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Tectonic Generation of Early Miocene Transgressive Fill of the Eastern Venezuela Basin (Maturin Sub-Basin)*
By
Nubia Santiago1, Kaolu Parra1; and Ron Steel2
Search and Discovery Article #30053 (2007)
Posted November 3, 2007
*Adapted from extended abstract prepared for presentation at AAPG Annual Convention, Long Beach, California, April 1-4, 2007
1PDVSA,Venezuela, Pto. La Cruz ([email protected])
2UT, Austin, TX
The Eastern Venezuela Foreland basin formed by the oblique collision of the Caribbean plate against the South American plate. The lower Miocene succession, derived mainly from the craton and examined here in the Maturín sub-basin, shows a clear transgressive development trend caused by the earliest downwarping of the foredeep. Although the main paleo-Orinoco system drained into the area from middle Miocene, and the bulk of the thrust tectonics is from middle Miocene; the early Miocene saw the initiation of foredeep subsidence and the creation of a deepwater shelf margin.
The lower Miocene succession of the area is composed of six thick stratigraphic units that record discrete phases of subsidence and successive backfilling of the basin. Each of these units has a stacked architecture of higher frequency (4th-order) sequences created by northward-prograding deltas that drained the south-southwesterly craton.
The hierarchy of high-frequency, northward-building clastic wedges, and how they stack in response to the embryonic, southward-growing fold-and-thrust belt on the other side of the basin, form the basis of the stratigraphic analysis. Three prominent features reflect the evolving tectonic control on the development of the stratigraphy:
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(1) early regressive deltas (sequence S2 ) prograded the southerly shelf prism to its maximum regressive position northward at an early stage when sediment supply could still outmatch embryonic tectonic subsidence driven from the north;
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(2) an aggradational to slightly backstepping phase of shelf-prism growth (sequences S3 to S5 ) that marks the main onset of increased, tectonic-driven subsidence in the growing foreland basin. This phase begins with widespread fluvial sands that may imply marked relative sea-level fall due to slight regional tectonic uplift. Later in this same phase tectonic subsidence became sufficiently rapid to create a shelf edge and bathyal slope in the northeast of the area;
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(3) greatly increased tectonic subsidence rates in the basin caused a rapid southward migration of the foreland bulge, a widespread drowning of the earlier shelf prism, and widespread accumulation of deepwater (sequence S6 ) shales that persist into the middle Miocene.
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Conventional stratigraphic correlations in the lower Miocene of the Eastern Venezuelan Basin are locally beneficial to exploration workflows, but insufficient to understand the early tectonic development of the region. The objective of the present study is (1) to show how a hierarchy of high-frequency deltaic transits, from the southerly craton toward the north, constructed a thick shelf-prism across the foreland bulge area and (2) to show that details of the shelf-prism architecture reflect the growing and changing tectonic influence from the north.
The Tacata area is located along the southern deformation front of the western Serranía del Interior in eastern Venezuela, in the footwall of the Pirital-Tala thrust (Figure 1). This structure has been defined by previous authors as a triangle zone involving mostly lower to middle Miocene units in its core (Manceda, 2005). The Pato area is a low deformation zone located in the south part of the study area (Figure 2).
The database used in the work consisted of some 15 wells with sedimentological , biostratigraphic,and well logs that penetrated the lower Miocene succession. In the northern area, many of these wells have a thrust-repeated stratigraphy, and the tectonic reconstruction of the locations of these repeated units allows extra well data points to the north, some 20 more of these. The present well locations plus the restored locations of individual stratigraphic segments are shown in Figure 3.
The tools used to define the depositional model and the stratigraphic framework are described, as follows: 1-The well log patterns allow identification of likely rock types and depositional environments. 2-Bathymetric data provided by the biostratigraphy (nonmarine, transitional, neritic, bathyal categories, and sub-divisions thereof). This provides important guidance on water depth in depositional environments and is especially important to identify where shales are bathyal (>600 ft of water) in character. 3-Core data gives details of the facies and trace fossils and provides direct input to testing and calibration of environmental intepretation. Cores also provide key information on the presence/importance of waves, tides, and currents in depositional systems.
Sequence Stratigraphic Framework Three second-order depositional sequences are previously known from the lower Miocene to Plio-Pleistocene interval of the Eastern Venezuela Basin. These sequences have now been correlated throughout the Tácata area defining a series of sixteen third-order sequences (Santiago et al., 2006). The depositional sequences of Neogene age have been drilled by wells in the Tácata structure, while depositional sequences of Cretaceous-Paleogene age are found in the autochthonous footwall, in the trailing edge of the Tácata thrusts, and in the overlying Tala and Pirital thrust sheets. Two significant features are the absence of Upper Miocene throughout the study area, and the lack of the lowermost Plio-Pleistocene sequences in the Tacata area.
Lower Miocene Succession in Pato-Tacata Area The lower Miocene succession is more than 4000 ft thick, but it forms a series of sandstone clastic wedges that collectively thin to the north. As a whole, the succession shows a significant change in its character from south to north. It change northward from being a mainly delta-plain succession dominated by distributary channels, brackish-water, inter-distributary embayments with sand splays, and muddy flood-basins to an open marine, delta-front and shelf succession dominated by marine sands and marine shale (Figures 4 and 5). There is an additional important lateral facies change eastward of well T19, from shelf environments to deepwater environments with turbidites (Figure 6). The entire succession can be seen as a very thick, northward-directed, shelf to shelf-margin prism that faced the growing fold-and-thrust belt that would advance southward toward it. Another important feature of the succession is its overall aggradational to transgressive geometry, within which there is a three-fold vertical development that reflects the clear and increasing influence of the rising and encroaching tectonic monster from the north, on the other side of the foreland basin. In addition to the general and overall northward change in the succession mentioned above, there are a series of 5 stratigraphic units or 3rd-order sequences (sequences S2-S6), convenient groupings of the more fundamental, 4th-order regressive-transgressive deltaic transits that built this thick shelf prism (Figure 5 and 6). Sequence S2 and part of sequence S3 portray a major phase of overall basinward (northward) progradation of the shelf platform, through the building of at least 6 regressive-transgressive transits of the delta system. Lower sequence S3 is a sand-rich, distributary-channel complex and represents the maximum northward progradation of the lower Miocene clastic wedge. During this phase of growth of the succession the sediment supply from the southerly craton clearly out-paced subsidence in the basin, and the erosive incisions in lower sequence S3 suggest that there may even have been some tectonic uplift of the area, or marked eustatic falls of sea level. Sequences S3 (upper), S4, and S5 show an initial vertical aggradation of repeated deltaic cycles, followed by a slight transgressive backstepping of the succession (seven 4th-order sequences of sequences S3 and S4), reflecting increased subsidence rates in the basin. This subsidence clearly succeeded in halting the regressive advance of the southerly derived clastic wedge. The succession then backsteps more markedly (four 4th-order sequences of sequences S5), as subsidence rates increased further or sediment supply decreased. The continued backstepping between sequence S4 and sequence S5 is also reflected in the change from neritic to bathyal water depths of shales in the northeastern parts of the study area. Sequence S6 shows a complete drowning of the previous shelf platform, as the clastic wedge retreated and became covered by deeper water shales, probably representing a culmination of subsidence rates in the developing foreland basin and clear signals of the advancing thrust fronts in the north. Thickness of sequence S6 shales reaches 1700 ft in the north where bathyal depths dominated. This 3-fold development of the lower Miocene stratigraphy clearly suggests that the onset of Miocene thrusting and subsidence and development of the foreland basin with southward migration of the foreland bulge area began in the early to mid parts of the early Miocene, rather than at the beginning of middle Miocene, as generally assumed..
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