Fan-delta Complex Research Articles

Structural Evolution of Chihuahua Tectonic Belt: ABSTRACT

The Chihuahua tectonic belt is a Laramide foreland fold and thrust belt that is in the initial phase of exploration. It can be compared to the Southern Overthrust belt prior to the Pineview discovery in 1975. The part within the United States is approximately 300 mi (483 km) long and extends from southeast of El Paso to Presidio, Texas. The frontal thrusts of the belt lie against the Diablo platform. The stratigraphic sequence involved in thrusting is Permian to Lower Cretaceous. Thrusting has been accomplished by decollement of Jurassic and Permian evaporates. The Malone Formation (Jurassic) is the proximal part of a fan-delta complex and contains marine, subtidal, intertidal, and supratidal facies. Two distinctive locally derived conglomerate facies are present: (1) clast-supported, bimodal, dolomite-pebble conglomerate (sheet-flood facies); and (2) a matrix-supported, dolomite-pebble conglomerate that lacks sorting (debris-flow facies). Probable correlatives of the Malone Formation occur at Sierra del Kilo and Sierra de la Alcaparra in Chihuahua 60 mi (97 km) to the southwest. By using new surface-mapping and surface-sampling plus interpretation of seismic and gravity data, a sequence of tectonic events can be inferred: (1) Late Triassic(?) faulting and formation of the Chihuahua trough; (2) Laramide folding and thrusting; (3) late Laramide en echelon left-lateral strike-slip folding; and (4) basin-and-range faulting. End_of_Article - Last_Page 548------------

Upper Jurassic-Lower Cretaceous Fan-Delta Complex--La Casita Formation of Saltillo Area, Coahuila, Mexico: ABSTRACT

The La Casita Formation represents a major influx of terrigenous sediment on the epicontinental shelf of northeastern Mexico during the Late Jurassic and earliest Cretaceous. Near Saltillo, the La Casita can be divided into three major units: (1) a basal unit of carbonaceous siltstone and mudstone, (2) a middle unit which is predominantly sandstone and pebble conglomerate, and (3) an upper unit of siltstone and fine-grained sandstone with thin limestone and dolomite layers becoming more abundant upward, and grading into the overlying Taraises Formation. The La Casita-Taraises terrigenous sequence is underlain and overlain by shallow-marine limestones. In the Saltillo area, the La Casita crops out in narrow canyons in breached anticlines of the Sierra Madre Oriental. Consequently, interpretations concerning the depositional framework of the La Casita must be made from vertical sequences exposed in widely spaced canyons. A first-order, single-dependence Markov analysis aids in identifying preferred vertical transitions in lithologically variable parts of the section. Important aspects of the La Casita in the Saltillo area include: (1) the large-scale depositional sequence of basal shallow-marine mudstones overlain by shallow-marine and alluvial sandstone, conglomerate, and mudstone, which are in turn, overlain by shallow-marine, fine-grained sandstone, mudstone, and carbonate rocks; (2) conglomeratic shallow-marine sediment; (3) textural and mineralogical immaturity; (4) the nonrandom nature of upward lithofacies transitions; and (5) a predominance of coarsening-upward sequences in the middle unit. These characteristics suggest that the La Casita in the Saltillo area records the progradation of a complex of fan deltas. The lower, fine-grained unit represents Late Jurassic prodelta deposits that accumulated on the submarine shelf prior to the influx of coarse sediment. The middle, coarse-grained unit was deposited during the period of maximum seaward advance of the fan-delta complex during latest Jurassic time. This unit contains predominantly coastal and shallow-shelf conglomerate, sandstone, and mudstone, with some distal alluvial-fan conglomerate and sandstone. The upper La Casita records the waning input of coarse sediment as the fan-delta system retreated in earliest Cretaceous time. End_of_Article - Last_Page 1429------------

Fan-Delta Sedimentology and Tectonic Setting--Yallahs Fan Delta, Southeast Jamaica

Fan deltas have been defined as alluvial fans that prograde into a standing body of water from an adjacent highland. Most large Holocene fan deltas are located at the edge of active continental margins and island-arc systems where high-gradient braided streams deposit their sediment load of coarse detritus. Such coastlines are usually wave dominated and receive between 100 and 300 cm of annual precipitation. The Yallahs fan delta along the southeast coast of Jamaica is a typical example of this type of fan-delta complex. The Yallahs River debouches from the Blue Mountains and has built a 10.5-sq km lobate fan delta composed of sand to boulder-size detritus. The morphology of the Yallahs delta is controlled in part by the foothills of the Blue Mountains which bound the delta on three sides and by a steep offshore profile which causes waves to break and expend most of their energy directly against the delta front. Environments comprising the subaerial delta plain include erosional and depositional beaches, abandoned and braided channels, flood plains, and salt-water ponds. The subaqueous delta environment is characterized by steep slopes and numerous submarine channels dominated by mass-gra ity processes resulting in patchy sediment-size distribution. Active coral growth occurs along the western delta margin. Sedimentologic characteristics of the Yallahs fan delta and published information on the southeast coast of Alaska provide data for the construction of two stratigraphic models that can be used for the recognition of ancient fan-delta deposits. The model based on the characteristics of the Yallahs fan delta is applicable to ancient fan-delta deposits that grade directly onto continental and/or island-arc slopes. These deposits are characterized by proximal, gravelly, braided-stream deposits that grade seaward into interbedded gravels and sands at the shoreline and to muddy gravels and muds of the slope. The second model, based on the characteristics of the southeast Alaska coastline, is applicable to fan-delta deposits that prograde onto continental and/or island shelves. These deposi s are characterized by gravelly, proximal, braided-stream deposits that grade seaward into sandy, distal braided-stream deposits, well-laminated sands of the beach and nearshore zone, and finally into burrowed shoreface muds. The two models represent end members of a spectrum of fan-delta types common in ancient sedimentary sequences ranging in age from Precambrian to Pleistocene.

Paleogeographic Evolution of Late Paleozoic Taos Trough, Northern New Mexico: ABSTRACT

The Taos trough (or Rowe-Mora basin) of northern New Mexico was one of several tectonically active cratonic basins associated with the late Paleozoic Ancestral Rockies. As the basin and adjacent uplifts evolved, the depositional systems and paleogeography varied in conjunction with changing tectonic stability, fluctuating End_Page 687------------------------------ sediment input, evolution and integration of sediment dispersal systems, and varying water depth. Morrowan time was characterized by a widespread transgression by a shallow sea flanked by low-lying, muddy strand plains. At this time the surrounding uplifts were relatively low-relief features and supplied only fine-grained sediment to the basin. By Atokan time, vertical movements elevated the Uncompahgre uplift to the west of the trough, and coarse alluvial fan, braided stream, and fan-delta complexes prograded eastward into the basin. During this episode of intense diastrophism to the west, the eastern side of the trough remained fairly stable and a broad, shallow-marine shelf sloped gently off the Sierra Grande arch. Farther south, carbonates accumulated on the Pecos shelf. This general depositional and paleogeographic pattern continued into Desmoinesian time. However, to the north the Cimarron arch had become a major positive feature by earliest Desmoinesian time and several braided stream-fan-delta systems prograded southward and southwestward into the basin. By middle Desmoinesian time an extensive coastal plain had developed along the western margin of the trough, with low-sinuosity rivers feeding lobate deltas. A major middle to late Desmoinesian transgression inundated this coastal plain and shallow shelf conditions prevailed throughout most of late Desmoinesian time. In latest Desmoinesian time, southward prograding fluvial-deltaic systems began filling the northern part of the basin causing a net southward regression. By Early Permian time the basin was dominated by continental sedimentation with southward flowing braided streams. End_of_Article - Last_Page 688------------

Petrology and Diagenesis of Arc-Derived Lithic Sandstones--Wagwater Trough, East-Central Jamaica: ABSTRACT

The Wagwater trough is a fault-bounded block of lower Tertiary sedimentary and volcanic rocks that cuts across east-central Jamaica in a southeast direction. Detrital sandstone and conglomeratic sequences within this block include the early Eocene and Paleocene(?) Wagwater and Richmond Formations. The Wagwater Formation is a red-bed sequence of breccias, conglomerates, sandstones, and mudstones with minor amounts of limestone and gypsum deposited as a subaerial fan-delta complex. Sedimentary rocks of the Wagwater Formation are laterally equivalent to sandstones, conglomerates, shales, and limestones of the Richmond Formation, which were deposited in submarine-slope and fan environments. Lithic arenites are the dominant sandstone type within the trough. Principal framework constituents include volcanic and carbonate lithic fragments, plagioclase, quartz, and fossil fragments with minor amounts of plutonic lithic fragments and opaque heavy minerals. Authigenic minerals are abundant in some sandstones and include phyllosilicates (chlorite and clay minerals), calcite, and iron oxide. The progressive sequence of diagenetic features from earliest to latest in marine sandstones of the Richmond Formation is: (1) development of clay coats around framework grains, (2) precipitation of calcite pore-fill cement, (3) development of a second clay coat on calcite cement in incompletely filled pores, (4) crystallization of radiating pore-fill chlorite after development of clay coats in either stage 1 or stage 3, and (5) late-stage calcite replacement. Despite minor differences in diagenetic features, the Richmond sandstones are similar to other arc-derived sandstones from five other basins. These separate occurrences suggest that sands deposited in arc basins have the same compositional range, and that postdeposition processes produce a common diagenetic sequence. End_of_Article - Last_Page 447------------

Basin Evolution of Late Paleozoic Taos Trough, Northern New Mexico: ABSTRACT

The Taos trough (Rowe-Mora basin of earlier workers) of northern New Mexico was one of several tectonically active cratonic basins associated with the late Paleozoic Ancestral Rockies. The basin was an asymmetric, fault-bounded feature flanking the Uncompahgre uplift. The basin fill was controlled primarily by local tectonics. As the basin and adjacent uplift evolved structurally, the depositional systems likewise evolved in conjunction with changing tectonic stability, fluctuating sediment input, evolution and integration of sediment dispersal systems, and varying water depth. The onset of the first mild structural deformation and encroachment of the sea during Morrowan time was marked by deposition of a complex sequence of fine-grained sandstone, shale, coal, and limestone. These sediments were deposited in a variety of environments including mud flats, marshes, strand plains, and shallow-water carbonate banks. During Atokan time, tectonic activity of the Uncompahgre uplift increased, and subsidence of the basin accelerated. During this time, coarse alluvial-fan, braided-stream, and fan-delta complexes began to fill the basin along its western margin. By middle Desmoinesian time, an extensive coastal plain and a more continuous sediment supply had developed. Meandering rivers with coarse-grained bed-loads fed lobate and wave-dominated deltas. Associated wi h this delta platform were local algal-mound and cross-bedded shelf-edge carbonate accumulations. Basinward, a mixed carbonate and terrigenous slope system developed in conjunction with continued subsidence of the basin. End_of_Article - Last_Page 824------------