Vitor Abreu

Sequence stratigraphy hierarchy and the accommodation succession method

We propose a framework for the hierarchy of sedimentary units observed in stratigraphic data that is based entirely on the geometric relationship of the strata. This framework of geometries is assumed to result from repeated successions of accommodation creation and sediment fill (here named accommodation succession). We have modified existing hierarchal frameworks to describe depositional units resulting from accommodation successions of varying magnitude and duration, across a depositional profile. Each full succession consists of component partial succession sets that are, sequentially, lowstand—progradation to aggradational; transgressive—retrogradation; and highstand—aggradation to progradation to degradation. The terms “highstand” and “lowstand” as originally defined to label systems tracts relative to a shelf edge, and with an implied relationship between sea level and systems tracts, have been the root of confusion. We propose that these terms be used in the strict sense of the original definition, because their meaning has been lost when applied to the many depositional settings and high-resolution data sets to which the concepts of sequence stratigraphy are now applied. We propose that the concept of accommodation succession stacking be used in the interpretation of stratigraphic data within a hierarchal framework of depositional sequences, sequence sets, and composite sequences. This will allow an interpreter to accurately categorize observations, provide a basis for predictions away from control points, and develop a framework that allows revisions as higher-resolution data become available.

Sequence stratigraphy of experimental strata under known conditions of differential subsidence and variable base level

Sequence stratigraphy has been applied from reservoir to continental scales, providing a scale-independent model for predicting the spatial arrangement of depositional elements. We examine experimental strata deposited in the Experimental EarthScape facility at St. Anthony Falls Laboratory, focusing on stratigraphic surfaces defined by discordant contact geometries, surfaces analogous to those delineated in the original work on seismic sequence stratigraphy. In this controlled setting, we directly evaluate critical sequence-stratigraphic issues, such as stratigraphic horizon development and time significance, as well as the internal geometry and migration of the bounded strata against the known boundary conditions and depositional history. Four key stratigraphic disconformities defined by marine downlap, marine onlap, fluvial erosion, and fluvial onlap are mapped and vary greatly in their relative degree of time transgression. Marine onlap and downlap contacts closely parallel topographic surfaces (time surfaces) and, prior to burial, approximate the instantaneous offshore topography. These stratal-bounding surfaces are also robust stratigraphic signals of relative base-level fall and rise, respectively. Marine onlap surfaces are of special interest. They tend to be the best preserved discordance, where widespread, allogenic-based onlap surfaces subdivide otherwise amalgamated depositional cycles amidst cryptic stacks of marine foresets; however, local, autogenic-based marine onlap discordances are present throughout the fill. A critical distinguishing feature of allogenic onlap is the greater lateral persistence of the discordance. Surfaces defined by subaerial erosional truncation and fluvial onlap do not have geomorphic equivalence because channel processes continually modify the surface as the stratigraphic horizons are forming. Hence, they are strongly time transgressive. Last, the stacking arrangement of the preserved bounded strata is found to be a good time-averaged representation of the mass-balance history.

Sea level influence on the nature and timing of a minibasin sedimentary fill (northwestern slope of the Gulf of Mexico)

This study focuses on the sedimentary fill of basin 4, the termination of the Brazos-Trinity minibasin slope system in the northwestern Gulf of Mexico. Results from multistratigraphic analyses of 15 giant piston cores provided (1) important information regarding the nature (hemipelagic versus gravity-induced mud and sand deposits) and the timing of the sedimentary fill; (2) some key chronostratigraphic constraints for the evolution of this system; and (3) strong links between well-known cycles of sea level change to clearly imaged deposits in the fill of basin 4. Gravity-flow–induced sedimentation in basin 4 occurred and increased in importance during the stepwise sea level regression that developed between 115 and 15 ka and clearly ceased just prior to the meltwater spike in the Gulf of Mexico dated at about 14 ka. The onset of gravity-induced deposition in basin 4 is dated at marine isotope stage (MIS) 5d (115 ka). This finding implies that sandy turbidity currents reached this distal setting as a consequence of a higher frequency sea level fall within a time of general high sea level (MIS 5). An interval of hemipelagic sedimentation lasting from 90 to 45 ka illustrates cessation of gravity-induced deposits in basin 4. Turbidite sandy deposits resumed in mid-MIS 3 and increased toward MIS 2 (approximately from 30 to 15 ka). The largest proportion of reservoir-grade sandy sediment was deposited during the maximum sea level lowstand of the last glacial maximum, consistent with the prevailing view of sequence-stratigraphic models for deep-water deposition.

Accommodation succession (δA/δS) sequence stratigraphy: observational method, utility and insights into sequence boundary formation

The future of sequence stratigraphy depends on stratigraphers making observations with a common method so that physical frameworks can be clearly separated from interpretations of driving mechanisms. Depositional sequence boundary selection is a well-known controversy that could be resolved with objective recognition criteria. Accommodation succession sequence stratigraphy refines traditional methods, using sedimentary facies, facies associations, vertical stacking, stratal geometries and stratal terminations as the objective record of competing rates of accommodation change and sediment fill through time. Observations are placed in context of lateral (transgression and regression) and vertical (aggradation and degradation) movement of shoreline through time, across multiple timescales in hierarchal stacks. The repeating motif consists of a subaerial unconformity and its correlative subaqueous surface overlain in coastal settings by a basinward shift in coastal onlap and strata with progradational to aggradation stacking, then retrogradation and aggradation–progradation–degradation stacking. These stacking patterns are bounded by key surfaces, recognized by stratal terminations and characteristic vertical successions of facies. This pattern is independent of time duration or position on a sea-level curve, but incorporates data resolution, regional extent and hierarchal stacking. Examples from multiple datasets show the utility and objectivity of the method and provide insights into sequence boundary formation.

Morphodynamics of supercritical turbidity currents in the channel-lobe transition zone

This study aims to resolve process-facies links at both bed and environmental scales for the channel lobe transition zone (CLTZ). Data comes from existing experimental and modern CLTZ studies and from new outcrop studies. The experiments show that the CLTZ architecture of supercritical turbidity currents is complex and different from their counterparts where flows are subcritical throughout. Supercritical CLTZ’s are characterised by erosive channels formed by supercritical turbidity currents, by offset stacked lobes deposited from subcritical turbidity currents and by hydraulic jump related mouth bar deposits and upslope onlapping backfill deposits at the down slope end of the transition zone. Erosive channels and backfill features can be resolved by high resolution seismic data, yet evidence for supercritical flow must come from facies analysis of core data. Outcrop examples of the CLTZ from the Tabernas submarine fan (SE Spain) and the Llorenc del Munt deep-water delta slope (N. Spain) are used to establish such links between seismic scale architecture and facies recognised in cores. The outcrops described here were mapped as transition zone, and show 100 m sized, spoon-shaped scours filled with sediment containing sandy to gravelly backsets up to 4 m in height. Their facies and architecture is indicative of deposition by hydraulic jumps, can be recognized from cores, and is a good proxy for further predicting CLTZ architecture constructed by supercritical turbidity currents.