Scour Features Research Articles

Submarine erosion on the anoxic sea floor: stratinomic, palaeoenvironmental, and temporal significance of reworked pyritebone deposits

Abstract Numerous black (organic-rich) shale units in the Palaeozoic of North America are marked by discontinuities along their bases which are characterized by acid-insoluble lag concentrations of reworked pyrite, phosphatic debris, and siliceous material. These contacts, ranging in magnitude from widespread disconformities (typically marine flooding surfaces) to local, within-outcrop-scale, scour features, were produced by submarine erosion under anoxic to lower dysoxic conditions. Dissolution of carbonate debris under conditions of carbonate undersaturation and/or low pH in bottom waters, was generally complete. Hydraulic transport coupled with dissolution led to the formation of laterally discontinuous and current-sorted lenses of pyritic grains, fish bones, condonts, quartz sand, chert nodules, and glauconite. Lags of reworked pyritic tubes and small spherules are typical of finely interbedded black and green shale successions (‘zebra facies’) observed in the Upper Devonian (Famennian). Under anoxic conditions pyrite was chemically stable, while carbonate debris was not. Herein, we examine lag deposits associated with a variety of black-shale-roofed discontinuities ranging in age from Middle Ordovician to Mississippian. Possible erosion mechanisms for producing these discontinuities and associated lags include deep-stormwaves, density currents and internal waves. In each case erosion occurred following an interval of condensation associated with flooding surfaces, particularly those timed with the beginning of highstand intervals.

San Jacinto Intrusive Complex: 1. Geology and mineral chemistry, and a model for intermittent recharge of tonalitic magma chambers

Geological mapping within the San Jacinto Mountains of southern California has delineated three major and numerous minor plutons of the Cretaceous Peninsular Ranges batholith. Early minor intrusives emplaced into quartz‐rich metasedimentary sequences span the compositional range olivine gabbro to granite. The three large (to 250 km2) plutons span a limited compositional range between mafic tonalite (color index (CI) > 15) and K‐feldspar‐poor granodiorite (CI ≃ 10). All units are composed of plagioclase (An30–40) (50–55%), quartz (20–30%), K‐feldspar (1–8%), biotite (10–15%), hornblende (0–5%), titanite (0–2%), and accessory zircon, apatite, allanite, and ilmenite. Variations in mineral abundances are geographically systematic only within the youngest major mass (unit III), which grades from marginal mafic tonalite to central K‐feldsparpoor granodiorite. Mineral foliations and banding, schlieren, and inclusion orientation within each unit usually parallel the nearest contact. Alignment of foliations and apparent flow‐sorting and scour features are interpreted as reflecting flow patterns within each chamber. Mafic synplutonic dykes (of quartz diorite and tonalite) intruded the tonalites and were broken up to form extensive inclusion trains. Dyke‐tonalite relations are interpreted as showing that (1) magma adjacent to the pluton wall had considerable yield strength, (2) magmatic flow adjacent to pluton walls was capable of moving material some distance (up to kilometers) to form the inclusion trains, and (3) the dykes represent conduits through which a considerable amount of liquid was added to the inflating magma chambers. Mineral compositions throughout the major plutons are relatively uniform. Mean plagioclase composition ranges from An40 in the most mafic tonalites to An30 in the most felsic granodiorites; the total microprobe‐observed range is An44 to An25 (and to An47 in a mafic inclusion). Mg/(Mg + Fe + Mn) of biotite and hornblende drop similarly from 0.44 to 0.36 reflecting changes in rock Mg/(Mg+Fe). The sole oxide mineral is almost pure ilmenite. The limited ranges of mineral compositions imply considerable stability of physico‐chemical conditions during the crystallization of each pluton. The field, petrographic, and mineral chemical data are interpreted in terms of an intermittently recharged continuously solidifying magma chamber.

Geologic setting and genesis of the Red Dog zinc-lead-silver deposit, western Brooks Range, Alaska

Red Dog is a Mississippian- to Pennsylvanian-age black shale-hosted, zinc-lead-silver deposit in the De Long Mountains, western Brooks Range, Alaska. It contains drilling-indicated reserves of 77 million metric tons of 17.1 percent zinc, 5.0 percent lead, and 82 g per metric ton silver.The De Long Mountains are characterized by eight stacked and folded thrust allochthons. The six structurally lowest allochthons are composed of Devonian through Cretaceous clastic and chemical sedimentary rocks; differing lithologic assemblages of coeval units distinguish the allochthons. The two uppermost allochthons contain Jurassic or older mafic and ultramafic igneous sequences.The Red Dog deposit occurs in the second lowest allochthon, in black siliceous shale and chert of the Mississippian to Pennsylvanian Kuna Formation. A distinctive interbedded light gray calcarenite and dark gray calcareous shale termed the Kivalina unit is the stratigraphic footwall to mineralization and defines the second order sedimentary basin in which the deposit formed.The Red Dog deposit is a strata-bound accumulation of silica rock, barite, and sulfides. Silica rock consists dominantly of growth-zoned mosaic aggregates of translucent quartz grains with accessory sulfides and occurs within and peripheral to the main mass of sulfides. The barite facies contains accessory sulfides, silica, and rare calcite and is concentrated toward the top and periphery of the deposit. Barite is commonly coarse grained and massive to poorly bedded. Sulfide rock textures vary from massive, chaotic, or fragmental to poorly bedded. Major sulfides in decreasing order of abundance are sphalerite, pyrite, marcasite, and galena. Rare disseminated chalcopyrite and pyrrhotite occur in sphalerite; boulangerite occurs in galena. The dominant gangue constituents are quartz, barite, and minor shale. The sulfides are generally fine grained, although coarse-grained crustiform and comb-textured sphalerite occurs in feeder veins which are best developed at the base and on the periphery of the deposit. Thin pyrobitumen-calcite veins form the latest stage fracture filling. The shales which host the deposit appear to have been silicified, and carbonate, which is a common accessory in the remainder of the upper Kuna Formation, is absent in the vicinity of Red Dog. The deposit is capped by silica- and sulfide-poor barite of the Pennsylvanian to Permian(?) lower Siksikpuk Formation.Deposit-scale structure mimics the structure of the De Long Mountains, that is, stacked thrust slices which were subjected to at least two directions of compression. The Main deposit consists of two major mineralized plates and one minor one with their associated capping postore rocks, lying within the Red Dog sequence of the Brooks Range allochthon. The deposit lies on a tectonic melange zone which separates it from the Cretaceous Okpikruak Formation of the underlying allochthon. Restoration of the two main mineralized plates to their relative prethrust position indicates that the Red Dog deposit was composed of a central core enriched in zinc and lead with a relatively iron-enriched halo. The deposit is weakly enriched upward in lead relative to zinc and more strongly depleted upward in iron relative to both lead and zinc. Limited sulfur isotope determinations indicate a fairly wide range and relatively light values for sulfide sulfur at Red Dog. These light values contrast sharply with heavier sulfur in basinwide sedimentary pyrite. There is some evidence to suggest that Red Dog formed partially by sediment replacement perhaps at a time when conditions in the basin were gradually becoming more oxidizing. The observed sulfur isotope values in the synsedimentary portion of the Red Dog deposit may, therefore, have been produced by constant mixing of a buoyant hydrothermal plume with suboxic seawater.A well-developed vent biota is preserved at Red Dog, particularly in silica rock. Slightly curved to sinuous tubular structures with organic-rich siliceous walls bear similarities to the alvinellid worms observed around modern-day sea-floor hydrothermal vents. Associated with the tubes are oblong chert pellets that contain very small carbon spheres of probable biogenic origin.The Red Dog deposit formed during the early stages of a long-lived starved sedimentary basin. Whereas some textures at Red Dog can be interpreted to be replacement in origin, the presence of conglomerates and scour features in sulfides indicates that at least part of the deposit formed at or near the same time as the enclosing stratigraphy. The deposit probably formed by a combination of sediment replacement and deposition on the sea floor. Tectonic instability at the time of ore formation is indicated by abrupt lateral facies changes evident in the Mississippian to Pennsylvanian basin. Small igneous intrusions of possible Mississippian to Pennsylvanian age provide evidence for locally elevated heat flow. The underlying permeable Upper Devonian to Lower Mississippian(?) fiuvial-deltaic rocks may have provided a basin aquifer through which the mineralizing fluids moved prior to exhalation on the sea floor.Pennsylvanian-age change to oxidizing conditions in the basin terminated the sulfide depositional event of Red Dog, followed by the cessation of major barite accumulation in Pennsylvanian to Permian(?) time.

Storm-Dominated Cyclic Shelf Deposits in Upper Ordovician of Cincinnati Arch Region: ABSTRACT

The Kope and Fairview Formations and the Bellevue Tongue of the Grant Lake Limestone (Upper Ordovician, Cincinnatian Series) exposed in the Cincinnati, Ohio, region represent storm-dominated regressive sedimentation on an intracratonic ramp or shelf. The Kope-Fairview transition zone (25 m) is an outer shelf lithofacies composed of numerous small-scale (1-3 m), shallowing-upward cycles. These cycles consist of two facies: (1) a shale-dominated component characterized by numerous, single-event distal tempestites, and (2) a fossiliferous limestone component characterized by amalgamated, proximal tempestites. Distal storm components contain abundant thin-bedded, parautochthonous packstones, rare autochthonous fossil accumulations, and abundant thin-bedded siltstones that commonly contain tool marks, gutter casts, parallel lamination, small-scale hummocky-cross-stratification, hummocky bed forms, mud-filled scours, and millimeter ripples. Proximal tempestites are thick-bedded, fine to coarse-grained packstones and grainstones that commonly have megarippled surfaces, cross-lamination, and shale lithoclasts, and contain a mixture of fossils showing degrees of abrasion and transport. Lower Fairview cycles are proximal equivalents of underlying Kope cycles, and typically show more scour features, bedding amalgamation, and proximal components. Cycles are laterally continuous across the study area (25 x 15 mi) and exhibit only minor variation. Cycle boundaries appear isochronous and mark the abrupt transition from proximal to distal storm sedimentation. Themore » widespread character and isochronous relationship suggest that these cycles are the product of an allocyclic mechanism involving episodic, but rapid, rises in relative sea level. Deposition of cyclic sequences occurred on a very gently dipping, northward-facing shelf.« less

Early Diagenetic Controls on Allochthonous Carbonate Debris Flows--Examples from Egyptian Lower Eocene Platform-Slope: ABSTRACT

Many of the models that have been proposed for the origin of carbonate debris flows are based upon examples from ancient carbonate slope settings. Few ancient slope environments have been described, however, where submarine cementation processes were prevalent. This early diagenetic phenomenon, common on many modern carbonate slopes and platforms, can control the mechanisms by which debris flows are generated, as well as the ultimate form of the beds they produce. Within the Duwi Trough of central Egypt, a basin to slope facies transition is preserved within the lower Eocene chalks and limestones of the Thebes Formation. Basinal facies are characterized by sequences of laminated chalks and limestones with thin, intercalated horizons of nodular limestone and limestone hardgrounds. Carbonate platforms developed on structural highs adjacent to the basin and periodically shed bioclastic detritus downslope in the form of fine-grained, skeletal turbidites. Nodular limestones and hardgrounds, that formed upslope, were in places dislocated and reworked into the basin as submarine debris flows. Individual debris flow beds preserved within the lower slope and basinal facies can be traced over 50 km down the trough axis and several kilometers laterally. > Nodular conglomeratic debris found within the flows range from 30 to 300 cm in thickness and are mud supported. Unlike most ancient debris flow breccias, larger clasts are unusually uniform in size and well rounded. This is not a reflection of textural maturity but a result of the primary nodular morphology of these clasts. Channels and basal scour features are poorly developed in these beds owing to the cemented (hardground) nature of the basin-floor during debris flow deposition. Sites of nodular limestone bed dislocation are not recognized within the slope facies. Neither large-scale rotational slides, nor slump structures associated with translational slides are developed. The proposed mechanism for the detachment of these nodular horizons is one of relatively shallow decoupling of the early-cemented surface layer from the underlying, unconsolidated sediment. This process would have been accelerated by the presence of high pore fluid pressures owing to an impermeable surface layer (hardground) and loading, resulting from both sedimentation and cementation. Where submarine cementation was a continuous process, as on the upper slope, and uninterrupted sequences of nodular limestone were developed, sediments were stable and debris flows were not generated. End_of_Article - Last_Page 995------------

Local Scour Around Bridge Pier in Tidal Current

A strong tidal current whose direction reverses periodically causes a complex local scour around a bridge pier and this scour has a bad influence on the stability of the bridge. This local scour is caused by an unbalance between the input and output sediment transport rates around the pier. In this paper some basic features of a local scour around a rectangular pier in a tidal current are investigated experimentally and, using these data, further analyses on the similarity between a model and its prototype are made.