Abstract
ABSTRACT: The present work aims to characterize the Neo-Jurassic to Neocomian succession of the Sergipe-Alagoas Basin, located in northeast region of Brazil, in order to discover the influence of tectonics on sedimentation in detailed scale and thus separating this sedimentary succession in tectono-stratigraphic units. Fieldwork observations and stratigraphic sections analysis allowed subdividing this rift succession into three depositional units that indicate different paleogeographic contexts. Unit I, equivalent to the top of Serraria Formation, is characterized by braided fluvial channel deposits, with paleocurrent direction to SE; unit II, corresponding to the base of Feliz Deserto Formation, is composed of anastomosed fluvial channel and floodplain facies associations; and unit III, equivalent to the major part of Feliz Deserto Formation, is characterized by delta deposits with polymodal paleocurrent pattern. The changes of depositional system, as well as paleocurrent direction, suggest that the previously described units were deposited in different evolutionary stages of rifting. Units I and II represent the record of a wide and shallow basin associated with the first stage of rifting. Unit I is characterized by incipient extensional stress generating a wide synclinal depression, associated to the low rate of accommodation and low tectonic activity. These two parameters progressively increase in unit II. The paleocurrent direction of unit I indicates that the depocenter of this wide basin was located at SE of the studied area. No conclusion could be done on paleocurrent from unit II because of the low amount of measurements. Unit III suggests a second stage marked by a deeper basin context, with a high rate of accommodation space associated with the lateral connection of faults and individualization of the half-graben. The scattering in the paleocurrent direction in this unit indicates sedimentary influx coming from several sectors of the half-graben. The boundary between these two stages is marked by a flooding surface that indicates an extremely fast transition and suggests a radical change in geometric characteristics of the basin due to the increase of tectonic activity.
Highlights
METHODSThe recent studies on rift basins focus on the influence of tectonic on basin geometry, on distribution and connection of faults and on the ratio between accommodation and sediment supply (A/S) during the stages of rifting (Prosser 1993, Bosence 1998, Gawthorpe and Leeder 2000, Morley 2002, Kuchle and Scherer 2010)
This paper aims to apply the concepts of sequence stratigraphy to analyze depositional architecture and the stacking patterns of the initial phases of the rift, which in Sergipe‐Alagoas corresponds to the Serraria and Feliz Deserto formations
It was possible to individualize three depositional units in the Neo-Jurrasic and Neocomian succession of Sergipe‐Alagoas Basin: unit I, which corresponds to the base of Serraria Formation, is characterized by multi-storey and multi-lateral, amalgamated sandstone bodies of braided fluvial channel system; unit II, equivalent to the base of Feliz Deserto Formation, is composed of anastomosed fluvial channel sandstones interlayered with floodplain mudstone deposits; Unit III, that corresponds to the major part of Feliz Deserto Formation and is composed of prodelta and delta front deposits
Summary
The recent studies on rift basins focus on the influence of tectonic on basin geometry, on distribution and connection of faults and on the ratio between accommodation and sediment supply (A/S) during the stages of rifting (Prosser 1993, Bosence 1998, Gawthorpe and Leeder 2000, Morley 2002, Kuchle and Scherer 2010). The Serraria Formation was deposited during the Neocomian and it is described as a succession of medium to coarse sandstones, white, greyish to reddish, poorly to moderately sorted with frequently subrounded feldspar and kaolinitic grains and locally conglomerate (Schaller 1969) These sediments represent deposits of braided fluvial channels systems with aeolian reworking (Campos Neto et al 2007, Chagas 1996). Medium- to very coarse-grained sandstone, badly-sorted, St granule and pebble quartz clasts, dispersed or at the base of sets; trough cross-stratification; 10 cm to 3.7 m thick sets; common intraformational mudstones clasts Interpretation The occurrence of sandstone bodies bounded by erosive surfaces, characterized by medium- to coarse-grained sandstones, moderately- to poorly-sorted, organized in stacked sets of unidirecionally-oriented, decimeter-scale trough and planar cross-strata forming fining upward cycles, suggests deposition in fluvial channels (Chagas et al 2007, Miall 1996). The abundance of cross-strata suggests a relatively constant discharge, UTM
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