This report is the result of a working group on continental sequence stratigraphy that was set up at the 1991 NUNA conference in Banff, Canada To date, sequence stratigraphic concepts have been applied mainly to the marine realm, but unconformity-bounded units have long been recognized in nonmarine strata Successful application of sequence stratigraphic concepts to continental strata requires careful consideration of controls on base level and sediment supply As with shallow marine environments, relative sea level can be considered as the stratigraphic as well as the geomorphic base level for coastal nonmarine settings Farther inland, stratigraphic base level, which determines accommodation space, is more complex and takes various forms, such as the graded profile for fluvial strata, groundwater tables for some eolian strata, and lake level for some intermontane sediments Sediment supply is also generally a more complex variable for nonmarine environments than in the marine realm because of the proximity to the source area The influence of climate and tectonism on sediment supply can clearly be seen in many continental sediments The fact that the major controls of climate, tectonism, and eustasy are somewhat interdependent, and that a change in one parameter will most likely be reflected in others, is also more readily apparent in continental strata Although in its infancy, sequence stratigraphic concepts have been applied to a wide variety of continental settings in attempts to explain variations in facies architecture Of particular interest are studies that have linked fluvial architecture on coastal plains to variations in relative sea level as indicated by coeval marine strata, and studies of lacustrine environments that show marked variation in facies between highstand and lowstand deposits The application of sequence stratigraphy to continental strata will likely result in the development of better correlation techniques and models that better predict the location and nature of fluvial and eolian reservoirs

Perspectives on the Sequence Stratigraphy of Continental Strata

The ratios of dibenzothiophene to phenanthrene and pristane to phytane as indicators of depositional environment and lithology of petroleum source rocks

The three-dimensional geometry of fluvial channel bodies and valley fills has received much less attention than their internal structure, despite the fact that many subsurface analyses draw upon the geometry of suitable fluvial analogues. Although channel-body geometry has been widely linked to base-level change and accommodation, few studies have evaluated the influence of local geomorphic controls. To remedy these deficiencies, we review the terminology for describing channel-body geometry, and present a literature dataset that represents more than 1500 bedrock and Quaternary fluvial bodies for which width (W) and thickness (T) are recorded. Twelve types of channel bodies and valley fills are distinguished based on their geomorphic setting, geometry, and internal structure, and log-log plots of W against T are presented for each type. Narrow and broad ribbons (W/T 1000, respectively) are distinguished. The dataset allows an informed selection of analogues for subsurface applications, and spreadsheets and graphs can be downloaded from a data repository. Mobile-channel belts are mainly the deposits of braided and low-sinuosity rivers, which may exceed 1 km in composite thickness and 1300 km in width. Their overwhelming dominance throughout geological time reflects their link to tectonic activity, exhumation events, and high sediment supply. Some deposits that rest on flat-lying bedrock unconformities cover areas > 70,000 km2. In contrast, meandering river bodies in the dataset are < 38 m thick and < 15 km wide, and the organized flow conditions necessary for their development may have been unusual. They do not appear to have built basin-scale deposits. Fixed channels and poorly channelized systems are divided into distributary systems (channels on megafans, deltas, and distal alluvial fans, and in crevasse systems and avulsion deposits), through-going rivers, and channels in eolian settings. Because width/maximum depth of many modern alluvial channels is between 5 and 15, these bodies probably record an initial aspect ratio followed by modest widening prior to filling or avulsion. The narrow form (W/T typically < 15) commonly reflects bank resistance and rapid filling, although some are associated with base-level rise. Exceptionally narrow bodies (W/T locally < 1) may additionally reflect unusually deep incision, compactional thickening, filling by mass-flow deposits, balanced aggradation of natural levees and channels, thawing of frozen substrates, and channel reoccupation. Valley fills rest on older bedrock or represent a brief hiatus within marine and alluvial successions. Many bedrock valley fills have W/T < 20 due to deep incision along tectonic lineaments and stacking along faults. Within marine and alluvial strata, upper Paleozoic valley fills appear larger than Mesozoic examples, possibly reflecting the influence of large glacioeustatic fluctuations in the Paleozoic. Valley fills in sub-glacial and proglacial settings are relatively narrow (W/T as low as 2.5) due to incision from catastrophic meltwater flows. The overlap in dimensions between channel bodies and valley fills, as identified by the original authors, suggests that many braided and meandering channel bodies in the rock record occupy paleovalleys. Modeling has emphasized the importance of avulsion frequency, sedimentation rate, and the ratio of channel belt and floodplain width in determining channel-body connectedness. Although these controls strongly influence mobile channel belts, they are less effective in fixed-channel systems, for which many database examples testify to the influence of local geomorphic factors that include bank strength and channel aggradation. The dataset contains few examples of highly connected suites of fixed-channel bodies, despite their abundance in many formations. Whereas accommodation is paramount for preservation, its influence is mediated through geomorphic factors, thus complicating inferences about base-level controls.

http://www.zenzebra.net/fluvial/gibling.pdf

Width and Thickness of Fluvial Channel Bodies and Valley Fills in the Geological Record: A Literature Compilation and Classification

▪ Abstract Avulsion is the natural process by which flow diverts out of an established river channel into a new permanent course on the adjacent floodplain. Avulsions are primarily features of aggrading floodplains. Their recurrence interval varies widely among the few modern rivers for which such data exist, ranging from as low as 28 years for the Kosi River (India) to up to 1400 years for the Mississippi. Avulsions cause loss of life, property damage, destabilization of shipping and irrigation channels, and even coastal erosion as sediment is temporarily sequestered on the floodplain. They are also the main process that builds alluvial stratigraphy. Their causes remain relatively unknown, but stability analyses of bifurcating channels suggest that thresholds in the relative energy slope and Shields parameter of the bifurcating channel system are key factors.

River avulsions and their deposits

Using modern and ancient examples we show that river-dominated deltas formed in shallow basins have multiple coeval terminal distributary channels at different scales. Sediment dispersion through multiple terminal distributary channels results in an overall lobate shape of the river-dominated delta that is opposite to the digitate Mississippi type, but similar with deltas described as wave-dominated. The examples of deltas that we present show typical coarsening-upward delta-front facies successions but do not contain deep distributary channels, as have been routinely interpreted in many ancient deltas. We show that shallow-water river-dominated delta-front deposits are typically capped by small terminal distributary channels, the cross-sectional area of which represents a small fraction of the main fluvial "trunk" channel. Recognizing terminal distributary channels is critical in interpretation of river-dominated deltas. Terminal distributary channels are the most distal channelized features and can be both subaerial and subaqueous. Their dimensions vary between tens of meters to kilometers in width, with common values of 100–400 m and depths of 1–3 m, and are rarely incised. The orientation of the terminal distributary channels for the same system has a large variation, with values between 123° (Volga Delta) and 248° (Lena Delta). Terminal distributary channels are intimately associated with mouth-bar deposits and are infilled by aggradation and lateral or upstream migration of the mouth bars. Deposits of terminal distributary channels have characteristic sedimentary structures of unidirectional effluent flow but also show evidence of reworking by waves and tides.

Terminal Distributary Channels and Delta Front Architecture of River-Dominated Delta Systems

A geomorphic and stratigraphic investigation of lacustrine delta deposits in the Atchafalaya Basin, Louisiana, revealed that lacustrine delta formation is rapid and cyclic in nature. During the Holocene, thin, coarse-grained, regionally extensive, fluvially dominated lacustrine deltas filled interdistributary basins in the transition zone between the alluvial valley and the marine delta plain of the Mississippi River. One delta, Lake Fausse Pointe delta, prograded 6.5 km, partially filling the lake and covering more than 29 km2 in area in twelve years. Depositional processes in Lake Fausse Pointe ranged from suspension-settling of mud and organic matter during low sediment input to traction deposition of sand during floods. Hyperpycnal flow conditions, set up by the introdu tion of sediment-laden river water into the freshwater lake, induced underflows that scoured the lake bottom and deposited upward-coarsening lobes. The low relief of the Mississippi basin, a constant large volume of fine-grained sediment supply, and prevalent fluvial processes have formed fluvially dominated lacustrine deltas that differ sedimentologically and stratigraphically from "Gilbert-type" and brackish-water lacustrine deltas. Parallel-laminated prodelta mud, rippled to cross-laminated delta-front silty sand, and very finegrained to medium-grained distributary-mouth-bar sand are characteristic of rapidly deposited sediment, whereas rooted and burrowed sediments signify periods of minimal deposition. In a fluvially dominated setting, delta geometry and sediment distribution patterns are controlled by river-mouth processes, basin shape, and bathymetry. Thickest sand accumulations occur in linear, dip-elongate distributary-m uth-bar and natural-levee lobes that are separated laterally by mud-filled channels and interdistributary troughs. Lacustrine-delta deposition is driven by basin subsidence (regional downwarping and sediment compaction) and the concomitant development of distributary channels in the interdistributary basins. Numerous lacustrine-deltaic wedges, each bounded by rooted backswamp clay, are preserved multilaterally in Atchafalaya Basin sediments. Although lacustrine-deltaic deposits constitute much of the basin's sedimentary fill, each individual delta sequence records depositional events of only 100 years duration. These deltaic deposits represent basin-filling (aggradational) episodes that preceded the formation of marine delta complexes such as the Maringouin, Teche, La Fourche, and Atchafalaya deltas of the Mississippi River.

Depositional Processes and Stratigraphy of Fluvially Dominated Lacustrine Deltas: Mississippi Delta Plain

Evolution of Atchafalaya lacustrine deltas, south-central Louisiana

Coastal wetland loss in Louisiana, now considered to amount to more than 100 km2/year, is receiving ever increasing amounts of attention. This loss is the result of a variety of complex interaction...

Wetland Loss in Louisiana

Modern depositional settings provide unambiguous geomorphic data facilitating the quantification of geologic interpretations for the numerical characterization of subsurface strata. Traditional three-dimensional geologic descriptions are limited by one- and two-dimensional data sources: generally well and outcrop data. However, geomorphic analyses of fluviodeltaic systems yield size distributions for discrete sedimentary units. These distributions provide constraints for conditioning the area (X and Y dimensions), shape, placement, and preferred orientation(s) of sedimentary units in reservoir models.Dimensional data for channel belts, channels, channel bars, crevasse splays, distributary channels, and distributary mouth bars from the fluvially dominated deltas of the Alaska North Slope and the Louisiana Gulf Coast reveal log-normal distributions for their lengths and widths. Modern analogs provide statistical constraints for conditioning data input for geologic facies associations in object-based reservoir models.Geomorphic data from modern fluviodeltaic analogs are linked with core and wire-line–log data to render conditioned, three-dimensional geologic models. Model accuracy relies on bed thickness and chronostratigraphic constraints imposed by cores and stratigraphic correlations, as well as the lateral extent of facies associations governed by geomorphology. These object-based reservoir models demonstrate the impact that varying the population of lengths and widths for geologic features has on sand body distribution and interwell continuity.

Geomorphology: An approach to determining subsurface reservoir dimensions

Geochemical characterization of crude oils from Wilcox reservoirs in central Louisiana and southwest Mississippi suggests that they represent a single crude oil family that is distinct when compared to crude oils in deeper Tuscaloosa and Smackover reservoirs. This observation is consistent with geologic constraints that suggest an origin of crude oil from within the Wilcox Group itself. Although shales of the shallow Wilcox Group in central Louisiana and southwest Mississippi contain gas-prone kerogen and are thermally immature, a more oil-prone source facies is present in marine shales of the deep Wilcox Group in southcentral Louisiana. Thermal maturity measurements based on pyrolysis suggest a broad area of effective Wilcox source rock in southcentral Louisiana. Migration distances from source to reservoir rocks of the downdip Wilcox Trend of southcentral Louisiana appear to be relatively short. However, long-range updip migration (sometimes greater than 100 km) from deeply buried Wilcox source facies provides the best explanation for emplacement of crude oil in the shallow Wilcox Trend of central Louisiana and southwest Mississippi.

Robert S. Tye

Papers

Depositional Processes and Stratigraphy of Fluvially Dominated Lacustrine Deltas: Mississippi Delta Plain

Evolution of Atchafalaya lacustrine deltas, south-central Louisiana

Wetland Loss in Louisiana

Geomorphology: An approach to determining subsurface reservoir dimensions

Origin of Crude Oil in the Wilcox Trend of Louisiana and Mississippi: Evidence of Long-Range Migration