A conceptual sequence stratigraphy model for ...

Ber. Inst. Erdwiss. K.-F.-Univ. Graz - ISSN 1608-8166 - Band 21 - 2015

A conceptual sequence stratigraphy model for continental rift successions based on the Recôncavo Basin, Cretaceous, Brazil Michael Holz, Felipe Moreira, Edric Troccoli - Universidade Federal da Bahia, Brazil [email protected]

Rift basins are caused by crustal stretching prior to continental break-up, forming elongate crustal depressions bounded by normal faults cutting the pre-rift succession and the crystalline basement.

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The model was applied to the Recôncavo Basin, a Cretaceous rift basin in northeastern Brazil with a sedimentary record from Late Jurassic to midCretaceous spanning a period about 25 m.a. � ��

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(A) rift initiation systems tract, which is characterized by isolated and restricted fault which create incipient half-grabens, filled with fluvial and deltaic facies; (B) rift development systems tract, during which the initial rift faults tend to link and to form a larger and deeper depositional area, recording lacustrine facies with an overall retrogradational trend, and (C) Rift termination systems tract, when the accommodation rate decreases, the sedimentation regime is progradational, and the rift basin is filled with fluvial, deltaic and eolian facies.

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Accommodation space within a rift basin is variable, and zones with high accommodation develop close to zones with low accommodation or even erosion. This implies in Main rift phase synchronous increase and decrease in accommodation. Hence, the passive-marginbased sequence stratigraphic scheme, where the LST, TST, HST and FRST develop followRift initiation phase ing a predictable order during a complete cycle of base-level change, is not applicable. Hence, for a practical and useful stratigraphic analysis of rift basins, conceptual and methodological adaptations are required in order to understand the basins sedimentary infill, especially concerning the prediction of specific economically important facies and depositional systems (Holz et al., 2014). Herein we exemply these adaptations, integrating the scheme of tectonic systems tracts of Garthorpe & Leeder (2000) with the classic stratigraphic model of Prosser (1993), resulting in a three-fold scheme of tectonic systems tracts with very distinct accommodation vs. supply ratios: Zone of increasing accommodation

Zone of decreasing accommodation

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The three-fold scheme of tectonic systems tracts is RIST linked to the subsidence SRU - Sin-rift unconformity RDS - Rift development surface curve of a typical rift MRS - Maximum rifting surface basin, where a phase of RTU - Rift termination unconformity within-trend facies contact (deltaic-fluvial) initial slow subsidence is followed by increasing rates, and ends with slow rates; hence the accomodation-supply ratio varies greatly during the rift basin´s infill.

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The crustal stretching, which has rates up to 20 mm/year, produces asymmetric halfgrabens where subsidence is controlled by one major boundary fault zone, which may have an extension of tens of kilometers and a dip up to 70 degrees. A continental rift system develops when one branch of a three-armed rift system fails to open and does not reach the passive margin stage, developing a system of linked half-grabens inAlluvial fans filled with continental systems, such as alDelta luvial fans, eolian, fluvial, deltaic, lacustrine debrites and turbidites.

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The systems tract rift termination scheme has been systems tract depositional system: deltaic used for correlaDatum = MRS depositional system: lacustrine rift development tion throughout systems tract the basin, using a t c a r t s m e yst s n o i database of ~100 t a i t i n i rift well logs (plus 2D seismic data). The color bar in each graph displays the thickness range of the sedimentary succession (in meters) Regional mapping of the tecInterpretation: (a) rift initated with two a- RIST minor half-grabens (north and south) tonic systems tract has shown significant thicknesses evolving during rift development (b) to only in restricted areas that the rift initiation systems two main and very deep depocenters. During rift termination (c), the lack of tract is in fact characterized by subsidence (= few accommodation) leads to widespread and relatively unia “spotty” occurrence, with two form infill of the basin. main areas of inicital rift depoc - RTST sition, reflecting the initial b - RDST significant thicknesses half-grabens, holding the over two huge areas record of fluvial-deltaic deposition, which occurs interfinsignificant thickness gered with lacustrine facies. over the central area The ongoing and increasingly rapid subsidence overwhelms the sedimentation rate, leading to high accommodation within the Rift development systems tract, characterized by lacustrine gravity-driven deposits like sandy debrites and turbidites; finishing with fluvial-deltaic and eolian facies of the Rift termination systems tract. Conclusion - sequence stratigraphy is a useful and functional tool even in rift basin, where conceptual adaptation of the basic ”four-systems-tract” model must be made. Mapping is more reliable and gives more insight to the depositional history of the basin using systems tracts boundary rather than lithostratigraphic units and limits. depositional system: fluvial and eolian

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Gawthorpe, R.L. ; Leeder, M.R. 2000. Tectono-sedimentary evolution of active extensional basins. Basin Research, 12, 195–218. Holz, M.; Troccoli, E.B; Vieira, M.P. 2014. Sequence Stratigraphy of Continental Rift Basins I: A Conceptual Discussion of Discrepant Models In: STRATI 2013.1 ed.: Springer International Publishing, 2014, p. 9-13 Prosser, S., 1993. Rift-related linked depositional systems and their seismic expression. In: Williams, G.D.; Dobb, A. (Eds.), Tectonics and Seismic Sequence Stratigraphy. Geological Society Special Publication, v.71, p. 35-66.