Abstract The upper Miocene (late middle to early late Miocene) depositional episode (UM depisode) records a long-lived family of sediment dispersal systems that persisted for nearly 6 Ma with little modification. In the central Gulf of Mexico basin, this depisode records extensive margin offlap, primarily centered on the paleo-Tennessee River and Mississippi River dispersal axes, that began immediately following the Textularia W/Textularia stapperi flooding and is terminated by a regional flooding event associated with the Robulus E biostratigraphic top. Thickest sediments are deposited in the paleo-Tennessee River delta beneath modern southeast Louisiana, where three major depocenters are recognized. These depocenters have migrated in both strike and dip directions, and margin progradation is very prominent. The composite fluvial-dominated paleo-Tennessee and Mississippi delta system rapidly built beyond the subjacent middle Miocene shelf margin to construct a sandy delta-fed apron. Margin outbuilding was locally and briefly interrupted by hyper-subsidence due to salt withdrawal and consequent slope mass wasting. Sediments also continuously bypassed into the Mississippi Canyon, Atwater Valley and Green Canyon OCS areas throughout the entire upper Miocene, forming two secondary depocenters composing the McAVLU submarine fan system at the base of the paleo-continental slope. A broad, but relatively thin, sandy strandplain and clastic shelf succession, supplied by reworking of the deltaic deposits, extended eastward and westward from the delta system. Abundant strike-reworked sediment locally prograded the strand plain to the shelf edge, and slope offlap exceeds 30 mi (50 km). The presence of extremely large volumes of high-quality shelf margin delta and deep-water fan sandstone reservoirs results in the great productivity of the central Gulf of Mexico upper Miocene, and upper Miocene production is dominated by a major deltaic oil and gas trend straddling the southeast Louisiana coast.

Abstract The Wilcox Group contains most of the documented large submarine canyons and slump scars within the northern Gulf of Mexico Paleogene section. Indeed, comparable abundance and scale of canyons is not seen again in the Gulf until the late Neogene. Four styles of shelf margin excavation morphologies can be differentiated based on published examples: Simple slump scars. Submarine canyons created by retrogradational slumping. Graded, mature submarine canyons. Valley-form cross-shelf gorges. These styles appear to be part of a continuum; many examples share elements of two or more types. However, most canyons have been mapped using limited well control, and their illustrated morphology may largely reflect the interpreter’s use of fluvial morphologic analogues as much as data constraints. Wilcox canyons typically occur in geographically localized clusters, which are centered beneath the central and upper Texas coastal plain and the central Louisiana coastal plain. Two of the clusters occur on the flanks of the lower–middle Wilcox Houston delta system; the third lies on the progradational front of the lower Wilcox Holly Springs delta system. Canyon clusters appear to occur at or near major tectonicstratigraphic domain boundaries of one or more Wilcox deposodes. Several of the largest canyons, including the Yoakum canyon, correlate with two thin, regional marine flooding units, the Yoakum and Big shales. However, known canyons also occur at several additional stratigraphic positions at the base of and within the Paleocene lower Wilcox genetic sequence. Recurrent shelf margin mass wasting events could have been triggered by seismic shocks initiated along the Laramide tectonic front, which lay along the west and northwest margin of the Gulf basin. Detailed analyses of the Yoakum and Lavaca canyons showed that they were excavated (at least in their late stage of development) across the Wilcox shelf and deltaic platform during times of local to regional transgression by processes of headward slumping and erosion by submarine currents. Canyon cutting and filling occurred sufficiently rapidly that steep (up to thirty degree) unstable canyon walls consisting of unconsolidated slope and prodelta muds were buried and preserved. Infilling occurred during subsequent progradational advance of the shore line. The obvious canyons were largely mud filled. However, sand bodies were present within the fill of both the Yoakum and Lavaca canyons. Fill of the Lavaca canyon was extensively cored and consists of turbidite channel and levee facies suspended within volumetrically dominant, muddy debris flow deposits. Intact slump blocks of canyonbounding delta front successions were also found. Updip reaches of canyons typically included a lower onlap fill and a superimposed, mud-dominated progradational fill. The presence of clusters of canyons along the updip Paleocene continental margin suggests a genetic relationship to turbidite channel and lobe sand bodies that constitute the reservoirs for the recent deep-water Paleogene discoveries. Successive canyons would have served to collect and focus sediment transport across the shelf and down the nascent Cenozoic continental slope, creating a submarine canyon—fan couple. Such highly evolved sediment transport systems would allow efficient separation of bed load (sand) from suspended load (silt/clay) as sediment gravity flows traveled from the shelf margin and upper slope, through mature canyons, and onto large, abyssal plain fan systems with well segregated upper, middle, and lower fan provinces. The high net/gross sections of channeled turbidite lobe sand bodies penetrated in the Mississippi Fan and Perdido fold-belt fairways likely have accumulated in the sand-rich middle fan. Paleogeographic reconstruction places the sand-rich middle fans more than 200 km basinward from the slope toe. Although impressive, such dimensions are typical of many sandy Quaternary abyssal fan systems.

Abstract Winker (1982 ; 1984 ) provided the first summary overview of the depositional evolution of the Gulf of Mexico continental margin, along with a suite of criteria for margin recognition. The subsequent more than 20 years of exploration deep drilling in the Gulf has substantiated both his methodology and synthesis. This paper updates Winker’s map and locates the paleo-continental margin at the termination of 17 principal Cenozoic depositional episodes (deposodes). Northern Gulf of Mexico margins are characterized by a family of attributes, including transition from outer shelf to upper slope faunal assemblages, syndepositional extensional growth structures, rapid thickening along margin-parallel depoaxes, change from relatively continuous, progradational delta and shelf to highly discontinuous, aggradational slope facies successions, local development of erosional submarine canyon heads and slump scars, and increased regional dip. Cenozoic shelf margin types include: (1) stable progradational margins; (2) unstable progradational margins, typically associated with high-rates of sediment supply by extra-basinal fluvial systems to large shelf-margin deltas and their associated shore-zone systems; (3) retrogradational margins created abruptly by rapid sub-regional salt withdrawal, commonly accompanied by submarine mass wasting and erosion or slowly by long-term compactional subsidence; and (4) perched margins formed by progradation onto foundered continental shelves. Rates of shelf edge offlap vary greatly in both time and space along the northern Gulf of Mexico margin. Highest rates exceed 30 km/Ma and are associated with Oligocene, middle Miocene, and Plio-Pleistocene depocenters.

Abstract Sequence stratigraphic application has emphasized the recognition and use of subaerial (fluvial entrenchment) or shallow marine/shoreface (regressive ravinement) surfaces as critical boundaries for defining sequences. These surfaces are variously objectively or conceptually associated with times of onset, maximum rate, and/or lowest position of relative sea level fall. However, well-dated Quaternary analogues demonstrate that the fluvial entrenchment surface is neither inherently synchronous nor regional, and that low-stand facies associations and their bounding surfaces are highly dependent upon the vagaries of paleogeography and sediment supply. Furthermore, some basin fills display stratigraphy in which demonstrable subaerial or ravinement surfaces correlative to fall events are poorly preserved or entirely lacking, but in which sequences can be defined by use of various combinations of transgressive ravinement, marine deflation, and marine starvation surfaces. These surfaces may not and need not correspond to a relative fall (or rise) of sea level. Selection of stratigraphic surfaces as sequence boundaries and interpretation of sequence systems tract compositions and relationships both require understanding of the overall depositional systems tract and of the full array of regime variables: sediment supply, sediment composition, base level change, and energy regime. Functional, reproducible, and chronostratigraphic “… genetically related successions of strata bounded by unconformities or their correlative conformities…” can be defined, correlated, mapped, dated, and interpreted through the use of a variety of regional stratigraphic surfaces of non-deposition and erosion.

Abstract Sedimentary facies, reflecting original depositional environment, define the trend, dimensions, connectivity and internal heterogeneity of transmissive zones within clastic aquifer systems. Three heterogeneity styles—layer cake, jigsaw puzzle and labyrinth—reflect increasing degree of complexity. Style is directly determined by the depositional origin of the aquifer. Heterogeneity occurs over a broad range of scales. Megascopic heterogeneity is determined by the external dimensions, trends and degree of interconnection of permeable units. Macroscopic heterogeneity, which occurs at the depositional facies scale, includes (1) compartmentalization due to flow barriers between specific facies within larger permeable units, (2) vertical and lateral permeability gradients created by patterns of grain size and sorting, and (3) stratification and low-permeability mud baffles, which create anisotropy. Mesoscopic heterogeneity reflects lithofacies, sedimentary structure and lamina-scale variability. All three scales of heterogeneity structure can be efficiently described, quantified, interpolated and predicted within the context of a well understood depositional system framework.

Abstract Fluvial depositional systems constitute major aquifers in closed and semiclosed terrestrial basins, alluvial valleys and intracontinental sags containing axial river systems, and coastal plains containing fluvial, deltaic shore-zone and sometimes fan and fan-delta systems. Fluvial systems can be grouped into a spectrum defined by trunk-stream bed-load, mixed-load and suspended-load channel types. Each channel type produces a predictable range of external aquifer geometries and internal heterogeneities. Because of the strong contrasts in fluvial systems of permeable, transmissive channel fill facies and confining flood-basin facies, flow patterns are controlled by channel connectivity, which correlates to fluvial system type and overall sand percentage. Bed-load (generally braided) and mixed-load (generally meandering) fluvial systems typically deposit “jigsaw-puzzle” aquifers systems. Heterogeneity increases with decreasing scale of facies units and increasing shale (or mud) in the system. Small meandering rivers and stable mud-rich, low-sinuosity channel systems create “labyrinthine” aquifer systems. In mixed- and suspended-load channel fills, vertical and along-channel textural changes and lithofacies partitioning and the dipping shale baffles in the upper point bar commonly create separate permeability units within a single point-bar sand body. Low-sinuosity suspended-load channels typically consist of a lower active fill and an upper abandonment phase fill—creating two units of differing aquifer properties. Sand-body width-to-thickness ratios are greatest for braided (largely bed-load) channels and are least for low-sinuosity, stable (largely suspended-load) channels and delta distributaries. Facies dimensions control both lateral extent of permeable units in labyrinthine systems and continuity of the more permeable units within jigsaw-puzzle systems.