Thomas F. Moslow

Origin, evolution, and distribution of shoreface sand ridges, Atlantic inner shelf, U.S.A.

Abstract A computer mapping system was employed to document the location of 259 shoreface-attached and detached sand ridges in water Although shoreface-attached sand ridges appear to have several different modes of formation, a two-step process for the development of most shoreface-attached and detached sand ridges along U.S. Atlantic barrier island and cape coastlines is proposed: (1) sand is deposited as ebb-tidal deltas or river deltas along the lower shoreface and/or inner continental shelf prior to or during transgression, followed by (2) further transgression, which reworks the deltaic sand bodies into linear sand ridges at the base of the shoreface by shelf processes. The best-developed shoreface sand ridge fields along the U.S. Atlantic shelf lie adjacent to shorelines characterized by all of the following: (1) transgression, (2) mixed energy, wave-dominated barrier islands, and (3) laterally migrating tidal inlet systems. Tidal inlet systems are natural sediment sinks that capture sand carried by longshore sediment transport. Ebb-tidal delta deposits associated with these migrating tidal inlets provide the initial sand body for the development of shoreface-attached sand ridges. The oblique orientation and linear form of shoreface-attached sand ridges appear to be a function of shoreline transgression, lateral inlet migration, and wave reworking of ebb-tidal delta deposits concentrated along an ebb-tidal delta retreat path. Shelf processes act as modifying agents in the evolution of sand ridges during and after ebb-tidal delta deposition. In general, shoreface-attached sand ridges are poorly developed or absent along eroding headlands, spits, and barrier island shorelines characterized by naturally stabilized tidal inlets. These latter inlets and shoreline types are dominated or influenced by one or all of the following: (1) antecedent topography, (2) higher tidal ranges, (3) larger tidal prisms, (4) lower wave energies, or (5) finer grain sizes. The coupling of shoreline and shallow marine sedimentary processes during a transgression is critical to the origin, evolution, and distribution of shoreface sand ridges in the study area. Modern shoreface-attached sand ridges are also known to occur in different coastal and shelf settings where large amounts of sediment were supplied to the shoreface and inner shelf during, or immediately before, transgression. Commonly observed vertical and lateral interrelationships of Holocene shoreface-attached sand ridges and tidal inlets or distributary channels have important ramifications for the development of shelf sandstone facies models. In addition, the geometric relationship documented in this study provides predictive petroleum and hard mineral exploration models of the spatial and temporal distribution of shoreface and shelf sand ridges.

The Outer Banks of North Carolina

The 320 km long Outer Banks of North Carolina comprise two cuspate forelands with morphologically and stratigraphically contrasting barrier limbs in a microtidal (0.9 m tidal range) high wave energy (1.7m average wave height) hydrographic setting. Holocene sedimentation and coastal evolution have been a function of five major depositional processes: (1) eustatic sea-level rise and barrier-shoreline transgression; (2) lateral tidal inlet migration and reworking of barrier island deposits; (3) shoreface sedimentation and local barrier progradation; (4) storm washover deposition with infilling of shallow lagoons; and (5) flood-tidal delta sedimentation in backbarrier environments.

Characterization of minerals in oil sands by reflectance spectroscopy

Abstract Diffuse-reflectance spectra (0.3–2.6 μm) of oil sands and mineral separates display a number of absorption bands which can be assigned to the spectrally significant minerals present in the samples. Minerals with strongly featured spectra such as kaolinite, illite and siderite can be identified even when they are minor constituents. Absorption bands in the 1.4 and 1.9 μm regions are most useful for identifying the presence and types of clay minerals, whereas the 1.0–1.2 μm region is most useful for identifying the presence of siderite.

Sediment Transport in a Submarine Channel System: Fraser River Delta, Canada

ABSTRACT The Sand Heads Seavalley is a submarine channel system seaward of the mouth of Fraser River, Canada. Soundings and sonographs reveal that the Seavalley is composed of tributary channels that coalesce downslope into a single sinuous channel that splits into distributary channels at the base of the slope. The channels are inferred to result from turbidity currents generated by periodic liquefaction of sandy sediment at the mouth of the river. Bathymetric surveys of a river mouth slope failure in 1985 are used to estimate sediment transport rates for a turbidity current generated by the failure. These rates are compared to predictions using a turbidity current model based on relationships between settling velocities of surface sediments and shear velocities required to transport the sediment as suspended and wash loads. Model predictions that assume suspended load transport are much lower than estimates from the landslide, but predictions based on wash load transport show reasonable agreement. The better agreement with the wash load model may be because grain sizes in the deposits are controlled by the sizes of sediment supplied to the distributary mouth bar by the river, rather than by the competency of the turbidity curre t itself.

Origin and Distribution of Holocene Temperate Carbonates, Hecate Strait, Western Canada Continental Shelf

ABSTRACT The surficial sediments in an area of 4000 km2 in Hecate Strait, western Canada continental shelf, are composed predominantly of CaCO3. The deposit is Holocene and forms a discontinuous unit consisting of coarse shell debris, typically < 1 m thick, overlying Tertiary bedrock. Carbonate sediments are largely restricted to water depths of < 50 m and are closely associated with rock and gravel substrates. The faunas yielding these sediments are predominantly bivalves, barnacles, and bryozoans. Radiocarbon dates on surficial material range from modern to 1500 yr B.P. Vigorous water circulation through tidal and wave action, combined with rocky or coarse-grained substrates in some areas, favors suspension-feedin biogenic carbonate producers. The presence of carbonates in Hecate Strait results from the glacial and postglacial history of the region. Glaciation and subsequent subaerial erosion produced abundant rock and gravel substrates; subsequent transgression starved the region of terrigenous clastic sediments. Carbonate sedimentation on high-latitude continental shelves is restricted to areas where conditions favorable to production of biogenic carbonate coexist with low supply of terrigenous sediment. High-amplitude fluctuations of sea level, such as those produced during glaciations, may favor development of temperate carbonates during transgressive and highstand intervals.

Exploration potential of the Falher G shoreface conglomerate trend: evidence from outcrop

ABSTRACT The lower Cretaceous Falher Member shoreface conglomerate trends are the most prolific natural gas reservoirs of the Alberta and British Columbia Deep Basin. Individual pools can be in excess of 100BCF with discovery wells yielding AOFs of more than 100 MMCFD. Since the 1980s, five conglomerate shoreface trends (Falhers A, B, C, D and F) have been successfully drilled and mapped in the subsurface through observations of cores, cuttings, and well logs. Subsequent to their initial discovery and drilling in the subsurface each trend has been correlated to an outcrop equivalent in the Rocky Mountain Front Ranges of northeastern British Columbia. Until recently, no other Falher conglomerate shorelines have been recognized. South (paleolandward) of the Falher A-F trends, an older shoreface conglomerate fairway, designated the Falher G, has been discovered through outcrop observations at Holtslander Ridge near Belcourt Creek, British Columbia, 110 km southeast of the southernmost described Falher shoreface conglomerate outcrop. This trend, which is oriented oblique to younger trends, can be extrapolated into the subsurface plains at the Narraway Field, Alberta, approximately 35 km south of the nearest (Falher F) shoreline conglomerate reservoir fairway. Detailed analyses of measured sections, gamma log profiles, and photo mosaics show that the Falher G is a prograding shoreface facies association of clast-supported and sand matrix chert pebble conglomerate 8 to 12 m thick. The outcrop is approximately 5.0 km in length, along a north-south-trending depositional dip exposure that grades basinward from clast B supported conglomerate to pebbly sandstone. The Falher G shoreface outcrop contains several internal ravinement surface that dip at 5-10° to the NNW (320°). Analysis of locally available coal exploration core limits the paleoshoreline width of the Falher G conglomeratic shoreface trend to less than 5.0 km, with a paleoshoreline orientation of 285°/105°. Extrapolation of this trend into the subsurface may yield a new and significant shoreface conglomerate reservoir. 1 Also at: Department of Geology and Geophysics. University of Calgary, 2500 University Drive NW, Calgary, AB T2N IN4 End_Page 23-------------------------

Depositional origin and facies variability of a Middle Triassic barrier island complex, Peejay Field, northeastern British Columbia

Middle Triassic strata of the Peejay field in northeastern British Columbia are composed of four regressive shorefaces, the youngest having been reworked by tidal inlets. Tidal-inlet sublitharenites and bioclastic grainstones form the best reservoir facies. These deposits form a series of shoreline-parallel, narrow, sharp-based, linear sand bodies that eroded the paleoshoreface. The orientation, geometry, and internal sedimentology of these tidal-inlet facies suggest that the paleoshoreline was subjected to a wave-dominated paleohydrographic regime. Determining the processes responsible for forming a specific tidal inlet can provide information regarding waves, tides, and storms characteristic of the depositional setting. Knowledge of this paleohydrographic regime can aid in predicting the orientation and internal characteristics of tidal-inlet reservoir facies. Improved predictability of reservoir facies geometry and quality can have direct implications on hydrocarbon exploration and development strategies of these and similarly formed hydrocarbon plays of the Triassic in the Western Canada sedimentary basin and elsewhere.

Tidal Inlet Reservoirs: Insights from Modern Examples

The coasts of North Carolina, South Carolina, and Louisiana are excellent examples of the range of sand-body types deposited along a terrigenous-clastic barrier island shoreline (Fig. 4.1). Submergence during the Holocene and the presence of reworked Holocene and Pleistocene sediment sources resulted in the formation of barrier-island, tidal-inlet, flood- and ebb-tidal delta, estuarine, and complex backbarrier environments. Hayes (1975), Nummedal et al. (1977), and Davis and Hayes (1984) determined that the geomorphic variability of barrier islands and tidal inlets along the southeast U.S. coast is controlled by regional changes in wave regime, tidal range, and tidal prism. In addition, Nummedal et al. (1977) and Hubbard et al. (1979) noted the geomorphic differences among wave-dominated, transitional, and tide-dominated tidal inlets.

Lithostratigraphy and Production Characteristics of the Wilcox Group (Paleocene-Eocene) in Central Louisiana (1)

Multiple, discontinuous sandstones of the Wilcox Group in central Louisiana were deposited in fluvial, deltaic, and marine sedimentary environments in the Tertiary Gulf of Mexico basin. Prolific hydrocarbon reservoirs developed in association with positive structural features or where sedimentary characteristics were favorable. Regional stratigraphic correlations are difficult because the Wilcox Group contains numerous complex depocenters. To delineate the occurrence of hydrocarbon-bearing sediments and to promote further exploration, a five-fold lithostratigraphic framework (units I-V, in descending order) is proposed for the Wilcox Group on the basis of its sedimentary character as interpreted from well logs. Lithostratigraphic unit thickness and sandstone content were mapped within 22 parishes. All units produce hydrocarbons, although production is geographically variable. Production in the updip Wilcox is the greatest in unit III, whereas units I and II are most productive in the downdip Deep Wilcox shelf-margin trend. Units V and IV represent the initial deposition of sand-rich sediment into the Gulf of Mexico following the Midway transgression. Transgression marked by unconformities and the Cane River Marl above unit I terminated Wilcox deposition. Wilcox sediment sources were located to the northeast and northwest. All units display a strong north-south isopach grain in the northern two-thirds of the study area; however, east-west-oriented sandstone packages are present along the paleoshelf margin. Alluvial valley (progradational and aggradational) and valley interfluve (retrogradational-coastal onlap) deposition can be discerned from well logs.

Patterns and variability in sediment accumulation rates, Fraser river delta foreslope, British Columbia, Canada

Minimum sediment accumulation rates on the Fraser River delta foreslope exhibit a high degree of spatial variation, with accumulation rates ranging from 0.50 to 3.0 cm/yr. Accumulation rates generally increase towards Sand Heads channel, the active foreslope depocenter. Sedimentation rates and patterns and micropaleontological assemblages are interpreted to reflect reintroduction of older sediment from upslope via slumping and sedimentgravity flow processes. Such processes account for the bulk of sedimentation in much of the subaqueous delta. These processes provide a mechanism for sand bypassing of the delta plain and foreslope and for delivery of coarse-grained sediment directly to prodelta and basinal environments.