Jeffrey A. May

Slope readjustment: A new model for the development of submarine fans and aprons

In a new model proposed for the formation of submarine canyons and submarine fan-aprons, numerical simulations of basin fill are utilized to illustrate simple concepts of slope grading. This modeling suggests that erosional truncation, sediment bypass, and marine onlap of submarine fan-apron complexes are formed in response to changing basin physiography. Two kinds of basin margins are identified. (1) Progradational margins represent the basinward advance of graded depositional profiles formed when diffusive and sediment gravity-flow processes are in equilibrium with sediment supply, basin subsidence, and basin physiography. (2) Erosional margins form when upper slope gradients exceed an equilibrium grade, and are characterized by erosion, slumping, and sediment bypass to lower slope environments via sediment gravity-flow processes. Erosional margins are transformed into progradational margins when the bathymetric escarpment (i.e., an oversteepened margin) is buried by onlapping and aggrading fanapron deposits. Progradational margins can develop bathymetric escarpments and become erosional in response to a rapid rise in relative sea level, structural deformation of basin profiles (e.g., faulting), and/or a transition from carbonate to siliciclastic deposition. Whereas relative falls in sea level play a major role in bringing the source of sediment input to the shelf edge, the development of slope unconformities and onlapping submarine fan-apron systems is controlled primarily by slope- readjustment processes triggered by changing basin physiography.

Insights from Stratigraphic Modeling: Mud-Limited Versus Sand-Limited Depositional Systems

Quantitative stratigraphic modeling provides new perspectives on the controls on basin-fill stacking patterns, gross lithology distributions, and submarine fan deposition in siliciclastic systems. Our modeling relates basin-fill geometries to the ratio of space creation vs. sediment supply. Facies components differentially distribute sand/mud mixtures below rising base-level surfaces across a basin profile. We model alluvial architecture, shoreline position, shelf width, and the occurrence of turbidite sands as a function of relative base-level rise and varying receiving-basin geometries. Slope unconformities and onlapping submarine fans are the erosional and depositional responses to changing basin physiography. Sediment budgeting reveals two types of siliciclastic systems. Sand-limited systems develop in shallow basins or on basin-margin platforms; have broad, muddy, wave-graded shelves; and are subject to exposure and major basinward shifts of shoreline with minor sea level falls. Mud-limited systems develop during progradation into deep-water basins, have narrow shelves with turbidites in a base-of-slope position, and are not particularly susceptible to major basinward shifts in facies. We emphasize the fundamental importance of basin physiography and lithology budgeting, as well as sea level, in contrast to eustatically based models. In addition to accommodation and sediment supply, alluvial architecture and basin depth also act as primary controls on shelf width, basin-fill geometry, and submarine fan deposition.

Eustatic control on synchronous stratigraphic development: cretaceous and eocene coastal basins along an active margin

Abstract Field studies document an apparent eustatic control on facies patterns along a tectonically active margin. In the San Diego Embayment and northern Baja California, progradational-retrogradational shoreline sequences characterize Late Cretaceous and Eocene forearc stratigraphy. Extensive benthonic foraminifera and nannoplankton data provide control on the age and distribution of facies changes along these depositionally compact, bathymetrically steep-gradient margins. The complete stratigraphic package is arranged into three scales and patterns of depositional sequences. Timing and geometry of the two largest sequences provide relative sea-level curves that correlate exceptionally well with worldwide sea-level curves. The major depositional cycle is asymmetric, hundreds of meters thick, characterized by a thin basal retrogradational sequence overlain by a thick progradational sequence; each cycle correlates to a coastal-onlap “supercycle”. Smaller scale stratigraphic rhythms, controlled by global “cycles” and “paracycles”, compose depositional cycles. Local depostional pulses overprint these two larger order sequences of sedimentation. Coeval cycles and depositional rhythms in isolated coastal basins from Oregon to Baja California further indicate a primary eustatic control on sedimentation. Field-based facies analysis thus supports the use of the “Vail curve” or other coastal-onlap and global sea-level curves as a predictive tool in basin analysis.

Abstract: Sequence Stratigraphic Framework and Depositional Variations of Miocene Lowstand Systems Tracts, South Louisiana

Abstract Facies of Miocene lowstand systems tracts comprise a large portion of the remaining exploration potential for deep untested objectives I southwest Louisiana. This complex and highly variable section is contained within 22 major depositional sequences deposited from 21.9 to 7 million years ago; each sequence ranges in duration from 0.5 to 1.5 million years. Regional well-log, seismic, and biostratigraphic correlation and mapping of the sequence stratigraphic framework defines the occurrence and extent of lowstand facies that filled intra-slope basins across south Louisianas coastal zone. From oldest to youngest, the sequence encompass the Cristellaria R through Cristellaria K biozones. Within onshore and state water areas, 14 sequences contain regionally correlative lowstand systems tract components. Almagamated, massive turbidite facies of basin floor fan complexes are not recognized within intra-slope basins in association with rapid depositional cycles. Deep-water fan and channeled levee facies of slope fan complexes and deltaic facies of prograding complexes are well-developed throughout the early and middle Miocene section. Late Miocene lowstand deposits are best developed in the offshore trend. Syndepositional fault movement and salt withdrawal controlled thickness trends, facies patterns, sandstone content, and the position and trend of the shelf-slope break. Sandstone-rich basin floor fans, slope fans, and prograding deltaic complexes are best developed in intraslope basins where paleobathymetric confinement and accommodation were greatest. End_of_Record - Last_Page 349-------

Diagenetic/Stratigraphic Modeling: Wilcox Group, Texas Gulf Coast: ABSTRACT

The basic premise of diagenetic/stratigraphic modeling is similar to that of simple stratigraphic modeling, i.e., facies interpretations are based upon (1) the depositional process recorded and (2) the lateral variation and vertical sequences of these processes. Stratigraphic models depend primarily upon physical processes, whereas diagenetic/stratigraphic modeling requires additional knowledge of chemical processes. For example, depending on the energy and character of the depositional process, each facies will contain a different subset of the original provenance mineralogy, and exhibit a different set of textural characteristics. In addition, chemistry of bottom waters during deposition governs formation of specific early authigenic clay and carbonate cements. Therefore, petrographically determined compositions can be used to complement, and test, traditional methods of defining depositional environments. The authors apply this method to the Wilcox Group of the Texas Gulf Coast.

Shelf-Edge Conduit: Channelized Sediment Transport Across Eocene Fore-Arc Basin Margin, Southern California: ABSTRACT

A complete shallow-to-deep marine transition is exposed in Eocene rocks at San Diego, California. Well-exhibited facies relations, precise biostratigraphic control, and paleobathymetric indicators allow comparison of these continental margin outcrops with similar sequences most commonly observed only by seismic-stratigraphic methods. Even in this active tectonic setting, eustasy appears to be the dominant factor in cutting shelf-edge unconformities, forming and reactivating submarine canyons, and controlling the distribution Figure of facies down the shoreline-to-basin conduit. Broad, shallow outer-shelf channels funneled coarse-grained littoral sediments to the gullied upper slope and canyon head. These channels were filled with massive to laminated sandstone and shell lag, bioturbated mudstone, and rare slump-folded mudstone. An early Eocene canyon head was cut at the shelf edge during a eustatic lowstand, then eroded landward during subsequent sea level rise. The canyon is floored with a thick, massive sandstone that may be amalgamated or crudely graded, containing clasts of canyon-wall debris up to 6 m long in cross section, as well as conglomeratic to pebbly or gravelly sandstone. It exhibits convolutions, flame structures, and other evidence of rapid End_Page 954------------------------------ deposition and dewatering. Deposits above the basal sand accumulated in broad, anastomosing channels 10 to 15 m deep with variegated fill; some still contain sand and conglomerate, but most were evacuated and filled with lower energy thin-bedded sands and muds or massive hemipelagic muds, indicating significant volumes of bypassed sand. At the base of slope, a major 100-m deep leveed channel was floored with conglomerates and large canyon-wall clasts, and filled with massive to convoluted sandstone. The channel fed a system of shallow, crosscutting, conglomeratic channels, interpreted as inner fan that extended into the canyon mouth. Paleobathymetric relief exhibited across this ancient shelf break is minimally 600 m (outer shelf to mid-bathyal or deeper) within a lateral outcrop distance of 3,000 m. End_of_Article - Last_Page 955------------