Braided Fluvial Deposits Research Articles

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.

Late Cretaceous Fluvial Systems and Inferred Tectonic History, Central Utah: ABSTRACT

Upper Campanian nonmarine sedimentary rocks exposed between the Wasatch Plateau and the Green River in central Utah record a tectonic End_Page 1346------------------------------ transition from thin-skinned deformation in the thrust belt to basement-cored uplift in the foreland region. Thick sections of the Mesaverde Group in the Wasatch Plateau on the west and the Book Cliffs on the east are separated by the San Rafael swell, a basement uplift across which the group is erosionally thinned. Strata in the west (Castlegate Sandstone and Price River Formation) were deposited by east to northeast-flowing braided rivers. Time-equivalent eastern sections comprise a lower sequence of mixed braided fluvial deposits (Castlegate Sandstone and Bluecastle Tongue of Castlegate, coastal swamp and meander-belt deposits (Neslen Formation), and nearshore marine deposits (Buck Tongue of Mancos Shale and Sego Sandstone), and an upper sequence that coarsens upward from meander-b lt deposits (Farrer Formation) into pebbly braided river deposits (Tuscher Formation). Paleocurrent data indicate that rivers of the lower sequence flowed east, while those of the upper sequence flowed northeast. Figure Sandstones within the section consist of two distinct compositional suites, a lower quartzose petrofacies and an upper lithic petrofacies. The compositional boundary occurs at the top of the Bluecastle Tongue and can be correlated across the San Rafael swell. The quartzose suite contains mostly compositional quartzarenites and sublitharenites; the lithic suite is composed of litharenites and feldspathic litharenites. Lithic grain populations of the upper petrofacies are dominated by sedimentary fragments in sections of the Wasatch Plateau and volcanic fragments in sections near the Green River. The sedimentary lithic grains were transported generally eastward from miogeoclinal strata uplifted within the thrust belt. The volcanic lithic grains of the Farrer and Tuscher Formations were erived from more distal arc sources to the southwest, and transported through the thrust belt somewhere west of the Kaiparowits region, where time-equivalent sedimentary rocks are also rich in volcanic lithic fragments. Disappearance of volcanic lithics and appearance of pebbles at the top of the Tuscher Formation is interpreted to reflect a latest Campanian reorganization of drainage patterns that marked initial growth of the San Rafael swell and similar basement uplifts to the south of the swell. Contemporaneous fluvial systems that deposited the uppermost part of the Price River Formation in the Wasatch Plateau were apparently unaffected by the uplift and continued to flow northeast. Depositional patterns thus indicate that initial growth of the San Rafael swell was probably concurren with late deformation in the thrust belt. Depositional onlap across the Mesaverde Group by a largely post-tectonic assemblage of fluvial and lacustrine strata (North Horn Formation) indicates a minimum late Paleocene age for growth of the San Rafael swell and deformation within the thrust belt. End_of_Article - Last_Page 1347------------

Gallup Sandstone: Complex of Wave-Dominated Deltaic and Low-Sinuosity Braided Fluvial Deposits, Gallup Sag, New Mexico: ABSTRACT

The Upper Cretaceous Gallup Sandstone in northwestern New Mexico is interpreted as coastal-barrier or delta-front deposits. A study of 160 closely spaced sections along nearly continuous exposures in the Gallup Sag, New Mexico, suggests that these deposits are related to wave-dominated delta and low-sinuosity braided fluvial systems. The Gallup Sandstone in this area is divided into three prograded depositional packages. The lowermost package consists of shoreface sheetlike sandstones, siltstones, and shales, which coarsen upward into coalesced distributary-mouth-bar and beach sandstones that are locally dissected by fluvially and tidally influenced channel sandstones. This package is overlain by constructional delta-plain deposits and coastal back-barrier deposits, the latter being associated with the destructional phase of the delta system. The constructional delta-plain deposits consist of meandering distributary channels interspersed with interdistributary crevasse-splay sandstones and backswamp coals, carbonaceous shales, siltstones, and shales. The destructional coastal back-barrier deposits consist of heavi y bioturbated lagoonal or bay sequences of sandstones, siltstones, shales, and carbonaceous shales. Coal beds associated with this destructional facies are as much as 4 ft (1.2 m) thick and extend laterally as much as 4.2 mi (6.7 km). These coal beds are blanketlike in contrast to the constructional delta-plain coal beds, which are thin, discontinuous, and lenticular. This overall package is overlain by an alluvial depositional package consisting of numerous stacked, overlapped, lenticular, varicolored, pebbly sandstones. These sandstones, which are blanketlike in extent and contain a few poorly developed coals, probably represent low-sinuosity braided-stream deposits. End_of_Article - Last_Page 561------------

Sedimentology of Fluvial Upper Devonian Kanayut Conglomerate, Brooks Range, Alaska: ABSTRACT

The Kanayut Conglomerate, which extends across most of the Brooks Range in northern Alaska, is a widespread nonmarine clastic sequence as thick as 2,000 m. It records southwestward growth of a major fluvial-dominated coarse-grained delta. The Kanayut is underlain and overlain by fossiliferous marine strata of the Upper Devonian Hunt Fork Shale and Lower Mississippian Kayak Shale, respectively. It has been subdivided into four members, in ascending order: (1) a lower marine member, 560 m thick, consisting chiefly of sandstone; (2) a lower nonmarine member, 550 m thick, consisting chiefly of fining-upward cycles of sandstone to shale; (3) a middle nonmarine member, 450 m thick, consisting of massive interbedded conglomerate and sandstone; and (4) an upper nonmarine member, he Stuver Member, 400 m thick, consisting chiefly of fining-upward cycles of sandstone to shale. The distribution of maximum size of conglomerate clasts suggests a source area to the northeast, and paleocurrent measurements indicate sediment transport dominantly toward the southwest. The conglomerates are texturally and compositionally mature, containing primarily clasts of chert with lesser amounts of quartz, quartzite, and argillite. The sandstones are also compositionally and texturally mature, composed chiefly of subrounded grains of quartz, chert, and argillite, with negligible amounts of feldspar. The Hunt Fork Shale, Kanayut Conglomerate, and Kayak Shale record a major progradational-retrogradational deltaic cycle. The lower marine member of the Kanayut and underlying and overlying marine units represent prodelta, delta-margin, and delta-front deposits, the lower nonmarine member and the Stuver Member of the Kanayut represents meandering fluvial delta-plain deposits, and the middle nonmarine member of the Kanayut represents braided fluvial delta-plain deposits. End_of_Article - Last_Page 758------------