--> ABSTRACT: Seismostratigraphic Study of a Miocene-Pleistocene Subsurface Interval, Northern Monagas, Eastern Venezuela Basin, by Lorena G. Moscardelli J. and María Antonieta Lorente; #90906(2001)

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Lorena G. Moscardelli J.1, María Antonieta Lorente2

(1) Universidad Central de Venezuela
(2) PDVSA EPM

ABSTRACT: Seismostratigraphic Study of a Miocene-Pleistocene Subsurface Interval, Northern Monagas, Eastern Venezuela Basin

The study area is located in the northern Monagas fields near the Serranía del Interior foothills of the eastern Venezuela basin. The interval of interest is restricted to a subsurface tertiary section which includes the Morichito, Las Piedras and Mesa formations. Well logs, two-dimensional seismic data (time and depth) and biostratigraphic data were used to reconstruct the geological history through Miocene-Pleistocene time.

Taking into account the characteristics of these units, two alloformations were defined, the Morichito and Las Piedras-Mesa alloformations. These units are mappable stratiform bodies of sedimentary rock that are defined and identified on the basis of their bounding discontinuities (NASC, 1983).

The older, Morichito Alloformation was dated as Middle Miocene age, based on the identification of Crassoretitriletes and Grimsdalea palynological zones (Lorente, 1986). The unit is composed by conglomerates and coarse sandstones interbedded with siltstones. The palynological assemblages, electrofacies analysis and seismostratigraphic features, suggest that this unit was deposited in an alluvial fan environment.

A Late Miocene to Recent age was assigned to the Las Piedras-Mesa Alloformation, based on identification of the Asteraceae (Bombacacidites and Fenestrites) palynological zone (Lorente, 1986). The unit is composed by interbedded sandstones, siltstones and thin coals. Two allomembers were defined in this unit, Las Piedras and Mesa. The palynological assemblages, electrofacies analysis and seismostratigraphic features show that the Las Piedras Allomember was deposited in shallow water marine environments (barrier islands and lagoons), and the Mesa Allomember was deposited in fluvial environments.

Statistical analysis of palynological assemblages, interpretation of well log stacking patterns and the age diagnostic fauna and flora, allowed identification of 400 ka and 600 ka cyclic periodicity. This periodicity was associated with Milankovith cycles (excentricity) and tectonics effects.

Four phases of geologic evolution were identified from Middle Miocene to Recent time for this northern part of the eastern Venezuela basin:

Middle Miocene (16 to 11.7 My): Proto-Serranía del Interior overthrust due to the beginning of transpression. Dominance of alluvial fan related environments (deposition of Morichito Alloformation).

Middle to Late Miocene (11.7 to 9.3 My): Renewed structural deformation that generated the upper erosional boundary of Morichito Alloformation.   

Late Miocene to Early Pliocene (9.3 to 5.2 My): Maximum flexural effect on the basin. Dominance of marine environments (deposition of Las Piedras Allomember).

Early Pliocene to Recent (5.2 to 0 My): Filling of the basin. Dominance of fluvial environments (deposition of Mesa Allomember).

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado