Sequence Stratigraphy Applied to Continental Rift Basins<subtitle>Example from Recôncavo Basin, Brazil</subtitle>

Abstract

The Reconcavo Basin in northeastern Brazil records more than 20,000 ft (7000 m) of clastic sediments deposited in eolian, shallow and deep lacustrine, deltaic, and fluvial environments during the Early Cretaceous rifting process, at the onset of the Gondwanaland breakup. During rifting, basin fill was controlled by the combined effects of tectonic activity and high-frequency climatically driven changes in lake level. Different sedimentation styles developed during the Neocomian as a function of the intensity of tectonic activity. During the phase of more intense tectonic activity, the basin was a half graben with a shelf area separated from a deep lacustrine depocenter by a narrow fault-controlled steep slope. During this stage, two main depositional systems developed in the half graben: (1) alluvial fans and deep-water sediment gravity flows from the rift's eastern border fault and (2) immature fluviodeltaic systems in the half-graben shelf area (flexural zone) with associated deep-water prodeltaic gravity flows on the down-thrown block of the fault-controlled slope zone. The rapidly subsiding depocenter and fault activity during this period led to a sedimentary record characterized by a fining-upward stacking on the slope and depocenter and erosion or bypass in large areas of the half-graben shelf. The sedimentary strata related to this stage are herein called slope-controlled deep-water systems (SC-SS) fed by a fluviodeltaic system. With the decrease in fault activity, gradually, the feeder systems moved from the flexural zone and an axial fluviodeltaic system developed from the north, with deep-water aggradational sedimentation followed by shallow-water aggradational-to-progradational amalgamated distributary mouth bars dominated by interpreted hyperpycnal flows. The shallow lacustrine environment paired with a high sediment load caused rapid progradation during this stage. The strata related to this stage are herein called the axial deep- to shallow-water systems (AX-SS). Interaction between climate and tectonics controlled the rate of accommodation creation, sedimentation rate and sourcing in the area, forming the basis for a sequence-stratigraphic interpretation. A composite sequence boundary is interpreted at the base of the deep-water sediments in the SC-SS with a lowstand sequence set extending from local units B to D. Sedimentation at the axial area of the half graben (AX-SS) started later, corresponding to the unit C in the SC-SS. A composite transgressive surface is interpreted at the base of unit E, marking a long-term trend of decreasing sedimentation energy (transgressive sequence set), culminating in a region abandonment surface interpreted as a composite maximum flooding surface in unit G. Highstand sequence set is represented mostly by the aggrading and prograding deltaic sequences that correspond for the major part to the Marfim Formation. Another composite sequence boundary caps this composite sequence, corresponding to the local marker 15 at the top of the Marfim Formation.