Braided Stream System Research Articles

Depositional Environment of Poison Canyon Sandstone in Gulf Mariano Lake Mine: ABSTRACT

A study of the depositional environment of the Poison Canyon Sandstone (Brushy Basin Member of the Upper Jurassic Morrison Formation) in the Mariano mine was undertaken to determine the relation of uranium occurrence to the depositional environment. An analysis of the sedimentary structures, paleocurrent measurements, isopach data, and electric-log shapes has led to the interpretation that the Poison Canyon Sandstone in the Mariano Lake mine vicinity is a sequence of bed-load deposits of a fluvial system. Paleocurrent measurements taken in the mine and at three outcrops of the Poison Canyon confirm findings from earlier work that indicated eastward current flow. The dominance of tabular cross-beds, channel scours, and trough cross-beds, the geometry of the Poison Canyon s ndstones, and the chaotic sedimentary structures corroborate deposition in a braided-stream environment. The Poison Canyon sandstones may be subdivided into two lithofacies. A cross-bedded unit is characterized by large-scale tabular cross-beds, scour surfaces, medium to coarse grain size, poor sorting, angular to subangular grains, an abundance of feldspar grains, and clay rip-up clasts. A second unit is dominated by laminar bedding, fine grain size, moderate sorting, subrounded grains, and an abundance of quartz. The source area for the sands had been dominated by plutonic rocks. These facies cannot be separated on the basis of electric-log shape. Uranium ore occurs in both facies but is more abundant in the cross-bedded unit. The complexities of the braided-stream system greatly complicate any study of the interrelation of the uranium and the depositional environment. Obvious conclusions are that the grain size does not greatly influence the emplacement of the uranium, that the orebody at the Mariano mine is epigenetic, and that the sheetlike nature of the Poison Canyon sandstones suggests that the limiting factors on the ore boundaries are geochemical rather than stratigraphic. End_of_Article - Last_Page 690------------

Reservoir Geometry of a Fluvial Sheet Sandstone

Fluvial sheet sandstone provides an ideal reservoir for hydrocarbons. Such a sandstone body is large enough to contain giant hydrocarbon reserves as well as an aquifer that could flush the hydrocarbons. The outcrops of the Westwater Canyon Member in the Morrison Formation (Jurassic), northwestern New Mexico, show representative attributes of a fluvial sheet sandstone. This sandstone body consists of coalescing fluvial-channel systems that in turn are composed of still smaller coalescing individual channels. The sheet is more than 63 mi (100 km) wide transverse to the paleocurrent direction and averages 200 ft (61 km) in thickness; the downcurrent dimension is not preserved but may have been as much as 100 mi (160 km). Channel systems internal to the sheet average 7 mi (11 km) wide and 50 ft (15 m) deep. The individual channel deposits that form the systems average 600 ft (183 m) wide by 12 ft (4 m) thick. Any one vertical column through the sheet sandstone is usually more than 90 percent sandstone. Further, each vertical column consists of repetitive incomplete (top-missing) fluvial sequences that show upward-decreasing grain sizes and scales of sedimentary structures. Fluvial sheet sandstone probably is deposited by aggrading and coalescing braided-stream systems.

Transverse Braid Bars in the Upper Triassic Trujillo Sandstone of the Texas Panhandle

Exposed within the Upper Triassic Trujillo Sandstone of the Texas Panhandle are excellent examples of transverse braid bar deposits. The bars are 0.3-1.0 m high with a distance of 5-10 m between bar crests. They are composed of fine- to medium-grained sand with planar cross stratification overlain by a thin layer of small-scale trough cross stratification or linguoid ripples. A plot of stratification ratio versus bed relief index indicates a predominance of transverse bars over longitudinal bars for the Trujillo Sandstone. This indicates deposition in the distal portions of a braided stream system.

Syntectonic Conglomerates in Southwestern Montana: Their Nature, Origin, and Tectonic Significance

During the Late Cretaceous (late Albian-Cenomanian) through Paleocene and probably early Eocene, southwestern Montana and adjacent Idaho comprised the depocenter for thick accumulations of syntectonic quartzite conglomerate, limestone conglomerate, and minor amounts of sandstone collectively known as the Beaverhead Formation. In this study, the depositional and deformational history of the Beaverhead is documented in detail in an attempt to understand the tectonic and topographic development of southwestern Montana and east-central Idaho. Clast imbrication and composition measurements suggest that two fundamentally different source areas existed for the Beaverhead. The limestone conglomerate clasts were derived primarily from Mississippian and Triassic carbonate rocks exposed by uplift of the Black-tail-Snowcrest basement arch and the “Ancestral Beaverhead Range” along the present-day Montana-Idaho border. These deposits, which derived their detritus from an area less than 15 mi away, probably represent coalesced alluvial fans. Although the quartzite conglomerate beds were deposited simultaneously with the limestone clasts, the quartzite was derived from a more distant source. The vast quantity of Belt quartzite clasts which initially reached what is now southwest Montana in late Albian time apparently originated from the large area of Belt exposure on the northeast side of the Idaho Batholith approximately 50 mi to the west and northwest of the study area. Gradual expansion of this source area by active uplift continuously provided steep slopes needed for the transport of gravel by braided stream systems to the adjacent subsiding alluvial plain. Recycling of the Belt clasts owing to Late Cretaceous uplift in the present eastern Snake River Plain carried them as far east as western Wyoming, where they became incorporated in the Harebell and Pinyon Conglomerates of western Wyoming. The Beaverhead Formation exhibits two distinct structural patterns: (1) an earlier cratonic pattern composed of northeasterly trending, gently plunging, open folds and associated high-angle faults, and (2) a northwest-trending “geosynclinal” pattern characterized by major upthrusts such as the Tendoy, Cabin, and Fritz Creek faults, and large, gently plunging, open folds. The older structural pattern seems to be related to the tectonism of the Late Cretaceous to early Paleocene that created the Blacktail-Snow-crest basement arch. The younger, more pervasive northwest trend may have a causal connection with the large uplift in the region of the Idaho Batholith. It is plausible that gravitative energy created by this uplift was available not only for eastward fluvial transport of Belt clasts, but also for northeastward down-slope tectonic transport. According to this hypothesis, large sheets of bedded rock slid eastward along detachment surfaces within the Belt, and as these surfaces were folded and locked, new ones developed to the east. This eastward migrating tectonic front reached the Lima, Montana, region soon after major Beaverhead sedimentation had ceased during middle to late Paleocene or early Eocene time. At this time, the Beaverhead rocks, which in part had inherited the earlier northeast trends,

Recognition and Delineation of Fluvial Sedimentary Bodies in Subsurface: ABSTRACT

A new method of recognition and delineation of ancient stream channels is based on the interpretation of subsurface sedimentary structures. A study of the Columbia Formation (Pleistocene) was conducted in a small area in northern Delaware. Forty holes were drilled through the formation and 486 samples were collected. In the absence of definite correlation between sedimentary units in various holes, and in the absence of any dependable horizon markers, the Columbia sediments have been thought of as divided into horizontal layers spaced 5 ft apart, and referred to their height above sea level. Primary sedimentary structures (crossbedding in sands and gravels, horizontal bedding in coarse sands, and horizontal lamination in clayey silts) determined from the drilling samples were mapped for each layer separately. This mapping made possible the recognition and delineation of Pleistocene stream channels. The behavior of these ancient streams, interpreted from the maps, is suggestive of low, flat topography, easily eroded banks, shallow and wide channels, frequent change in water and sediment discharges, and flooding. All these are the characteristics of a braided stream system. In spite of frequent lateral shifts of stream courses, the channel bodies of the Columbia Formation are vertically continuous. The method of subsurface investigation of fluvial sedimentary bodies described here could be applied to the exploration for oil and gas in areas where sufficient well control and sampling are available. End_of_Article - Last_Page 655------------

Geology of Jurassic, Flomation-Jay Area, Alabama and Florida: ABSTRACT

Flomaton field, in Escambia County, is the first major gas condensate discovery from the Jurassic Norphlet Formation in Alabama. Structurally the field is a NW-SE trending low-relief salt anticline bounded on the north and east flanks by a major down-to-the-basin fault which is part of the Pickens-Gilbertown-Pollard regional fault system. The Norphlet sandstone reservoir is about 60 ft thick within the field and produces CO2 and sour gas with a high condensate yield. The paleostructural history of the area indicates that early Louann Salt movement and faulting occurred, probably as a result of gravity slide and basinward salt creep, forming structures necessary for trapping hydrocarbons. Jurassic deposition was affected by these early structural features and pre-salt topography. Norphlet clastics were derived from the northeast and deposited by braided stream systems. As the Smackover seas transgressed the area, the upper part of the Norphlet was partly reworked. In the Flomaton area, the Smackover Formation is a dark-brown, dense, micritic limestone. The overlying Haynesville Formation can be subdivided into an upper member consisting of predominantly red, coarse clastic material and a lower member consisting of fine, red clastics and evaporites. At Flomaton, over 300 ft of bedded salt has been drilled in the lower Haynesville causing many drilling and completion problems. The Cotton Valley Group marks the top of the Jurassic and consists primarily of coarse gravelly clastics. Exploration activity in southwest Alabama during 1969, 1970, and 1971 led to additional discoveries from the Jurassic. Among these is Jay field in Santa Rosa County, Florida, 7 mi southeast of Flomaton field. Jay produces oil from Smackover dolomite and is the first Jurassic discovery in Florida. The discovery well produced 1,710 BOPD and has resulted in one of the most active exploration plays in the United States. End_of_Article - Last_Page 1695------------

Jurassic Geology of Flomaton Field Area of Southern Alabama: ABSTRACT

Flomaton field, in Escambia County, Alabama, is the first major gas condensate discovery from the Jurassic Norphlet Formation in Alabama. Structurally the field is a NW-SE trending, low-relief salt feature bounded on the north and east by a major down-to-the-basin fault which is part of the Pickens-Gilbertown-Pollard regional fault system. The Norphlet sandstone reservoir is about 60 ft thick and produces CO2 and sour gas with a high condensate yield. The paleostructural history of the area indicates that movement of Louann salt and faulting occurred, probably as a result of gravity slide and basinward salt creep, forming structures capable of trapping hydrocarbons. Jurassic deposition was affected by these early structural features and by presalt topography that existed updip from the Flomaton area. Norphlet clastics were derived from the northeast and deposited by braided stream systems. As the Jurassic Smackover seas transgressed the area, the upper part of the Norphlet was partly reworked. In the Flomaton area, the overlying Smackover Formation is a dark-brown, dense, micritic limestone. Above the Smackover, the Haynesville Formation can be subdivided into upper and lower members with the upper Haynesville consisting of predominately red, coarse clastics and the lower member being fine, red clastics and evaporites. At Flomaton, over 300 ft of bedded salt has been drilled in the lower Haynesville causing many drilling and completion problems. The Cotton Valley Group marks the top of the Jurassic and consists primarily of coarse, gravelly clastics. End_of_Article - Last_Page 347------------