William E. Galloway

Genetic Stratigraphic Sequences in Basin Analysis I: Architecture and Genesis of Flooding-Surface Bounded Depositional Units

Marine basin margins are characterized by repetitive episodes of progradation punctuated by periods of transgression and flooding of the depositional platform. The resultant stratigraphic units consist of genetically related (1) depositional systems and their component facies sequences; (2) bypass, nondepositional, and erosional surfaces; and (3) in thick sequences affected by gravity tectonics and crustal response to loading, syndepositional structural discontinuities. Units are bounded by hiatal surfaces preserved as submarine unconformities or condensed sedimentary veneers and that record maximum marine flooding of the basin margin. The repetitive stratigraphic architecture is the product of the ongoing interplay among sediment supply, basin subsidence (and uplift), an eustatic sea level change. Each of these three variables may dominate depositional evolution; furthermore, stratigraphic architecture is very similar regardless of the dominant control. A genetic stratigraphic sequence is the sedimentary product of a depositional episode. The sequence incorporates and reconciles depositional systems, bedding geometries, and bounding surfaces within the framework of cycles of basin-margin offlap and flooding. Each sequence consists of the progradational, aggradational, and retrogradational or transgressive facies deposited during a period of regional paleogeographic stability. The defining genetic stratigraphic sequence boundary is a sedimentary veneer or surface that records the depositional hiatus that occurs over much of the transgressed shelf and adjacent slope during maximum marine flooding. The genetic sequence paradigm emphasizes preserving the stratigraphic integrity of three-dimensional depositional systems and does not rely n widespread development of subaerial erosion surfaces caused by eustatic falls of sea level to define sequence boundaries. The physical stratigraphic record of transgression and flooding--distinctive thin but widespread facies sequences, prominent erosional surfaces, and superjacent marine condensed intervals or sedimentary veneers--provides readily recognized, regionally correlative, easily and accurately datable, and robust sequence boundaries that commonly define times of major basin-margin paleogeographic reorganization in terrigenous clastic basins.

Sequence stratigraphy: methodology and nomenclature

The recurrence of the same types of sequence stratigraphic surface through geologic time defines cycles of change in accommodation or sediment supply, which correspond to sequences in the rock record. These cycles may be symmetrical or asymmetrical, and may or may not include all types of systems tracts that may be expected within a fully developed sequence. Depending on the scale of observation, sequences and their bounding surfaces may be ascribed to different hierarchical orders. Stratal stacking patterns combine to define trends in geometric character that include upstepping, forestepping, backstepping and downstepping, expressing three types of shoreline shift: forced regression (forestepping and downstepping at the shoreline), normal regression (forestepping and upstepping at the shoreline) and transgression (backstepping at the shoreline). Stacking patterns that are independent of shoreline trajectories may also be defined on the basis of changes in depositional style that can be correlated regionally. All stratal stacking patterns reflect the interplay of the same two fundamental variables, namely accommodation (the space available for potential sediment accumulation) and sediment supply. Deposits defined by specific stratal stacking patterns form the basic constituents of any sequence stratigraphic unit, from sequence to systems tract and parasequence. Changes in stratal stacking patterns define the position and timing of key sequence stratigraphic surfaces. Precisely which surfaces are selected as sequence boundaries varies as a function of which surfaces are best expressed within the context of the depositional setting and the preservation of facies relationships and stratal stacking patterns in that succession. The high degree of variability in the expression of sequence stratigraphic units and bounding surfaces in the rock record means ideally that the methodology used to analyze their depositional setting should be flexible from one sequence stratigraphic approach to another. Construction of this framework ensures the success of the method in terms of its objectives to provide a process-based understanding of the stratigraphic architecture. The purpose of this paper is to emphasize a standard but flexible methodology that remains objective.

Cenozoic depositional history of the Gulf of Mexico basin

A Geographic Information System (GIS) database incorporating information from 241 publications, theses, and dissertations; well logs and paleontologic reports; and interpreted University of Texas Institute for Geophysics (UTIG) deep-basin seismic lines was used to map and interpret 18 basinwide genetic stratigraphic sequences that form the Gulf of Mexico basin Cenozoic fill. Eight principal extrabasinal fluvial axes provided the bulk of the sediment infill in the basin. First-order temporal and spatial use of these axes reflects four continent-scale phases of crustal uplift. Abundant sediment supply has prograded the northern and northwestern basin margin 150 to 180 mi (240 to 290 km) from its inherited Cretaceous position. Margin outbuilding has been locally and briefly interrupted by hypersubsidence due to salt withdrawal and mass wasting. Three depositional systems tracts characterize Cenozoic genetic sequences: (1) fluvial --> delta --> delta-fed apron, (2) coastal plain --> shore zone --> shelf --> shelf-fed apron, and (3) delta flank --> submarine fan. One or more examples of the fluvial --> delta --> delta-fed apron systems tract occur in each of the major genetic sequences. Immense volumes of sand have bypassed the shelf margin to be deposited in slope and base-of-slope systems, primarily within fluvial --> delta --> delta-fed apron system tracts, during all major Paleogene and Neogene depositional episodes. Deposition and preservation of volumetrically significant coastal plain --> shore zone --> shelf --> shelf-fed apron tracts is typical of Paleogene through Miocene depositional episodes only. Fan system origin was commonly associated with major continental margin failures, but large submarine canyons occur mainly in Pleistocene sequences. Thick, potential reservoir sand bodies occur in offlapping delta-fed slope and subjacent basin floor aprons, in autochthonous slope aprons and related infills of slide scars and canyon cuts, and in submarine fans.

History of Cenozoic North American drainage basin evolution, sediment yield, and accumulation in the Gulf of Mexico basin

The Cenozoic fill of the Gulf of Mexico basin contains a continuous record of sediment supply from the North American continental interior for the past 65 million years. Regional mapping of unit thickness and paleogeography for 18 depositional episodes defines patterns of shifting entry points of continental fluvial systems and quantifies the total volume of sediment supplied during each episode. Eight fluvio-deltaic axes are present: the Rio Bravo, Rio Grande, Guadalupe, Colorado, Houston-Brazos, Red, Mississippi, and Tennessee axes. Sediment volume was calculated from digitized hand-contoured unit thickness maps using a geographic information system (GIS) algorithm to sum volumes within polygons bounding interpreted North American river contribution. General age-dependent compaction factors were used to convert calculated volume to total grain volume. Values for rate of supply range from >150 km to <10 km3/Ma. Paleogeographic maps for eleven Cenozoic time intervals display the evolving matrix of elevated source areas, intracontinental sediment repositories, known and inferred drainage elements, and depositional fluvial/deltaic depocenters along the northern Gulf of Mexico basin margin. Patterns of sediment supply in time and space record the complex interplay of intracontinental tectonism, climate change, and drainage basin evolution. Five tectono-climatic eras are differentiated: Paleocene late Laramide era; early to middle Eocene terminal Laramide era; middle Cenozoic (Late Eocene–Early Miocene) dry, volcanogenic era; middle Neogene (Middle–Late Miocene) arid, extensional era; and late Neogene (Plio–Pleistocene) monsoonal, epeirogenic uplift era. Through most of the Cenozoic, three to four independent continental-scale drainage basins have supplied sediment to the Gulf of Mexico.

Siliciclastic Slope and Base-of-Slope Depositional Systems: Component Facies, Stratigraphic Architecture, and Classification

Subaqueous slope and base-of-slope depositional systems are a major component of most marine and many lacustrine basin fills, and constitute primary targets for hydrocarbon exploration and development. Seven basic facies building blocks comprise slope systems: (1) turbidite channel fills, (2) turbidite lobes, (3) sheet turbidites, (4) slide, slump, and debris-flow sheets, lobes, and tongues, (5) fine-grained turbidite fills and sheets, (6) contourite drifts, and (7) hemipelagic drapes and fills. The grain size of supplied sediment is a primary control on channel and lobe morphologies and on the scale and importance of slump and debris-flow deposits. Two general families of siliciclastic slope systems occur. Constructional (allochthonous) systems, including fans, aprons, and basin-floor channels, are built of sediment supplied from superjacent delta, shore-zone, shelf, or glacial systems. The facies architecture of allochthonous systems is determined jointly by the sediment texture and pattern of supply to the shelf margin. Point sources of supply create fans; line sources create strike-elongate prisms of slope sediment called slope aprons. Shelf-margin deltas provide a particularly common intermediate source geometry, forming offlapping delta-fed aprons. Autochthonous systems, including retrogressive aprons, canyon fills, and megaslump complexes, record slope reworking and resedimentation.

Genetic Stratigraphic Sequences in Basin Analysis II: Application to Northwest Gulf of Mexico Cenozoic Basin

The northwest Gulf of Mexico Cenozoic sedimentary wedge illustrates the application of genetic stratigraphic sequence analysis and documents several general conclusions. (1) Sequences defined by regional marine flooding are the principal genetic stratigraphic units of the basin fill. Continental margins are characterized by repetitive episodes of basin-margin offlap punctuated by periods of transgression and marine flooding of the depositional platform. (2) Continental margin outbuilding is concentrated at one or more shelf-edge deltaic depocenters separated by interdeltaic bights. Depocenters remain fixed during a depositional episode but commonly relocate during transgression and flooding. (3) A distinct syndepositional structural style in prograding continental margins results in sporadic uplift of a basin-fringing peripheral bulge and accentuates preservation of shelf-margin facies along zones of extensional normal faulting and enhanced subsidence. (4) Genetic stratigraphic sequences commonly reflect an evolving interplay among two or even three variables. For example, early Cenozoic Gulf sequences are most closely related to tectonic events of the intraplate source terrane, which, in turn, affect rate and location of sediment supply and basin-margin response to loading. Late Cenozoic sequences more closely reflect proposed eustatic cycles.

Deposition and Diagenetic Alteration of Sandstone in Northeast Pacific Arc-Related Basins: Implications for Graywacke Genesis

Sedimentary basins characterized by continental to marine shelf depositional regimes are common adjuncts to active lithospheric plate junctions of the northeast Pacific area. Most of these basins are elongate troughs and may form in both fore-arc and back-arc positions. Andesitic composition of volcanic and plutonic rocks that form the major source lithologies in active or recently active arcs produces petrologically distinctive sands that are dominated compositionally by volcanic rock fragments, plagioclase feldspar, and mafic heavy minerals. Quartz content of the sands is low and rarely exceeds 50 percent. Such sands are mineralogically unstable and react upon shallow to moderate burial and consequent increase in temperature and pressure to produce a recurrent sequence of authigenic cements. Three progressive stages of diagenesis are differentiated in sample suites from the Bristol, Queen Charlotte, Grays Harbor, and Chehalis basins: stage 1, early diagenetic calcite pore-filling cement; stage 2, authigenic clay rims and coats around detrital grains; stage 3, authigenic phyllosilicate and (or) laumontite pore-filling cement. Development of silicate overgrowths with chlorite and calcite replacement of rock fragments and plagioclase becomes more pronounced as burial is further increased. As the depth of burial increases, higher temperature, overburden pressure, and fluid pressure result. Of these three factors, geologic evidence favors temperature as the primary control of burial diagenetic reactions. Petrographic data support the concept of a diagenetic origin for graywacke matrix. The second and third diagenetic stages recognized here result in the authigenesis of clay minerals and micas and can reasonably be considered precursors of a graywacke lithology.

Problem of secondary porosity: Frio Formation (Oligocene), Texas Gulf Coast

Secondary porosity, formed by the dissolution of both carbonate and silicate minerals, especially K-feldspars, is widely developed in sandstones of the Frio Formation (Oligocene) in the Texas Gulf Coast. CO 2 produced by decarboxylation of organic matter is commonly suggested as the acid required for dissolution. Material balance calculations indicate that CO 2 produced by decarboxylation of organic matter in Frio Formation shales can account for a regional average of only 1% or 2% secondary porosity in Frio Formation sandstones, yet point-count data indicate an average of 10% secondary porosity. Long-distance fluid transport (many kilometres) and/or other mechanisms of acid generation should, therefore, be considered. Carbon isotopic data on dissolved inorganic carbon in formation water, CO 2 in produced natural gas, and carbonate cements indicate that CO 2 produced by decarboxylation had a minor impact on these carbon reservoirs. The reaction kerogen + water → methane + carbon dioxide can explain the isotopic data, but alone it is insufficient to account for all the secondary porosity unless long-distance material transport is involved.

Sediments and Stratigraphic Framework of the Copper River Fan-delta, Alaska

ABSTRACT The Copper River has prograded a marine dominated fan-delta onto the deep, tectonically active northern shelf of the Gulf of Alaska. The morphology and internal stratigraphy of the delta system are products of the sporadic influx of great volumes of bed load sediment into a basin characterized by a high wave, tide, and current energy flux. Subaerial morphology and processes of the delta are typical of humid region alluvial fans. The fan margin is, however, completely reworked and modified by wave and tidal action, and marine portions are skewed westward by longstore drift and marine currents. Major environments of the delta include (1) the subaerial deltaic plain, consisting of marsh and swamp organic muds, and braided to estuarine distributary channel fills; (2) the tidal lagoon, composed of tidal sand and mud flat sequences interlaced with a complex of tidal channel fills; and (3) the shoreface, which consists of marginal island, breaker bar, and middle shore face sands, lower shoreface sand and mud, and prodelta/shelf mud. Marine influenced tidal lagoon and shoreface facies are volumetrically dominant components of the system. The Copper River fan-delta is a working depositional model for tectonic clastic wedges prograded into an open marine basin.

Cenozoic evolution of sediment accumulation in deltaic and shore-zone depositional systems, Northern Gulf of Mexico Basin

Abstract Paleogeographic and volumetric lithofacies mapping of 18 Cenozoic genetic sequences within the Northern Gulf of Mexico Basin quantifies the proportional sequestering of sediment within wave-dominated shore-zone vs. deltaic systems through time. Three long-term depositional phases are revealed by plots, based on paleogeographic and sediment isochore maps, of total shore-zone system area and volume to total delta system area and volume (SZ/D). (1) SZ/D area and volume ratios are highly variable in Paleocene through Eocene sequences. However, typical volume ratios for major genetic sequences (Upper, Middle, and Lower Wilcox; Queen City (QC), and Yegua) range between 0.2 and 0.6. Minor sequences (Sparta (SP), Jackson (JS)), which record very low rates and volumes of sediment accumulation, have the greatest variability in their ratios. (2) Oligocene and Miocene sequences display consistently high proportions of shore-zone sediment. SZ/D area ratios range from 0.6 to 1.0, and volume ratios cluster between 0.4 and 0.8. (3) A substantial late Neogene decrease in SZ/D ratios is presaged in the late Miocene sequence. Pliocene and Pleistocene sequences are uniformly characterized by very low ratios of