condé; José Botelho Neto; Elisabete Pedrão and João Claudio Conceição - Petrobras/Cenpes

The integration of palynostratigraphy and quantitative analyses of organic matter has greatly enhanced the understanding of the depositional paleoenvironment in a sequence stratigraphy context of the upper Aptian section from the Almada Basin, a rift which has evolved to a passive margin during the separation of South America and Africa.

This section was subdivided into six biozones based on palynological data: Dicheiropollisetruscus (early Barremian), Sergipea Variverrucata (late Aptian), Exesipollenites tumulus (late Aptian), Dejaxpollenites microfoveolatus (late Aptian), Cardioangulina elongata (late Aptian) and an informal interval with a poor palynological assemblage (early Albian). Three hiatus were recognized: between lower Barremian and upper Aptian, in the upper Aptian and between the upper Aptian and Albian (Fig. 1).

The studied section represents a proximal restricted marine depositional environment. Palynostratigraphy and quantitative analyses of organic matter results allowed the subdivision of this section in two major third order sequences and nine fourth order sequences (I to IX). The lower third order sequence comprises siliciclastic and evaporitic sediments, representing restricted environment (sequences I to V - fourth order). This sequence is characterized by the abundance in Classopollis, a pollen that indicates semi-arid to arid climatic conditions. The upper third order sequence (sequences VI to IX - fourth order) comprises only siliciclastic sediments deposited in a less restricted marine environment. There is a dominance of the continental over marine palynomorphs in all studied sections, nevertheless the upper sequence is characterized by higher relative proportion of marine palynomorphs (mainly foraminiferal linings). These features associated with the decrease of Classopollis indicate higher humidity conditions interpreted as a transgression over the lower third order sequence.

The ternary microplankton-spore-pollen palynomorph plot (Fig. 2a, b) of the lower third order sequence shows a concentration of points at the pollen corner, while for the upper third order sequence the same plot presents a tendency of dispersion toward the microplankton corner. These plots indicate the transgressive trend of the studied sections. The ternary phytociasts-amorphous organic matter-palynomorphs Previous HitkerogenTop plots (Fig. 2c, d) present a predominance of amorphous organic matter for both third order sequences. Nevertheless, the lower third order sequence presents a tendency of displacement of the data points toward the phytoclast corner what corroborates the paleoenvironmental interpretation.

Fourth order sequences are composed by a highstand and a transgressive systems tract, except sequences I and V that are represented exclusively by the evaporitic layers. In the lower third order sequence the upper boundaries of the HST are marked by the top of halita layers that represents the shallowing event. Sequence V represents the end of evaporitic event.

Maximum flooding surfaces are characterized by high local percentages of marine palynomorphs. Maximum percentages of total organic carbon are associated with transgressive systems tracts (Fig.1). Sequence boundaries are mainly characterized by minimum contents of marine palynomorphs and by a tendency of inversion in the relative proportions of amorphous organic matter and phytoclasts (Fig.1).

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil