Subaerial Erosion Research Articles

Subaerial Exposure and Erosion in Capitan Reef (Permian), Guadalupe Mountains, New Mexico: ABSTRACT

A prominent subaerial exposure surface has been identified within the Capitan reef in Rattlesnake Canyon. Equivalent to the middle Yates interval in the back reef, the exposure surface records planation of the reef during a sea level drop to below the shelf edge. This is the first exposure surface recorded within the reef, although several are known within the back reef facies. The surface is quite planar and can be traced shelfward for nearly 1 km before it is lost in near-back reef grainstones. The surface does not appear to correlate directly with one of the Yates sandstone beds. Channels cut into the surface have a maximum relief of nearly 2 m. Gypsum molds (now calcite filled) and mud cracks are found in the channel-filling sands. In nonchannel areas, a thin (1 to 5-cm) zone of reddened pebbles is sometimes present. Although the exposure horizon has only a thin sediment veneer, the extensive truncation of the underlying beds implies significant sediment transport across the surface. This is presumably reflected in one of several prominent clastic lowstand wedges found in the Bell Canyon Formation of the Delaware basin.

Genetic Stratigraphic Sequences in Basin Analysis I: Architecture and Genesis of Flooding-Surface Bounded Depositional Units

Marine basin margins are characterized by repetitive episodes of progradation punctuated by periods of transgression and flooding of the depositional platform. The resultant stratigraphic units consist of genetically related (1) depositional systems and their component facies sequences; (2) bypass, nondepositional, and erosional surfaces; and (3) in thick sequences affected by gravity tectonics and crustal response to loading, syndepositional structural discontinuities. Units are bounded by hiatal surfaces preserved as submarine unconformities or condensed sedimentary veneers and that record maximum marine flooding of the basin margin. The repetitive stratigraphic architecture is the product of the ongoing interplay among sediment supply, basin subsidence (and uplift), an eustatic sea level change. Each of these three variables may dominate depositional evolution; furthermore, stratigraphic architecture is very similar regardless of the dominant control. A genetic stratigraphic sequence is the sedimentary product of a depositional episode. The sequence incorporates and reconciles depositional systems, bedding geometries, and bounding surfaces within the framework of cycles of basin-margin offlap and flooding. Each sequence consists of the progradational, aggradational, and retrogradational or transgressive facies deposited during a period of regional paleogeographic stability. The defining genetic stratigraphic sequence boundary is a sedimentary veneer or surface that records the depositional hiatus that occurs over much of the transgressed shelf and adjacent slope during maximum marine flooding. The genetic sequence paradigm emphasizes preserving the stratigraphic integrity of three-dimensional depositional systems and does not rely n widespread development of subaerial erosion surfaces caused by eustatic falls of sea level to define sequence boundaries. The physical stratigraphic record of transgression and flooding--distinctive thin but widespread facies sequences, prominent erosional surfaces, and superjacent marine condensed intervals or sedimentary veneers--provides readily recognized, regionally correlative, easily and accurately datable, and robust sequence boundaries that commonly define times of major basin-margin paleogeographic reorganization in terrigenous clastic basins.

Origin and provenance of submarine volcaniclastic rocks in the Late Permian Drake Volcanics, New South Wales

The Late Permian, intermediate to silicic, calc‐alkaline Drake Volcanics of New England, NSW, consist largely of shallow marine, redeposited volcaniclastic rocks, which host epithermal silver‐gold deposits. Pervasive hydrothermal alteration is widespread. The 200–300 m thick Cataract River Member includes several sedimentation cycles composed of massive pumice breccias, overlain by laminated sandstone and siltstone, interbedded with marine, fossiliferous mudstone. Two distinct groups of clasts within the volcaniclastic beds include: (1) lithics of predominantly intermediate‐composition lavas that have undergone subaerial epiclastic erosion and transportation; and (2) pumice, quartz and plagioclase crystals that have not been reworked by epiclastic processes. The lithic clasts were probably derived locally in a subaerial environment within the Drake Volcanics, and pumice and crystal fragments from contemporaneous centres of silicic volcanism elsewhere in New England. The latter group of clasts was transpor...

Proterozoic canyons of the Flinders Ranges (South Australia): submarine canyons or drowned river valleys?

A number of kilometer-sized erosional incisions (canyons) occur within a specific stratigraphic interval in the late Proterozoic succession of the Adelaide Geosyncline, Flinders Ranges, South Australia. This paper describes in detail four adjacent east-west orientated incisions in the northern Flinders Ranges which are interpreted to be part of an ancient sinuously meandering canyon system. At least 600 m of siltstones and fine sandstones of the Bunyeroo and the Brachina Formations were removed during the erosional event that produced the canyon. Infilling sediments, which exhibit a similar succession in each incision, were deposited during emplacement of the Wonoka Formation. Although an identical sedimentary succession occurs within each incision, palaeocurrents determined from flute casts at the bases of sandstone beds alternate by close to 180° from one erosional feature to the next. Basal infilling sediments commonly contain lenticular breccias, interpreted as slumps derived from the Brachina Formation, which forms the incision walls. It is significant that clasts in these breccias preserve evidence of lithification predating the erosional event that produced the incisions. Channel-filling debris-flow units which occur mainly above the basal slumps, along with associated sandstones, were most likely emplaced episodically. Asymmetric ripple cross-laminae in the sandstone beds above and within debris-flow units commonly are wave modified. Isolated occurrences of hummocky cross stratification are observed in most units of the incision-fill. A thin ( < 100 cm) but extensive limestone unit, developed above the erosional unconformity east of the canyon and on southern canyon walls, has a non-marine isotopic signature which suggests subaerial exposure of at least upper portions of canyon walls. The final position of what is interpreted as a laterally migrating thalweg channel is located adjacent to the north wall of each incision. Current debate centers on whether the incisions represent the meandering portion of an ancient submarine canyon system, or whether they were cut subaerially and were subsequently drowned and filled by coastal onlap. Both models are discussed in this paper. Sinuous meanders can form on a variety of scales, both in subaerial as well as in submarine environments. The abundance of wave-generated and wave-modified sedimentary structures within the canyon fill and the presence of what appears to be a non-marine limestone precipitated on the canyon walls favour the subaerial erosion model.

Evolution of marine climates of the U.S. Atlantic coast during the past four million years

Marine climatic and sea-level changes in the eastern United States show two distinct modes: a gradual, directional Pliocene warming that ended with an abrupt regression, and a quasi-cyclic, high-amplitude, high-frequency middle-late Pleistocene pattern of alternating glacials and interglacials. Pliocene marine sediments of the Duplin Formation, deposited during a period of high sea level between 4.0 and 2.8 Ma BP, contain increasing percentages of tropical and subtropical ostracods, signifying a gradual warming. After maximum warm-water temperaturesca. 3.2-2.8 Ma BP, sea level dropped; this was followed by extensive subaerial erosion between about 2.8 and 2.0 Ma BP. This series of events reflects the emergence of the Isthmus of Panama between about 3.5 and 3.0 Ma BP, concomitant intensification of warm Gulf Stream flow along the eastern U.S.A., and initial Pliocene glaciation in the Northern Hemisphere. In the middle-late Pleistocene, glacial-interglacial cycles occurred with a periodicity ofca. 100 ka. Four (possibly five) emerged interglacial marine sequences correlate with deep-sea oxygen-isotope stages 13/11, 7, 5, and 1. During some interglacials, however, climatic conditions ranged from full interglacial warmth to cool, nearly interstadial conditions; this observation indicates short-term regional climatic variability.

Shattuck Member of Queen Formation (Permian), Northwest Shelf, Permian Basin: Eolian Sandstone Deposited During a Lowstand of Sea Level: ABSTRACT

The Queen Formation is a sequence of carbonates, evaporites, and sandstones of Permian (Guadalupian) age that is found across the subsurface Northwest shelf of the Permian basin. The formation is a major reservoir for oil and gas, and one of its producing zones is a thick sandstone (equivalent to the Shattuck sandstone) that defines the top of the Queen. Studies of logs and cores of the Shattuck from Caprock and Central Corbin fields (Chaves and Lea Counties, New Mexico) were undertaken to determine its sedimentary history. The Shattuck is a wedge-planar sand body that thickens downdip (Caprock to Corbin) from 6 to 19 m. It generally has sharp (in some places clearly erosive) contacts with dolomite and anhydrite that lie above and below it. The Shattuck is composed of well-sorted arkosic to subarkosic silty sand, and it contains the following sedimentary structures (in order of decreasing abundance): (1) high-angle cross-bedding, (2) planar wind-rippled laminae, (3) wavy and discontinuous adhesion laminae, (4) wavy and lenticular-bedded water ripples, and (5) contorted and massive beds with small nodules of anhydrite. The cross-bedded and wind-rippled sandstones are generally reservoirs for oil and gas; the remaining lithologies are not reservoirs. The Shattuck was deposited inmore » continental eolian environments during a lowstand of sea level. This lowstand exposed highstand marine and shoaling sediments (carbonates and evaporites) beneath the Shattuck to subaerial erosion and weathering, and they were soon buried by windblown sand. The sand accumulated in desert dunes (cross-bedded sandstone), sand sheets (erges) that were alternately dry (planar-laminated sandstone) and wet (adhesion laminae, water ripples), and sabkhas (anhydritic sandstone). Deposition of the Shattuck was terminated by a sudden rise in sea level and the return of marine depositional environments.« less

Geology and Hydrocarbon Potential of Dawson Bay Formation Carbonate Unit (Middle Devonian), Williston Basin, North Dakota: ABSTRACT

The Middle Devonian Dawson Bay Formation carbonate unit is present in the subsurface of North Dakota except where truncated by postdepositional erosion. The carbonate unit thickens from the erosional limit to a maximum thickness of 47.5 m (156 ft) in Renville County and reaches a maximum depth of 3798 m (12,460 ft) below the surface in McKenzie County. In North Dakota, a submarine hardground separates the carbonate unit from the underlying second red bed member of the Dawson Bay Formation. The upper contact with the Souris River Formation is conformable except in those areas where the Dawson Bay Formation was exposed to subaerial erosion prior to deposition of the Souris River sediments. The Dawson Bay carbonate unit is predominantly dolomitic and fossiliferous limestone or fossiliferous dolostone. The carbonate unit can be subdivided into five lithofacies on the basis of characteristic fossil fauna, flora, and other lithologic features. Lithofacies analysis of the Dawson Bay carbonates suggests a shallowing-upward succession of depositional environments and associated energy zones as follows: shallow epeiric sea (very low energy), stromatoporoid biostrome/bioherm (low energy), very shallow epeiric sea (very low energy), restricted shallow epeiric sea (extremely low energy), and shallow epeiric sea shoreline (variable energy). Eogenetic diagenesismore » includes color-mottling, dolomitization of micrite to microcrystalline dolomite with penecontemporaneous anhydrite replacement of cryptalgal mudstones and boundstones, cementation by sparry calcite, and vuggy porosity development. Mesogenetic diagenesis includes formation of mosaic dolomites, cementation by blocky equant calcite, neomorphism, pressure-solution, fracturing, halite cementation, and hydrocarbon emplacement.« less

Evolution and petroleum geology of Amlia and Amukta intra-arc summit basins, Aleutian Ridge

Amlia and Amukta Basins are the largest of many intra-arc basins formed in late Cenozoic time along the crest of the Aleutian Arc. Both basins are grabens filled with 2–5 km of arc-derived sediment. A complex system of normal faults deformed the basinal strata. Although initial deposits of late Micocene age may be non-marine in origin, by early Pliocene time, most of the basinfill consisted of pelagic and hemipelagic debris and terrigenous turbidite deposits derived from wavebase and subaerial erosion of the arc's crestal areas. Late Cenozoic volcanism along the arc commenced during or shortly after initial subsidence and greatly contributed to active deposition in Amlia and Amukta Basins. Two groups of normal faults occur: major boundary faults common to both basins and ‘intra-basin’ faults that arise primarily from arc-parallel extension of the arc. The most significant boundary fault, Amlia-Amukta fault, is a south-dipping growth fault striking parallel to the trend of the arc. Displacement across this fault forms a large half-graben that is separated into the two depocentres of Amlia and Amukta Basins by the formation of a late Cenozoic volcanic centre, Seguam Island. Faults of the second group reflect regional deformation of the arc and offset the basement floor as well as the overlying basinal section. Intra-basin faults in Amlia Basin are predominantly aligned normal to the trend of the arc, thereby indicating arc-parallel extension. Those in Amukta basin are aligned in multiple orientations and probably indicate a more complex mechanism of faulting. Displacement across intra-basin faults is attributed to tectonic subsidence of the massif, aided by depositional loading within the basins. In addition, most intra-basin faults are listric and are associated with high growth rates. Although, the hydrocarbon potential of Amlia and Amukta Basins is difficult to assess based on existing data, regional considerations imply that an adequate thermal history conducive to hydrocarbon generation has prevailed during the past 6-5 my. The possibility for source rocks existing in the lower sections of the basins is suggested by exposures of middle and upper Miocene carbonaceous mudstone on nearby Atka Island and the implication that euxinic conditions may have prevailed during the initial formation of the basins. Large structures have evolved to trap migrating hydrocarbons, but questions remain concerning the preservation of primary porosity in a sedimentary section rich in reactive volcaniclastic debris.

Sediments of ice-dammed, self-draining Ape Lake, British Columbia

The glacier damming Ape Lake has withdrawn from its Neoglacial terminal moraines in the lake since early in this century at an average rate of 15 m/a. As a result, the first known drainings of Ape Lake occurred in October 1984 and August 1986. In each event, about 54% of the volume of the lake was lost through a tunnel in the damming glacier. Most of the remaining water was held in the deep basin of the lake behind partially breached Neoglacial terminal moraines. As the glaciers have withdrawn, the character of the sediments has changed. Sediments in the deep basin of the lake are varved, but the grain size, especially of the summer deposits, has decreased and rates of sedimentation have decreased from about 4 mm/a to less than 2 mm/a. In shallow water, deposition of varved sediments has given way to the deposition of massive sediments at rates of less than 1 mm/a. Ice-rafted debris is rare in deep water, despite the presence of calving bergs.During and following the first draining, significant subaerial erosion occurred as a result of the event itself, the drawdown and steepened gradients, and the action of waves on the exposed sediments as the lake refilled. In proximal areas, distinct deposits within the normal winter deposits are recognised. In deep water, deposition of massive, highly underconsolidated, fine-grained sediments occurred. Organic debris released from shallow deposits by erosion has become concentrated in both shallow- and deep-water sediments. Within a year of the first draining, sediment loads in the lake water were returning to normal.

Morphology from subaerial erosion of a Mediterranean seamount

The morphology of a seamount in the Valencia Trough of the Western Mediterranean has been investigated with side-looking sonar (Sea MARC I) and swath-mapping sonar (Sea Beam). The seamount is characterized by deep reentrants on its flanks, radiating spurs and a broad summit plateau. Presently its summit lies 1020 m below sea level. Avalanche scars and debris flow deposits are detected on re-entrant floors and slopes. Fragments of aphanitic basalt and andesitic tuff, sampled at DSDP Site 122 attest to the volcanic origin of the Valencia Seamount. First constructed as a subsea volcano about 20–25 m.y. ago, its flanks became almost entirely emerged 6 m.y. ago at the time of the evaporitic drawdown of the Mediterranean. Valencia Seamount has some features in common with young and old seamounts on the floor of the Pacific Ocean that have never emerged above sea level and other features which may be attributed to subaerial exposure. Two working hypotheses, erosional and constructional, are presented to explain the observed morphology. The erosional hypothesis better accounts for the known paleogeographic and paleoceanographic history of the Mediterranean. The mapped spurs are interpreted as flank rift zones which give many seamounts a starfish-like appearance. Fountain eruptions of hyaloclastites from vents could explain the broadly arched summit plateau. Peaks might be secondary parasitic cones or remanent erosional features of stocks and dykes which are more resistent to mass-wasting than an extrusive cover of vesicular lava flows and deposits of tuff and ash. Gullies are most abundant on the middle and lower flanks. Whereas the gullies might be breached calderas, it is more reasonable to expect that they are dominantly subaerial erosional in origin, considering the emergence history of this particular seamount.

Hylton Northwest Field's Tectonic Effect on Suggs Ellenburger Producing Area, Nolan County, Texas: ABSTRACT

An evaluation of the geology of Hylton Northwest field in southeastern Nolan County, Texas, indicates that the pre-Pennsylvanian tectonics associated with this field may have affected the producing zone of Suggs Ellenburger field 6 mi (9 km) west. Both fields are located along the Fort Chadbourne fault system of the Eastern shelf of the Midland basin. The study of the depositional environment of the Suggs Ellenburger field reveals some interesting aspects of the tectonostratigraphic terrane that appears to have in part influenced the development of the reservoir rock. The tectonics of the Cambrian-Ordovician (Ellenburger) period in Hylton Northwest field created a southwest-trending fault system with associated fractures. The fractures allowed percolating surface waters to leach carbonate rocks in the area, creating vuggy secondary porosity in the intercrystalline rock fabric. The faults were modified to a karst topography by periods of subaerial erosion of the Cambrian-Ordovician depositional plain. Sea level fluctuations that occurred in the area were associated with the alternating uplift and subsidence of the Hylton Northwest field's tectonic feature. As a result, environmental zones of porosity with varying vertical subaerial erosion formed within the overall Cambrian-Ordovician (Ellenburger) interval. The producing zone of the Suggs Ellenburger field occurs at approximatelymore » 6,400 ft (1,951 m).« less

Strontium-isotope stratigraphy of Enewetak Atoll

/sup 87/Sr//sup 86/Sr ratios determined for samples from a 350 m core of Neogene lagoonal, shallow-water limestones from Enewetak Atoll display a remarkably informative trend. Like the recently published data for Deep Sea Drilling Project (DSDP) carbonates, /sup 87/Sr//sup 86/Sr at Enewetak increases monotonically but not smoothly from the early Miocene to the Pleistocene. The data show intervals of little or no change in /sup 87/Sr//sup 86/Sr, punctuated by sharp transitions to lower values toward greater core depths. The sharp transitions correlate with observed solution disconformities caused by periods of subaerial erosion, whereas the intervals of little or no change in /sup 87/Sr//sup 86/Sr correspond to intervals of rapid accumulation of shallow-water carbonate sediments. When converted to numerical ages using the published DSDP 590B trend, the best-resolved time breaks are at 282 m (12.3 to 18.2 Ma missing) and 121.6 m (3.0 to 5.3 Ma missing) below the lagoon floor. At Enewetak, Sr isotopes offer a stratigraphic resolution for these shallow-marine Neogene carbonates comparable to that of nannofossil zonation in deep-sea carbonates (0.3-3 m.y.). In addition, the correlation of times of Sr-isotope breaks at Enewetak with times of rapid Sr-isotope change in the DSDP 590B samples confirms the importance ofmore » sea-level changes in the evolution of global-marine Sr isotopes and shows that the Sr-isotope response to sea-level falls is rapid.« less

The transition from a passive margin to an Upper Cretaceous foreland basin related to ophiolite emplacement in the Oman Mountains

The Upper Cretaceous Muti Formation in the Oman Mountains exemplifies the transition from a passive continental margin to a foreland basin related to thrusting and ophiolite emplacement. Field data largely substantiate theoretical models that depict a flexural forebulge moving continentward of an advancing thrust-load, followed by flexural downwarping and collapse as nappes override a continental margin. Five units of the Upper Cretaceous (? late Cenomanian–late Campanian) Muti Formation depositionally, or structurally, overlie Mesozoic passive-margin paleoenvironments ranging from abyssal plain to continental rise, slope, and carbonate platform. The Arabian continental margin and Tethys ocean rifted in Permian and Triassic time, giving rise to a mature passive margin in the Jurassic and Lower Cretaceous. At the end of Lower Cretaceous time, northeastward subduction apparently began, and the Semail ophiolite formed, probably in a marginal basin setting. As subduction continued, the trench migrated toward the Arabian continent preceded by a flexural fore-bulge that soon reached the adjacent Arabian continent (Cenomanian-Turonian; ca. 90−88.5 Ma), halting deposition regionally (“Wasia-Aruma break”). Flexural uplift was greatest near the shelf edge, with removal of as much as 600 m of the platform edge succession, and redeposition onto the base of slope as slump sheets, debris flows, and lithoclastic turbidites. As the advancing thrust-load impinged on the edge of the Tethys ocean, the crust was downflexed (late Coniacian–Campanian time; 88.5−73 Ma), creating a foredeep that partly filled with terrigenous clastics, olistostromes, detached blocks of rift-related carbonate build-ups, seamount volcanics and continental basement rocks. The basin depocenter migrated inboard with time, collapsing, in turn, Mesozoic abyssal plain, base of slope, and slope settings to establish a foreland basin on the foundered shelf. Terrigenous sediments accumulated in deep water below the carbonate compensation depth (CCD), together with lithoclastic limestones and detached blocks shed from submarine fault scarps. Successive foreland basins were overridden by the advancing nappes. During the later stages of thrusting onto the continent, Mesozoic sediment decollement nappes were bulldozed ahead of the Semail nappe, ploughing into the existing foreland basin. In conjunction with the final emplacement of the Semail ophiolite, the foreland basin migrated to its most continentward location and filled with clastics derived by subaerial erosion of the Semail nappe and other units. The nappes were submerged again in late Maastrichtian time, possibly in response to flexural relaxation, followed by re-establishment of a carbonate platform. Further compression and thrusting in the Paleocene-Eocene gave rise to a successor foreland basin adjacent to the northern Oman Mountains front.

Morphology from subaerial erosion of a mediterranean sea-mount : Barone, A.M. and W.B.F. Ryan, 1987. Mar. Geol., 74(3–4):159–172

Morphology from subaerial erosion of a mediterranean sea-mount : Barone, A.M. and W.B.F. Ryan, 1987. Mar. Geol., 74(3–4):159–172

Capital Reef Complex (Permian), Guadalupe Mountains, Southwestern United States: A Classic Sedimentologic Model in Flux: ABSTRACT

The Capitan reef complex of west Texas and New Mexico has been an important sedimentologic model since a reef origin was proposed for the Capitan Limestone 1929. The Capitan's magnificent exposures the Guadalupe Mountain area; its large scale; its variety of carbonate, sandstone, and evaporite facies; and its relationship to major petroleum resources of the Permian basin have made it a justly famous sedimentary geologic model for academic and industrial geologists alike. Since 1950, extensive research has yielded markedly contrasting sedimentologic interpretations of key features, such as the nature and origin of the Capitan massive (reef wall); the back-reef pisolite, sandstone, and evaporite facies; the depositional profile of the shelf and shelf edge; the impor ance and magnitude of sea level fluctuations; and the role of submarine, vadose, and phreatic diagenesis. Early views of a barrier reef depositional profile have been replaced by a shelftop marginal mound profile, which the mound's gentle crest coincided with the backreef pisolite and tepee facies. The Capitan massive, earlier considered an ecologic barrier reef, is now interpreted as an outstanding example of massive limestone formed at a submerged shelf edge where extensive submarine cementation lithified sponge wackestones and formed massive cement boundstones. Permian vadose diagenesis, earlier accorded much importance and inferred as being related to major sea level falls, appears negligible. Phreatic diagenesis by mixed meteoric and marine fluids was at least locally important the Capitan massive and foreslope strata. The famous Guadalupe pisolite, interpreted until the mid-19 0s as lagoonal, and then widely accepted as Permian caliche of vadose origin, is now reinterpreted as largely synsedimentary, formed by subaqueous precipitation from hypersaline waters of a peritidal shelf crest; associated vadose fabrics are minor, overprint isopachous pisolite fabrics and are unrelated to major intraformational erosion surfaces. Subaerial erosion surfaces within the reef complex are largely localized high on the shelf marginal mound. Unequivocal evidence of emergence of the Capitan massive or its underlying foreslope has not been recognized, suggesting that any sea level lowering during Capitan deposition did not exceed a few tens of meters. Although a sabkha origin of the back-reef evaporites and eolian transportation of sand across a sabkha surface has conceptual appeal, a lagoonal origin of the evaporites and subaqueous deposition of the back-reef sandstone sheets better fits the available field evidence. Vast amounts of siliciclastic sand had to bypass the Capitan massive and upper foreslope facies, but neither channels nor scoured erosion surfaces have been identified. The Capitan reef complex can serve not only as a world class sedimentologic model, but also as a much studied model not yet well understood. The model is one whose scientific investigators have frequently been afflicted by over-reliance on Holocene models (modern model mesmerism), by overthrust of established authorities or dogma (dogma reverence), by uncritical acceptance of new concepts (bandwagonitis), by overuse of superficial look-alike features for End_Page 1441------------------------------ genetic analogies, and by just not discerning what was available for viewing the field or laboratory. Capitan remains a model in flux awaiting more critical field and laboratory research. End_of_Article - Last_Page 1442------------

Mixing zone dolomitization of Devonian carbonates, Guangxi, South China

Summary Regional facies relationships indicate that dolomitization around the Lower-Middle Devonian boundary in Guangxi region is largely restricted to platform margin facies. In the Liujin area dolostones are represented by the Tangding and succeeding Yingtang Formations. The latter comprises alternations of laminated fine-grained dolostones (type A dolomite) and massive, more coarsely-crystalline dolostones (type B dolomite). Type B is richer in marine fossils and other allochems, and increases in importance upwards. The Formation is bounded above by a disconformity marked by a haematitic marl, interpreted as a subaerial erosion surface, and is succeeded by limestones lacking dolomite. Petrographic observations indicate that the two dolomite types represent the dolomitization of originally different sedimentary facies by a single dolomitization process. The dolomite is dominantly replacive, but also partly cementing. Variably negative δ 13 C and δ 18 O values (interpreted as original) and covariation of these parameters with Sr and Na points to dolomitization by mixed meteoric-marine fluids. Stratigraphic variation in geochemical data is inconsistent with a single episode of subaerial exposure and therefore points to repeated emergence. Dominantly replacive dolostones can form by the mixing-zone mechanism given sufficiently stable conditions.

World slope map

A colored slope map (scale 1:80,000,000) was generated from digital land and sea floor elevations at 5‐min spacings from data tapes from the National Geophysical Data Center. Slope analysis is facilitated by examining the average slope in 250‐m altitude‐depth intervals both globally and in each of eight tectonic plates. The most striking feature of the map is the multiple parallel bands of steep slope at subduction zones. Submarine volcanic chains produce zones of steep slope that show little degradation at the scale of the map since the Cretaceous. The average slope, or roughness, of spreading ridges decreases exponentially with increasing spreading rate. The passive continental margins generate a steep zone at 1–2 km depth. The lowest average global slope (0.2°) occurs at sea level and reflects the base level that the sea provides for subaerial erosion and the upper limit for marine sedimentation. Slope minima also are caused by the abyssal plains at 4–6 km depth and by glacial ice caps at 3 km elevation.

Lower Pocono Coastal Embayments, in Western Maryland and Vicinity, Related to Latest Devonian Eustatic Sea Level Rise: ABSTRACT

New data from an exposure of upper Hampshire-lower Pocono in western Maryland document a transgressive-regressive sequence of three distinct depo-facies in this outcrop. A deltaic-alluvial plain facies of unfossiliferous Hampshire red beds is conformably overlain by a brackish fossiliferous facies of gray and green Pocono beds, disconformably overlain by a fining-upward, point-bar facies of unfossiliferous gray Pocono beds. The fossiliferous facies coarsens up from sideritic, Planolites-burrowed shales and siltstones bearing Lingula and Camaro-toechia to fine-grained, Skolithus-bearing sandstones. This subtidal to intertidal depo-facies accumulated in a protected, shallow, brackish bay affected only by tides and storms. An eastward-thinning, lower Pocono transgressive tongue occurs throughout the western Maryland area. This transgression, which halted the westward progradation of the Hampshire delta complex, resulted from a base-level rise. Resultant submergence of the delta complex created several estuaries and bays. Coastal sand barriers in West Virginia protected these embayments. Base-level drop caused westward regression, incision of major streams near their mouths, and some subaerial erosion of the emerged coastal area. Regional stratigraphy supports correlation of the lower Pocono transgression in Maryland with the latest Devonian transgression in Ohio (Cleveland shale) and Pennsylvania (Oswayo shale). The late Fammenian eustatic cycle (IIf) from the Devonian eustatic seamore » level curve may explain the lower Pocono transgression-regression. Locally, the Devonian-Mississippian boundary may be the disconformity beneath the Pocono point-bar facies.« less

Quantitative Analyse von Korallengemeinschaften des Sanganeb-Atolls (mittleres Rotes Meer). I. Die Besiedlungsstruktur hydrodynamisch unterschiedlich exponierter Außen- und Innenriffe

The Sanganeb-Atoll off Port Sudan is an elongate annular reef which rests on a probably raised block in the fracture zone along the Red Sea-graben. Its gross morphology was most likely formed by subaerial erosion during low sealevel conditions. Features of its topography and hydrography are described. The prevailing wind waves are from NE, Hence, the outer and inner reef slopes are exposed to different hydrodynamic conditions. The sessile benthos was analysed using the quadrat method. Four test quadrats (5×5 m each) were selected on the outer and inner slopes at a depth of 10 m along a SSW-NNE transect across the atoll. Cnidaria were by far the most dominating group; coralline algae, Porifera, Bryozoa and Ascidia, however, counted for just under 3 % living cover. Light and temperature intensities did not differ significantly at the sites studied; water movement, however, decreased in the following order: TQ IV (outer NE side of the reef ring) was exposed to strong swell and surf; TQ II (inner side of the SW-ring) was met by a strong longreef current; TQ I was situated on the outer lee of the SW-atoll ring and TQ III in the inner lee of the NE-side. This hydrodynamic gradient correlates with the composition of the coral communities from predominantly branching Scleractinia (staghorn-like and otherAcropora species andPocillopora) in TQ IV, through aLobophyllia, Porites andXenia-dominated community in TQ II, and a mixed community with an increasing percentage of xeniid and alcyoniid soft corals in TQ I, to a community in TQ III which is dominated by the soft coralsSinularia andDendronephthya. The cnidarian cover ranged between 42.4 and 56.6 % whereby the two exposed test quadrats had a higher living coverage than the protected ones. In total, 2649 colonies comprising 124 species of stony, soft and hydrocorals were recorded by an elaborate method of accurate in-situ mapping. The 90 scleractinian species found include 3 species new to the Red Sea and 11 hitherto unknown species from its central part. Only 7 cnidarian species covered large areas, forming aXenia macrospiculata-zone in TQ I, a Lobophyllia corymbosa-zone in TQ II, aSinularia-Dendronephthya-zone in TQ III and anAcropora-Pocillopora verrucosa-zone in TQ IV. Hydrodynamic exposition and ratio of genus abundanceAcropora: Montipora are closely correlated: on the windward side (TQ IV) it is 943:4, but on the leeward side (TQ I) 18:208. Apart from the taxonomic and quantitative distribution of species and colonies, types of growth form and categories of the dead substrate are given. Since soft corals do not contribute to the reef framework, the distribution of Scleractinia and Alcyonaria indicates that at Sanganeb-Atoll reef substance is mainly generated on the windward side.

Seismic stratigraphy of the flexural moat flanking the Hawaiian Islands

The elastic thickness of oceanic lithosphere, based on flexure studies in the region of surface loads, is two to three times smaller than its seismic thickness1. Thus, as a load ages, the thickness of the mechanically supportive part of the lithosphere must decrease from its short-term (seismic?) thickness to its long-term mechanical thickness. Extrapolation of data from experimental rock mechanics suggests that thinning occurs by brittle failure in the uppermost part of the lithosphere and thermally activated creep in its lower part2. Modelling shows that for load ages less than about 1 Myr, flexural moats change shape and the peripheral bulges migrate in towards the load3. These transient changes in flexure are best manifest, we believe, in the stratigraphical sequences that infill flexural moats. The largest thicknesses of sediments occur in moats flanking orogenic belts4,5. However, the stratigraphical record in these basins is complicated by the effects of compaction, sub-aerial erosion and sea-level changes, and by the fact that the loading history extends over very long periods of time. Here we describe a multichannel seismic reflection profile across a flexural moat flanking the volcanic load of Oahu in the Hawaiian Islands. The advantages of Oahu are that it is located in the interior of a plate, the load is 2.7 Myr old and formed within 1 Myr6, and the flexural moat was not significantly affected by sea-level changes and erosion. We show that the observed stratigraphical patterns cannot be explained by changes in material influx, load distribution, and rapid relaxation of the lithosphere to its long-term mechanical thickness. Rather, the observed patterns suggest a prolonged relaxation (>1 Myr) due to either local or regional re-heating of the lithosphere. This re-heating has important implications for the relationship between elastic thickness and age of the lithosphere at the time of loadings.