Fan-delta Deposits Research Articles

Fan-Delta Deposition in the Cretaceous (Albian) Fortress Mountain Formation, Central North Slope, Alaska: ABSTRACT

The Albian Fortress Mountain Formation at Atigun syncline is interpreted to be a proximal fan-delta depocenter. This interpretation is based on the absence of shale and marine indicators, the presence of indicative sedimentary features, and the discontinuous occurrence of this conglomerate in thick localized accumulations. The Fortress Mountain Formation crops out discontinuously across the Brooks Range foothills belt from west of the Kukpowruk River to the Sagavanirktok River. It is among the first sediments shed north into the Colville trough during early uplift of the Brooks Range orogen. In the central North Slope area, the Fortress Mountain Formation overlies deformed sediments of the pebble shale, Okpikruak Formation, and Konqakut Group. This study examines the Fortress Mountain Formation at Atigun syncline, near the eastern limit of its outcrop belt, where a 1,700-m section was measured and described during the 1989 field season. The Fortress Mountain Formation at Atigun syncline is composed of interbedded conglomerate (20%) and pebbly sandstone (80%). Significantly, shale is almost completely absent. The conglomerate is clast supported and consists dominantly of rounded and spherical chert pebbles and cobbles in a sandy matrix. Conglomerate beds are commonly ungraded and unimodal, with occasional crude imbrication. Most beds have planar basesmore » and tops, but some channeling does occur. While most beds are featureless, some sandstone beds are planar and trough crossbedded. Plant fragments are common on bedding surfaces.« less

Thrust-Generated Fan-Delta Deposition: Little Muddy Creek Conglomerate, SW Wyoming

ABSTRACT The upper Cretaceous Little Muddy Creek conglomerate in southwestern Wyoming is an often-cited example of a thin-skinned, thrust-derived conglomerate that has been used to date absolute and relative timing of thrust sheet motion in the Idaho-Wyoming-Utah fold and thrust belt. Despite the regional tectonic significance of the Little Muddy Creek conglomerate, no quantitative sedimentological or compositional analyses are available in the literature. Sedimentological analysis indicates that the Little Muddy Creek conglomerate was produced by forelandward (eastward) progradation of a marine fan-delta that emanated from the thrust ridge above the old Absaroka thrust. Fan-delta deposits are overlain by nearshore, inner-shelf, outer-shelf and slope deposits which record sedimentation as motion on the old Absaroka thrust began to wane. A two-dimensional provenance modeling technique described herein compares hypothetical, synorogenic-sediment compositions with actual compositional data from the Little Muddy Creek conglomerate. The two-dimensional modeling is a refinement of previous, one-dimensional models in that it is based upon the reconstruction of syn-thrusting hanging wall and footwall geometries. The provenance mod ling shows that compositional data from the Little Muddy Creek conglomerate record uplift followed by gradual cessation thereof on the old Absaroka thrust. Sedimentological and compositional interpretations are integrated, allowing for an evaluation of the tectonic controls on deposition of this thin-skinned, thrust-derived conglomerate.

A Structurally Controlled Fan-Delta Complex at the Southern Margin of the Peninsular Range Forearc Basin Complex (Baja California): ABSTRACT

A confined trunk fan delta and its structurally controlled tributary fan deltas are extremely well exposed along the southern margin of the Rosario embayment. This fan-delta complex consists of nonmarine to deep marine deposits almost continuously exposed for over 20 km in the down-paleoslope direction. Facies and stratal patterns in the fan-delta complex were controlled by local faulting, climatic variation, and eustasy, resulting in relative sea level fluctuations. Basin bathymetry and drainage patterns were controlled by a series of half-grabens that formed along north-south-trending faults that lay along the northern margin of an east-west-trending depression. Breccias were initially shed into the north-south-trending half-grabens; axial drainage systems were later established within the grabens, making up the tributary fan deltas. These tributary fan deltas fed a voluminous trunk fan delta confined to the east-west-trending depression. The trunk and tributary fan delta deposits show two major progradational to retrogradational cycles that record relative sea level fluctuations. Progradation in the shallow-marine environment is represented by conglomerate channels cut into ripple-laminated or bioturbated siltstone and HCS sandstones, overlain by conglomerate mouth bar deposits interstratified with nearshore sandstone deposits. Retrogradation in the shallow marine environment is recorded by either a vertical clastic facies transition or amore » clastic-carbonate facies transition. The clastic facies transition consists of nearshore sandstone deposits overlain by offshore bioturbated siltstones. The clastic-carbonate facies transition consists of the development of red algal patch reefs and rhodoliths on top of fan-delta conglomeratic lobes, mudstone and sandstone bank channel margins, or paleobasement highs. The clastic-carbonate facies transition reflects low sediment supply controlled by climatic conditions.« less

High-Frequency Cyclicity in Quaternary Fan-Delta Deposits of the Andean Fore-Arc: Relative Sea Level Changes and Aseismic Ridge Subduction: ABSTRACT

High-Frequency Cyclicity in Quaternary Fan-Delta Deposits of the Andean Fore-Arc: Relative Sea Level Changes and Aseismic Ridge Subduction: ABSTRACT

The upper Cenozoic White River "tillites" of southern Alaska: Subaerial slope and fan-delta deposits in a strike-slip setting

The White River tillites crop out over 8 km 2 either side of the Totschunda fault in the northern foothills of the Wrangell Mountains in southern Alaska and record glacially influenced slope and fan-delta deposition in a high-relief basin close to an active volcano between 10 and 2.7 Ma. Two measured sections, 1,200 and 450 m thick, show diamictite and conglomerates interbedded with augite andesite lavas. Facies range from massive diamictites, deposited subaerially as debris flows, to crudely stratified sandstones and conglomerates formed by sheet-flow processes on alluvial fans, and well-stratified and normally graded sandstones and conglomerates that accumulated subaqueously as turbidites; large-scale southward-dipping foresets as much as 17 m high identify a fan delta that prograded into a temporary lake. The White River succession is identified as a valley infill complex consisting of subaerial slope and lacustrine fan-delta deposits; there are no in situ primary deposits present in the succession. The component of sedimentation was limited to the supply of coarse-grained debris identified by the presence of glacially striated and faceted clasts. The White River deposits may be typical of glacial sequences preserved as isolated local basin fills in high-relief areas such as occur in convergent or strike-slip plate-margin settings. They provide only a limited and fragmentary record of paleoenvironmental changes compared to coeval glacially influenced marine rocks preserved in the Yakataga Formation (5 km thick) along the Gulf of Alaska coast.

Architectural Elements of Fan-Delta Complex in Pennsylvanian Taos Trough, New Mexico: ABSTRACT

Identification of architectural elements within alluvial-fan and subaqueous fan-delta gravel units is fundamental to resolving depositional processes within fan-delta complexes of the Pennsylvanian Taos trough, New Mexico. Subaqueous fan-delta deposits consist of lenticular gravel-body complexes encased by black, basinal shales. Gravel-body complexes are composed of a series of stacked gravel lenses, each of which is enveloped by fifth-order bounding surfaces. The central portion of individual gravel lenses contains a channel complex. Channels are outlined by third- and fourth-order bounding surfaces and are infilled by high-density gravity flow deposits. The fringe of submarine gravel lenses consists of stacked, laterally continuous Bouma sequences separated by second-order bounding surfaces. Bouma sequences were deposited by dilute turbidity flows during evacuation of submarine channels. Subaqueous channel complexes within gravel lenses represent midfan channels, whereas the fringe of lenticular gravel lenses represent outer-fan lobes. Recognition of depositional processes and architectural elements of fan deltas in the Sandia Formation enables distinction between these and other types of coarse-grained deltas in the Taos trough. This, in turn, has implications for resolving evolution of the trough.

Profiles of fan deltas and water depth in the receiving basin

Summary Profiles of prograding fan deltas are largely controlled by the water depth in the receiving basin. The initial slope of the fan-delta front with respect to that of the fan-delta plain, which is sometimes detectable in stratigraphic packages, decreases with the increase in water depth at the toe of fan-delta front deposits. The resultant relationship provides important evidence and information for restoring the depositional architecture of ancient fan deltas that may reflect the marginal geometry of sedimentary basins. Relatively steep gradients in fan-delta accretion surfaces may lead to an overestimation of the vertical stratigraphic thickness, particularly if it is calculated using a simple cumulative bed thickness. Furthermore, fan-delta successions which appear to be anomalously thick may not be due to progressive or incremental basin subsidence, but rather to overestimated stratigraphic thickness because the fan-delta clinoform dips were ignored. A part of the thick fan-delta deposits, therefore, can best be interpreted in terms of lateral accretion of fan-delta front deposits on the dipping clinoform surfaces, responding to the shallowing in the receiving basin.

Minimal Shell Transport Detected in a Coarse-Grained Pleistocene Marine Terrace, Sonora, Mexico

Taphonomic features and the stratigraphic association of benthic fauna with their preferred substrate indicate minimal between-habitat transport of shells in a coarse-grained shallow-marine Pleistocene fan delta deposit. Well-preserved invertebrate shells occur in dense concentrations and as dispersed specimens in life position in a range of supratidal to subtidalfacies. The association ofp rimarily epifaunal hard-substrate species with gravel sediments and of infaunal molluscs with sand substrates is very strong, based on relative abundance data from 22 samples (R = 0.88). Hard-substrate fauna comprise 0.4-10% of the total fauna in fine-grained sediments, and 80-90% of the total fauna in clastsupported cobble beds. The strong association between fauna and substrate suggests that transportation among adjacent sandy and cobble habitats was minimal; therefore, fossils in this coarse-grained terrace were deposited under low-energy marine conditions. Such low-energy conditions are also supported by the following taphonomic and sedimentary features: a high percentage of bivalves in the terrace are articulated (up to 55%o), most shells are unworn and unabraded, highly convex disarticulated bivalve shells have a predominant concave-up orientation (50%-76%b), and the fine-grained shell beds are massive, lacking physical sedimentary structures indicative of highenergy conditions. The occurrence of coarse-grained sediments in low-energy environments may often be misinterpreted unless faunal and taphonomic features are also examined.

The Paleoenvironmental Significance of the Nearshore Curvolithus Ichnofacies

Newly discovered Curvolithus-rich trace-fossil assemblages in fan-delta deposits of the Pennsylvanian Minturn Formation of Colorado are remarkably similar to several other Carboniferous deltaic ichnofaunas, as well as those from Jurassic fan-delta environments and other marginal-marine deposits with freshwater influence. The Minturn trace fossils comprise the type assemblage of the Curvolithus ichnofacies, dominated by infaunal locomotion-feeding activity in environments characterized by relatively rapid deposition of sandy sediment exceeding physical reworking. The ichnofacies also occurs in sandy, low-energy, open-shelf environments. Thus, the ichnofacies is a paleo-energy and paleo-depositional rate indicator, and we predict that its occurrence in strongly localized deltaic and nearshore sandy facies recurs throughout the Phanerozoic.

The Bani Ghayy group; sedimentation and volcanism in pull-apart grabens of the Najd strike-slip orogen, Saudi Arabian shield

Abstract The cratonisation of the Arabian Shield involved the accretion of ensimatic arc terranes in the west and south followed by continental collisions along the Nabitah and Al Amar sutures and the onset of the Najd strike-slip orogen. Volcanosedimentary successions which post-date these collisions have been proposed as the products of magmatism in transitional volcanic arcs. One such succession, the Bani Ghayy group, lies unconformably above a series of volcanosedimentary rocks associated with an older active continental margin and was deposited in three separate, elongate, fault-bounded grabens. Each graben contains coarse fanglomerate and fan delta deposits along the margin and proximal greywackes toward the centre. Reef limestones occur locally whereas volcanogenic rocks are widespread. The latter are geochemically bimodal and with the sediments are typical of continental rifting environments although deformed ultramafic rocks within internal reverse faults suggests that the grabens were floored, in part, by primitive mantle derived crust. Deposition of the Bani Ghayy group post-dates incipient motions along fractures of the Najd fault system. The orientation of the Bani Ghayy grabens and the geometry of the boundary faults are consistent with development in a regime of northwest-trending dextral shear. However, the Bani Ghayy grabens are displaced and deformed by later movements of the same faults but in a sinistral sense. Other successions correlated with the Bani Ghayy also preserve features consistent with deposition in continental grabens and the possibility that they represent arc volcanism is discounted.

The Proterozoic Skoadduvarri Sandstone Formation, Alta, Northern Norway: A tectonic fan-delta complex

The Skoadduvarri Sandstone Formation, at least 1700 m thick, consists of eight commonly recurring lithofacies: mudclast conglomerate, pebbly sandstone with conglomerate, shale, and five sandstone lithofacies. The formation is divided informally into three distinct members, interpreted as: (1) prograding fluvio-marine and swash platform deposits, overlain by (2) prograding, shoal-water, sandy delta deposits, topped by (3) short-headed stream or fan-delta deposits. Widespread and pervasive soft-sediment deformation has affected the sandstones. The formation is considered to have been deposited in a tectonically subsiding basin by northward progradation of fluviomarine and alluvial-fan valley fill deposits. This conclusionsstrengthens a paleorift hypothesis for the tectonic setting of the Skoadduvarri Sandstone Formation and its associated volcanic and sedimentary rock units.

Pennsylvanian Fan-Delta Deposition Resulting from Tectonic Uplift Along Southwestern Margin of Anadarko Basin: ABSTRACT

Pennsylvanian sedimentation on the southwestern margin of the Anadarko basin was dominated by a clastic wedge of conglomeratic alluvial fan-delta deposits, referred to locally as washes. Chert, carbonate, and granite (arkosic) washes were shed basinward in response to thrusting and uplift of the Ancestral Wichita Mountains. Chert and carbonate washes were deposited in the Early Pennsylvanian, as lower Paleozoic limestones and dolomites eroded from the Wichitas. Arkosic washes predominated during the Middle and Late Pennsylvanian, as drainage basins composed primarily of Precambrian-Cambrian crystalline rocks eroded. The detailed stratigraphy available from well control in the Anadarko basin permits relatively precise timing of orogenic events along the Ancestral Wichitas. Wash deposition in the basin began in the Morrowan and continued episodically in seven major pulses through the Missourian. These wash sequences range from 200 to 1500 ft thick; each terminates with an unconformity. Wash sequences are further subdivided into as many as nine conformable subsequences, 100-400 ft thick and correlative for 10-20 mi, which appear to record individual drainage-basin erosional cycles. Subsequences generally coarsen upward abruptly, then gradually fine upward to the next subsequence. Basinal marine shales interfinger with the wash sequences and are a possible source of structurally and stratigraphically entrappedmore » gas and condensate. Optimal reservoir quality occurs in braided channels and fan-delta fronts. Reserves average 2-5 bcf and 50,000-200,000 bbl/well, making granite wash an economically attractive target at present Anadarko drilling costs.« less

Depositional Trends In Upper Paleozoic Terrigenous Clastic Rocks, Sacramento Mountains, New Mexico

Field analysis of terrigenous elastic units lying within the upper Pennsylvanian-lower Permian cyclic elastic/ carbonate sequence (Holder and Labore/ta Formations) of the northern Sacramento Mountains, New Mexico, illustrates a marine to non-marine environmental progression as infilling of the Orogrande basin occurred. The oldest units studied, flat-based sandstone bodies A, B and C (Virgilian), coarsen upward texturally and dis­ play large-scale (up to 5m thick) foresets, hummocky stratification, bimodal or polymodal cross-stratification trends, and abundant burrows. These characteristics indicate that siliciclastic shelf bars were present on a storm­ dominated and tide-influenced shelf. Higher in the section, units D (upper Virgilian) and E (lower Wolfcampian) exhibit fluvially influenced depositional features such as concave-up erosional bases, upward-fining textural sequences and unimodal or asymmetric, bimodal paleocurrent distributions. Units D and E are interpreted as estuarine and fan deltaic deposits, respectively Several upward-coarsening fan-deltaic sequences were recognized in the study area suggesting that delta shifting , rather than sea level fluctuations, was a significant local cause of cyclicity. The sequence of units A-E may represent the relatively continuous progradation of terrigenous elastic sediments from the Pedernal uplift into the Orogrande basin during late Paleozoic time.

Fan-Deltaic and Marine Siliclastic Facies of Laborcita Formation, Northern Sacramento Mountains, New Mexico: ABSTRACT

Fan-deltaic sequences in the Laborcita Formation, northern Sacramento Mountains, New Mexico, form a clastic wedge that prograded north and west. These Penn-Permian (Virgilian-Wolfcampian) deposits represent a facies tract of proximal to distal alluvial fan and fan-deltaic facies. Associated nearshore and shelf siliclastics are also developed. Red bed conglomerates interpreted as braided and ephemeral fluvial channel fills are the most proximal deposits. These limestone cobble conglomerates crop out near the Fresnal Canyon fault and are syntectonic deposits. Northern quartzite cobble conglomerates are less proximal and form more continuous sheet-like units. These thick (10 m, 33 ft) and laterally extensive (6 km, 4 mi) conglomerates that contain trough cross-bedded coarse sand lenses associated with intrachannel bars are interpreted as midfan deposits. Shales and thin (20-40 cm, 8-16 in.) progradational distal sands under conglomerates are highly variable in total thickness (20-60 m, 65-200 ft). The thin sandstones show evidence of marine processes and are interpreted as fan-deltaic foreset deposits. Gray limestone and chert conglomerates deposited directly on limestones are thin (0.5-2 m, 1.5-6.5 ft), deposited subaqueously, and exhibit reworking by marine processes. Northern sands are disassociated with conglomerates. These green arkosic sands are relatively thick (3 m, 10 ft) and contain landward (east) accreting tangential foresets. Geometry, stratigraphic position, and minor upward coarsening indicate shelf-bar deposition. Fan-delta source areas shifting during Laborcita deposition. Initial deposits were from the southeast and characterized by limestone/chert conglomerates. Later quartzite/rhyolite porphyry cobble conglomerates prograded from the east and northeast. Ultimately, alluvial fan red beds of the Abo Formation prograded over Laborcita fan-delta and shelf-bar deposits. End_of_Article - Last_Page 512------------

Sedimentation and tectonism in the Middle Ordovician of the Girvan district, SW Scotland

ABSTRACTThe Kirkland Conglomerate and the Benan Conglomerate and associated sediments of the Barr Group (Ordovician—Llanvirn-Llandeilo) of the Girvan district, SW Scotland record the development of two fan-delta systems situated on the northern margin of Iapetus. The intervening Stinchar Limestone represents a shallow marine fan-delta abandonment facies. Subaerial fan-delta deposits are seen at the lowest exposed horizons within the Kirkland Conglomerate. Transgression and eventual abandonment of the fan-delta system is recorded by (1) matrix-rich gravels forming the topmost horizons of the unit. (2) subaqueous distributary channel sands of the overlying transitional sandstone and associated shallow-marine carbonates of the Auchensoul Limestone and (3) shallow-marine sandstones of the Confinis Flags. Following a period of shallow-water sedimentation (Stinchar Limestone), a phase of rapid subsidence occurred during the upper Llandeilo, broadly synchronous with the Nemagraptus gracilis Zone transgression. Lowermost horizons of the succeeding Benan Conglomerate comprise re-sedimented gravels and laterally equivalent ‘outer shelf’ deposits (Superstes Mudstone). Renewal of coarse clastic sedimentation resulted from source area uplift related to granite plutonism, responsible also for the rapid subsidence of basinal areas. Fan-delta progradation is recorded by the occurrence of braided-fluvial deposits and shallow-marine carbonates at higher stratigraphical levels within the unit. Progradation of the fan-delta complex resulted from a gradual reduction in subsidence rates along basin-margin faults.Fan-deltas of the Barr Group prograded southwards, from a SW—NE-trending faultdelineated basin margin, across a narrow shelf area. To the S of the shelf area defined by Barr Group outcrop, sediments of the Tappins complex accumulated in outer shelf, ?slope and base of slope settings.The thickness of individual conglomerate units (>150 m—Kirkland Conglomerate, and up to 700 m—Benan Conglomerate), the associated high sedimentation rates, narrowness of the shelf area, and distribution and style of basin-margin faults indicate that oblique-slip motion along the northern margin of Iapetus may have provided a major control over Middle Ordovician sedimentation in the Girvan district.

Internal structure and environmental reconstruction of eocene transitional fan-delta deposits, monllobat-castigaleu formations, Southern Pyrenees, Spain

A detailed study has been made of a part (500 × 200 × 15 m) of the Eocene Monllobat Formation. The conglomeratic bottomset—foreset—topset build up of a small fan-delta passes into sand- and mudstone layers. Ultimately the sandstone layers wedge into the mudstone due to concave-upwards basal surfaces and convex top surfaces or due to thickening of intercalated mud layers. Sequences vary accordingly from coarsening upwards in the proximal parts to fining upwards distally. Both sequences are topped by a thin layer fining upwards. The proximal fan-deltaic part consists of interconnected lobes. Crescentic, transverse bars deposited gravel on a slightly inclined top. Conglomerates of the moderately inclined foreset show some imbrication. The bottomset consists of tabular cross-bedded and cross-laminated sandstone. Parallel-laminated and tabular cross-bedded sandstone layers drape rises of fine sediment in front of the fan-delta face. Continued progradation brought about truncation of these backset sediments. Sandstone lobes of the distal part are not connected. Tabular croos-bedding and parallel lamination are the main sedimentary structures. Climbing, large-scale bedforms are prominent on top surfaces. Most of the lobes are enveloped by blue-grey mud. Layers covered by mottled mud terminate in foreset faces. Highly energetic inflow took place from a shallow, braided stream with longitudinal bars. The low sedimentary relief caused jetflow. The high flow energy, the large grain-size range and the large quantities of sediment caused great differences from inflow models for the deltaic environment. Sedimentation was effected by flow deceleration. Expansion, inertial forces, friction or a combination of these phenomena were responsible for the deceleration. Expansion and inertial forces caused development of moderately inclined foreset faces. Due to expansion, steeply inclined foresets were formed. Sand was deposited in shallow marine water and in scour pits in continental deposits during flow deceleration caused by frictional and inertial forces. Parallel-laminated backsets connected with flat, tabular sets developed due to flow deceleration by expansion and frictional forces. The deposits were formed in the transition zone of a fan-delta under a tropical climate with marked dry and wet periods. Wide, braided streams entered a restricted, shallow marine basin at right angles to the basin axis. Aggradation exceeded water depth. Progradation processes were influenced by large- and small-scale tectonic activity.

Tectonic Control of Pennsylvanian Fan Delta Deposition, Southwestern Colorado: ABSTRACT

Cyclical deposits within the Lower Member of the Honaker Trail Formation (Desmoinesian), between Durango and Silverton, Colorado, have been studied in detail and indicate tectonically controlled sedimentation along the western flank of the Uncompahgre uplift. These cycles were previously considered the result of eustatic sea level fluctuation. Fan delta deposits form a thick wedge of coarse clastics and are interbedded with thin carbonates and siliciclastic shelf-bar systems. Significant lateral variation in depositional style and stratigraphic succession occurs along the strike of this faulted basin margin. Two major types of cyclic sequences are recognized. Clastics found in the northern and southern portion of the study area near Molas Lake and Engineer Mountain are dominantly thin (10 to 15 m, 33 to 50 ft), sheet-like, rapidly shifting fan delta complexes. Three subfacies within these fan deltas can be distinguished. (1) The bottomset beds, (2.5 to 8 m, 8 to 26 ft, thick) are parallel-laminated and rippled, moderately sorted, micaceous, fine (0.125 mm) to medium-fine (0.25 mm) sandstones. Large plant fragments, as well as macerated plant debris and bioturbation are common. (2) Forsets (2.5 to 4.5 m, 8 to 15 ft thick) are characterized by small to medium-scale trough cross strata and abundant soft sediment deformation in poorly sorted, arkosic, medium (0.25 mm) to coarse (0.71 mm) sand tones. (3) Topset beds (1.5 to 2.5 m, 5 to 8 ft thick) form a capping unit of very poorly sorted, arkosic, very coarse sandstones (2 mm) to conglomerates (4 mm +). Climbing units of topset beds are characterized by medium scale trough cross strata with occasional ripple stratification. In association with these fan delta units are thin carbonates (0.5 to 2 m, 1.6 to 6 ft of wackestones/packstones) and nonmigrating shelf sands (0.25 to 2 m, 10 in. to 6 ft) with subparallel laminations and ripple stratification. The carbonates apparently do not cap the deltaic sequences but are more closely associated with the shelf sands. These fan deltas are fluvially-dominated with little or no evidence of reworking by marine processes. The central region of the study area near Coal Bank Pass is characterized by marine-influenced high energy fan deltas (9 to 16 m, 30 to 52 ft) as evidenced by the abundance of hummocky cross strata at the base of the fan delta sequences and flanking shelf-bar systems. The topsets and forsets are similar in scale and in sequences of sedimentary structures and textures to those previously discussed. The bottomsets, however, are finer (0.17 mm), better sorted, and not as micaceous as those in lower energy areas. Closely associated with these delta complexes are flat-bottomed, lense-shaped shelf sand-bars. These coarsening-upward, laterally migrating bars are the cleanest, most well-sorted, and finest (0.06 to 0.125 mm) sandstones. Oscillation ripple stratification subparallel laminations with both broad synforms and antiforms are the dominant sedimentary structures indicative of the marine reworking of fan delta sediments. Variation in stratigraphic successions in adjacent areas and the areal distribution of fan delta types suggests major strike-slip motion along the bounding faults responsible for the development of upthrown and downthrown wedge-shaped blocks within the border fault zone. End_of_Article - Last_Page 514------------

Fan-Delta Deposition in Lower Cotton Valley Group Sandstones of Northeast Texas: ABSTRACT

Fan deltas have been defined as progradation of alluvial fans into a standing body of water from a proximal highland area. Sedimentary environments associated with fan delta complexes have been described in detail in Holocene examples. The subaerial fan is composed of braided channels, gravelly beaches, flood plains, and marshes. The subaqueous fan includes tidal lagoons, channel-fill complexes, marginal islands, breaker bars, and is also characterized by steep slopes and submarine channels where mass-gravity processes may dominate. Few fan delta complexes have been recognized in the subsurface. The Cotton Valley Taylor B Sandstone is interpreted as the distal part of a prograding fan delta based on the vertical sequence in cores from Kildare field, Cass County, Texas. Three End_Page 423------------------------------ major sandstone facies are distinguished on the basis of sedimentary structures, composition and texture. They are, in descending order, (1) a massive to laminated, well-sorted sandstone, (2) a laminated, pebbly sandstone, and (3) a very fine-grained sandstone interbedded with black shale. Facies 1 is fine-grained (0.21 mm), lacks bedsets, and is mostly massive or indistinctly laminated. Monocrystalline quartz content is high, reaching 95%. Other minerals are minor at 4%, and rock and shell fragments range from 5 to 15%. Facies 1 is only 10 ft (3.1 m) thick and is interpreted as bar or beach deposits formed by reworking of sand by waves at the margins of the delta plain. Facies 2 is composed of conglomeratic bedsets which fine upward from a pebbly erosive base to a parallel or cross-laminated top. Bedsets range from 0.5 to 3 ft (15 cm to 1 m) thick. There is an increase in rock and shell fragments and a decline in monocrystalline quartz from Facies 1, averaging 34% and 63% respectively. The facies remains fine-grained (0.19 mm); however, average grain size and bed thick ess increase upward. Facies 2 is interpreted as a complex of shallow, braided, distributary channels. Thickness is variable but ranges from 10 to 20 ft (3.1 to 6.2 m). Facies 3 bedsets are graded and the sequence of sedimentary structures reflects deposition under conditions of decreasing flow regime. The thin, 1 ft (0.3 m), graded sandstone beds are separated by black laminated shale. This facies is very fine-grained (0.09 mm), relatively poor in quartz, 70%, and rich in matrix which may exceed 30%. Facies 3 is interpreted as thin-bedded turbidites deposited in channels and channel margins downdip from the break in slope at the edge of the delta platform. The thin AE, ABE, BE and CDE bedsets alternate with intervals, 1 to 3 ft (0.3 to 1 m) thick, of bioturbated siltstone or rippled sand tone indicative of normal, shallow marine biotic and current activity. There is a sharp, erosive relationship between Facies 2 and 3. Resistivities are variable and SPs depressed due to extensive carbonate cement (up to 50% total rock) which is also responsible for low porosity and permeability. Dissolution of authigenic carbonates results in secondary porosity up to 17%. Cross-plots of resistivity (Rt) versus porosity are useful in characterizing facies and enable identification of facies in non-cored wells. End_of_Article - Last_Page 424------------

Minturn and Sangre de Cristo Formations of Southern Colorado--A Prograding Fan-Delta to Alluvial-Fan Sequence Shed from Ancestral Rocky Mountains: ABSTRACT

The Middle Pennsylvanian Minturn Formation and the Pennsylvanian and Permian Sangre de Cristo Formation of the northern Sangre de Cristo Range form a 4,000-m (13,000-ft) thick, progradational sequence of fan-delta and alluvial-fan deposits. This sequence was deposited along the western margin of the central Colorado trough during faulting and uplift of the late Paleozoic Uncompahgre highland of the Ancestral Rocky Mountains. The Minturn Formation is composed mostly of sandstone and shale deposited by fan deltas that prograded into the central Colorado trough. The Minturn of the northern Sangre de Cristo Range is divisible into (1) a turbidite-bearing facies, (2) a limestone-bearing facies, and (3) a red-bed facies. The turbidite-bearing facies is interpreted as deposits of fan deltas that prograded onto the sea bottom below wave base. The limestone-bearing facies is interpreted as deposits of fan deltas that prograded onto a shallow sea bottom above wave base. Turbidites and shallow-marine limestones, although they make up only a minor part of the Minturn Formation, are mutually exclusive deposits that serve to distinguish the two facies. In the limestone-bearing facies, sandstones containing deltaic fore ets overlie thin shallow-marine limestones and are considered diagnostic of that facies. Both facies contain thick intervals of sandstone and shale interpreted as deltaic and alluvial deposits. Where it onlaps the Uncompahgre highland, the lower part of the Minturn Formation contains quartzose and arkosic red beds of probable alluvial origin. The continental Sangre de Cristo Formation conformably overlies the Minturn Formation basinward, but it unconformably overlies Minturn Formation and Precambrian basement near the Uncompahgre highland. The Sangre de Cristo Formation contains (1) a sandstone facies deposited on the distal surfaces of alluvial fans, and (2) a conglomerate facies (Crestone Conglomerate Member) deposited on the proximal surfaces of alluvial fans. The sandstone facies consists of fining-upward cycles of red conglomeratic sandstone and siltstone interpreted as braided-stream deposits. The conglomerate facies consists of poorly sorted conglomerates interpreted as debris-flow and mudflow deposits, and sorted conglomerate and sandstone interpreted as streamflow and sheetflow deposits. End_of_Article - Last_Page 854------------

Pennsylvanian Fan-Delta and Carbonate Deposition, Mobeetie Field, Texas Panhandle

Fan deltas prograded repeatedly onto a shallow, carbonate-shelf environment in the Anadarko basin during Late Pennsylvanian (Missourian) time. These fan-delta sandstones were part of a thick wedge of coarse clastics which were derived from the Amarillo uplift. Similar fault-bounded basement uplifts found throughout the west-central United States at this time formed an intracratonic tectonic setting in which alluvial-fan and fan-delta sedimentation was common. Missourian reservoirs at Mobeetie field consist of three alternating limestone and sandstone units. Limestones accumulated along strike in elongate, phylloid-algal mounds that separated the open shelf from a more restricted lagoonal environment. Algal-mound cores consist of algae-foraminifer wackestone and packstone, and mounds are capped by coral boundstone and encrusting algae-foraminifer wackestone. Progradation of coarse-grained terrigenous clastics onto the carbonate shelf halted carbonate production. Braided streams breached low areas along the algal-mound trend and deposited fan-delta sediments on the shelf basinward of the mounds. Distal-fan-delta deposits in the two younger sandstones were reworked along strike into spits and bars. Carbonate deposition recommenced following w ning of clastic influx and subsidence of the fan-delta sediments. Algal mounds were reestablished preferentially on shallow platforms formed by terrigenous clastics of the previous cycle. The principal trapping mechanism in Mobeetie field is structural draping of Pennsylvanian beds over an underlying horst block, forming a doubly plunging anticline. However, within the area of closure, stratigraphy controls reservoir quality and production. Updip fluvial sandstones and undisturbed distal-fan-delta deposits have good porosity and permeability, and produce oil. Reworked fan-delta deposits are not productive, even within the area of closure, because of the loss of porosity by calcite cementation.