Abstract Sequence stratigraphy is a method to systematically place key stratal observations into a chronostratigraphic framework for more accurate predictions away from control points. The depositional sequence is its basic unit, defined as “a stratigraphic unit composed of a relatively conformable succession of genetically related strata and bounded at its top and base by unconformities or their correlative surfaces” ( Abreu et al., 2014 modified from Mitchum et. al., 1977a ), which forms as a result of transgressions and regressions. Sequence stratigraphy is first and foremost a method that guides observations in the stratigraphic record across an array of depositional settings, stratal attributes, and data sets, explicitly recognizing that the stratigraphic record is comprised of both rocks and surfaces in various forms. These observations are then summarized in models that generalize details to facilitate prediction away from data control points. For completeness, sometimes the models are interpreted in terms of mechanisms (e.g., eustasy, climate, etc.) that may help explain observations and enhance prediction. The accommodation succession method of sequence stratigraphy ( Neal and Abreu, 2009 ) assumes that these building blocks form in response to varying rates of coastal accommodation increase and decrease (δΑ) relative to the rate of sediment flux (δS).

Abstract Determining how autogenic and allogenic processes and responses in deltas scale up from meter-scale laboratory experiments to actual field examples remains a challenge. This study was devised to bridge that scale gap using field data from small, hundreds of meter-scale natural deltas. Ground-penetrating radar and core data were collected from four different river-dominated delta morphotypes developing at the margins of freshwater coastal lagoons in southern Brazil. Since the sediment supplying these deltas is sourced from a nearby dune field and is similar between the deltas, it is hypothesized that major morphological differences in the four deltas are primarily the result of differences in sediment discharge rates (sediment-water ratio). As observed in published tank experiments, channel cross-section and distributary channel patterns in the deltas vary as a function of sediment discharge, from shallow sheet-like flow at high discharge to well-established, stable distributary channels (i.e., birdsfoot pattern). A contributing factor may be the development of vegetation on the slower growing deltas influencing sediment cohesion, a key control in laboratory-scale deltas. As in many tank experiments, these lagoon deltas are steep and sandy, with the Froude number modulated to just below Froude critical flow (i.e., they are Froude-scaled). Ground-penetrating radar sections were processed, interpreted, and integrated with cores, allowing the definition of radar units. Analysis of the radar units demonstrates the presence of both allogenic and autogenic signals. Allogenic control is identified in the stacking of clinoforms and is perceptible in both sides of a single delta (delta 4), as well as in two other deltas (deltas 1 and 2). An autogenic signal varies according to delta planform shape and was identified by the stacking of lobe elements, both in dip and strike. Base-level change (lake level) and autogenic avulsion cycles occur on similar timescales, and therefore it is a significant challenge to separate these different processes in the stratigraphy. The potential uses of these types of data include understanding the link between delta dynamics, channel patterns, and stratigraphy to develop improved genetic models of steep sandy deltas common in the stratigraphic record.

Abstract The Reco^ncavo 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.

Abstract The oil industry has been active in Azerbaijan for centuries, and the Apsheron Peninsula, Apsheron sill, onshore, and the shelf margin of Azerbaijan are considered mature areas for exploration. However, large areas of the offshore Caspian, including the deep-water South Caspian, Turkmenistan shelf, and Central Caspian are still exploration frontiers. An understanding of the stratigraphy of reservoir rocks and seals in these areas could significantly reduce exploration risk. The interplay of the paleo-Volga, paleo-Amu Darya, and paleo-Kura deltas, since the late Miocene, provides the first-order controls on prospect distribution. A continuous trend of coastal onlap on the western margin exists for the South Caspian and in the Central Caspian basins from the upper Miocene to lower Pliocene, with onlap of these units over Miocene and Cretaceous rocks. These coastal onlap trends are associated with an overall rise in lake level from the lower to the upper productive series. The three delta systems exhibit significant differences in depositional style and timing, reacting in different ways to the rising lake level. Strong progradation of the paleo-Amu Darya delta occurred on the Turkmenistan shelf, on the eastern margin of the South Caspian Basin. This progradation is related to the Pliocene deposition of the Red series in Turkmenistan (equivalent to the Pereryva to Surakhany suites in Azerbaijan). Thus, the paleo-Amu Darya delta prograded during rising lake level, controlled primarily by sediment supply. During the deposition of the Pereryva suite, the paleo-Volga delta aggraded in the Apsheron region (northern margin of the South Caspian). A transgressive trend marks the Central Caspian Basin from the upper Balakhany to the Su-rakhany suites and may indicate backstepping of the paleo-Volga delta at that time. In the paleo-Kura system, on the southwest margin of the South Caspian Basin, a backstepping trend occurred during the deposition of the upper Ba-lakhany and Sabunchi suites (lower Pliocene). A downlap surface developed at the base of the paleo-Kura delta in the middle Surakhany suite. This downlap surface along the western Caspian margin correlates to the upper part of the progradational phase of the paleo-Amu Darya delta on the eastern margin of the basin. A paleo-Kura prograding wedge developed concurrent with the deposition of the upper Surakhany and Akchagylian (upper Pliocene). The impact of sediment supply from the Alborz Mountains in Iran could not be evaluated because of lack of data. Climatic fluctuations did exert a dominant control on the style of sedimentation in the South Caspian Basin through their direct impact both on lake levels and on sediment supply. The entire Productive Series reflects the Pliocene golden climate, when the Earth, overall, was much warmer than today. In addition, on shorter time scales, the stratal pattern is controlled by high-frequency climatic cycles. Late lowstand deposits are dominated by aggradational braided streams and braid deltas. Transgressive and highstand deposits consist of extensive lake shales interbedded with silts and sands. The transgressive shales can act as pervasive seals and permeability barriers and baffles in the reservoirs. Very little sand appears to have entered the lake during periods of falling lake level.