Erosional Escarpment Research Articles

Variations in the along‐axis segmentation of the Afar Rift system

Both oceanic and continental rifts show regular along‐axis segmentation, but the relationship, if any, between the two is poorly understood. The tectonically active East African Rift system encompasses systematic along‐axis variations in extension and magmatism, making it possible to explore the links between strain, magmatism, and the length scales of faulting and depositional systems during rift development. We summarize Quaternary along‐axis segmentation within the Afar Rift system, which is transitional to seafloor spreading, and compare it with segmentation in a “continental” rift sector, the Main Ethiopian Rift (MER) system to the south. We use high‐resolution satellite imagery calibrated by field studies and digital topography data to delineate faults and magmatic centers in unmapped areas and to compare with existing geological reports. From south to north we see a significant and systematic decrease in the lengths of young basin‐bounding normal faults (∼50 km long in MER to ≤ 15 km long in northern Afar), in the lengths and widths of rift basins (from 50 to 100 km long × 30 to 80 km wide in MER to ≤ 20 km long × ≤ 5 km wide in northern Afar), and in the relief of the uplifted rift flanks surrounding the basins (from ≥ 1000 m high in MER to ≤ 100 m high in northern Afar). Erosional escarpments along the older, outer rift margins of Afar reveal long, high border fault segments similar to those found in the less evolved MER, suggesting that the Afar Rift was originally bounded by much longer faults. In northern Afar, there is a drastic increase in the volume of Quaternary basalts, with segmentation now dominated by 50‐ to 80‐km‐long volcanic ridges, which are similar in size, morphology, and spacing to the second order, nontransform offset segmentation of slow‐spreading mid‐oceanic ridges. The along‐axis changes in rift segmentation are concomitant with decreasing crustal thickness (i.e., cumulative strain), decreasing effective elastic thickness estimates, and increasing magma supply. These temporal and spatial patterns suggest that segmented continental rifts can evolve into segmented oceanic rifts dominated by mantle sourced magmatic processes.

Erosional dynamics, flexural isostasy, and long‐lived escarpments: A numerical modeling study

Erosional escarpments are common features of high‐elevation rifted continents. Fission track data suggest that these escarpments form by base level lowering and/or marginal uplift during rifting, followed by lateral retreat of an erosion front across tens to hundreds of kilometers. Previous modeling studies have shown that this characteristic pattern of denudation can have a profound impact upon marginal isostatic uplift and the evolution of offshore sedimentary basins. Yet at present there is only a rudimentary understanding of the geomorphic mechanisms capable of driving such prolonged escarpment retreat. In this study we present a nonlinear, two‐dimensional landscape evolution model that is used to assess the necessary and sufficient conditions for long‐term retreat of a rift‐generated escarpment. The model represents topography as a grid of cells, with drainage networks evolving as water flows across the grid in the direction of steepest descent. The model accounts for sediment production by weathering, fluvial sediment transport, bedrock channel erosion, and hillslope sediment transport by diffusive mechanisms and by mass failure. Numerical experiments presented explore the effects of different combinations of erosion processes and of dynamic coupling between denudation and flexural isostatic uplift. Model results suggest that the necessary and sufficient conditions for long‐term escarpment retreat are (1) incising bedrock channels in which the erosion rate increases with increasing drainage area, so that the channels steepen and propagate headward; (2) a low rate of sediment production relative to sediment transport efficiency, which promotes relief‐generating processes over diffusive ones; (3) high continental elevation, which allows greater freedom for fluvial dissection; and (4) any process, including flexural isostatic uplift, that helps to maintain a drainage divide near an escarpment crest. Flexural isostatic uplift also facilitates escarpment retreat by elevating topography in the vicinity of an eroding escarpment, thereby increasing channel gradients and accelerating erosion which in turn generates additional isostatic uplift. Of all the above conditions, high continental elevation is common to most rift margin escarpments and may ultimately be the most important factor.

Flexural constraints on the denudation of assymetric mountain belts

The regional excavation of metamorphic rocks from mid and lower crustal levels is the consequence of erosion‐driven denudation of mountain belts that are in permanent isostatic equilibrium. If the erosion process occurs at different rates in different parts of a topographically asymmetric mountain belt, then the flexural properties of the lithosphere need to be considered in order to describe the features of the isostatic rebound. Such a situation is given in plateaulike mountain ranges which are likely to erode only at their steeply sloping edges whereas erosion on top stagnates. This paper investigates the flexural isostatic response of a plate that is unloaded due to the continuous retreat of an erosional escarpment. The isostatic response is tested for two limiting model cases: (1) for a plate with a broken edge and (2) for a continuous plate with variable flexural rigidity. It is shown that for both cases the rate of topography development on the exposed plate is not linear with the retreatrate of the escarpment. This feature is utilized to predict the nature of the subsequent erosion of the newly formed topography and ultimately of the denudation of the plate. Geological limitations of the theoretical model and its relevance to plateaulike topographies in southern Africa and the eastern highlands of Australia are then discussed. In these areas the origin of the apparently cyclic rebound forces creating “stairlike” topography at their continental margins and the old age of their present surface topography has previously been unexplained. Within the presented model these features can be explained satisfactorily. Two important conclusions may be drawn: (1) The distance of flexural swells from great escarpments and the origin of subsequent stair‐like topographies may be explained by simple flexural considerations. (2) The flexural rebound forces uplifting a plate exposed by a retreating escarpment are cyclic. Excavation times of metamorphic rocks under such mountain belts may therefore be prolonged not only by the nature of erosion of their initial topography but also by the nature of the subsequent isostatic rebound and ultimately their denudation history.

Submarine-fan development in the southern Chile Trench: A dynamic interplay of tectonics and sedimentation

Submarine fans are well developed in the southern Chile Trench, from 33°S to 41°S latitude. SeaMARC-II side-scan sonar and seismic reflection records image steep erosional escarpments, as much as 400 m in relief, extending seaward across the trench basin from the mouths of submarine canyons. The scarps bisect trench fans into paired lobes of contrasting morphology where gravity flows either follow or oppose the gradient of the axial trough. Fanlobes are depositional and constructional up-gradient (south) from the canyon mouths. They are composed of aggraded channel/levee complexes, smooth and conformable sediment drapes, and crescentic levees rimming the headwalls of erosional scarps. Fanlobes are carved and dissected by erosional processes down-gradient (north) from the canyon mouths. They exhibit amalgamated lag pavements, composite sediment lobes, longitudinal furrows, braided channels, and canyon-mouth bars. Thick, massive-to-laminated sand and gravel with abundant scour surfaces were sampled from the erosional fanlobes, whereas fine-grained turbidites with expanded hemipelagic intervals typify the depositional fanlobes. The texture and composition of the sediment supply, the onshore climate, and the tectonic perturbations of the axial gradient affect the morphologic development of trench fans. Stratigraphic intervals recording periods of intensified fan erosion and progradation of the axial channel are manifested by channeled, high-amplitude seismic facies: reflective lenticular bodies, truncation and scour surfaces, planar-amalgamation, and sigmoidaccretion structures. The severe erosional dissection of down-gradient fanlobes and the northerly encroachment of the axial channel are best developed in the surficial strata of the trench basin. Extensive sediment remobilization and efficient longitudinal transport sculpted the present fan surface and are correlated with the last glacial maximum. The trench fans may have been deposited on progressively steeper trench gradients, because the buoyant Chile Rise migrates northward as it subducts beneath the Andean continental margin. Fan distributary channels, axial channels, slump scars, and erosional gullies are largely localized along structural features. Normal faults propagate through the sedimentary cover and create elongate depressions on the sea floor that capture the high-velocity mainstreams of turbidity currents. Orthogonal fault sets within the deposits of the trench basin, which parallel the spreading and transform structures of the extinct Pacific-Farallon Rise and the Chile Rise, are evidently reactivated during subduction by flexure of the oceanic basement along the outer wall of the trench basin. Uplifted thrust ridges, generally restricted to a narrow zone along the base of the deformation front, are dissected by distributary channels, and channel courses are locally deflected seaward of these propagating structures. Transform-oriented basement ridges, associated with strike displacements of the axial channel and vertical faults in the trench basin, may accommodate renewed strike-slip motion as they enter the subduction zone and thereby influence the debouchment points of submarine canyons to the trench basin.

Potential Geologic Hazards near the Thistle Landslide, Utah County, Utah

In mid-April, 1983, an old landslide near Thistle, in Utah County, Utah, began to move, and within days had blocked Spanish Fork Canyon. As the slide9s movement continued, construction crews gradually converted the toe of the slide into an earth-filled dam—Thistle Dam—that impounded northwest-flowing Spanish Fork River. The resultant reservoir, known as Thistle Lake, was subsequently drained because of uncertainty about the stability of the dam. Recently, officials of Utah County have explored various alternatives for a water-retention structure in the area, including utilization of Thistle Dam. The Thistle Slide Committee, established by the State of Utah to evaluate the suitability of using Thistle Dam, suggests that construction of a new dam upstream from the present one might be a more reasonable and cheaper solution than investigating the stability of the present dam. Two potential geologic hazards that could impact a dam site upstream from the Thistle Dam, however, may be in the area: the Thistle Canyon fault and the Thistle Creek diapiric fold. Uncertainty shrouds the existence of both. The Thistle Canyon fault is a postulated high-angle normal fault that trends about N20°E through Thistle. The fault, downthrown on the east, separates an erosional escarpment formed on the Charleston-Nebo thrust plate from younger overlying Cretaceous and Tertiary sedimentary rocks. The Thistle Creek diapiric fold theoretically trends about N30°E through the area. Tenuous evidence suggests that the Middle Jurassic Arapien Shale, an evaporite-rich intrusive sedimentary unit that forms the core of the fold, was overridden by the upper plate of the Charleston-Nebo thrust fault. Since then, the Arapien has welled upward, arching both the thrust plate and the overlying younger sedimentary cover. Additional field investigations should be completed to determine the existence of these and other geologic hazards prior to any final decision about a new dam. The presence of either or both of these hazards, however, does not necessarily preclude the construction of a safe and stable dam that would impound a multi-purpose reservoir.

Carbonate platform depositional sequences, Upper Cretaceous, south-central Pyrenees (Spain)

The Upper Cretaceous (Cenomanian-Maastrichtian) of the south-central Pyrenees shows five carbonate platform sequences where the major parameters are tectonism, relative sea-level fluctuations and inherited depositional profile. Depositional geometries and basin analysis permit an understanding of the depositional history. Five depositional sequences have been recognized: (1) The Santa Fe sequence (Middle-Upper Cenomanian), a ramp to a skeletal rimmed shelf with an escarpment bypass margin. The lower boundary is an angular unconformity and the upper one records a sea-level drop. The platform location of the margin was determined by a listric normal fault. (2) An abrupt sea-level rise drowned the former platform. The Congost sequence (Turonian-Lower Coniacian), a distally steepened ramp with erosional distal deep slope. The depositional model was largely controlled by pre-existing basin morphology. Cessation of platform development was due to a relative sea-level drop. (3) The Sant Corneli sequence (Upper Coniacian-Lower Santonian), a mixed terrigenous-skeletal homoclinal ramp with upright margin, deep slope and dysaerobic basin. The slope results from the backstepping by 24 km of the previous margin and gentle basin tilting. The platform margin remained more or less at the same position, and relief between platform and slope increased indicating continuous relative sea-level rise. The upper boundary is an angular unconformity at the platform margin produced by an abrupt sea-level rise and platform drowning, and by listric normal faulting. (4) The Vallcarga sequence (Upper Santonian-Campanian), a distal-steepened skeletal homoclinal ramp, erosional escarpment and turbidite basin, which corresponds to the Mesozoic maximum marine expansion. A listric normal fault created two depositional areas: a more or less flat footwall block with a north-northwest prograding carbonate ramp.

Red Bluff, Marion County, Mississippi: A Citronelle Braided-Stream Deposit: ABSTRACT

Red Bluff is an erosional escarpment located on the western margin of the Pearl River flood plain in northwestern Marion County, Mississippi. The bluff shows approximately 30 m (100 ft) of relief and is composed of alternating units of red-to-yellow sand and sandy gravel. The sand grains are composed primarily of quartz, with small amounts of heavy minerals and feldspar. The gravel is composed of varying percentages of chert, flint, jasper, rip-up clasts, quartz, and tripoli, including a small fraction of silicified Paleozoic fossils. Grain-size analysis of the sediment and investigation of the sedimentary structures suggest a braided-fluvial environment of deposition. Particle sizes in the medium sand to pebble range predominate in all units; very little silt and clay is present. The largest particles present are boulder-size rip-up clasts. The most conspicuous sedimentary structures at Red Bluff are graded bedding, low-angle to medium-angle cross-bedding, and well-developed paleochannels. A statistical comparison (discriminant analysis) of the seven most abundant heavy minerals of Red Bluff, with the same suite of heavy minerals found at the type section of the Citronelle Formation (Pliocene-Pleistocene), and outcrops of a known Miocene coarse clastic unit indicates a correlation of Red Bluff to the Citronelle Formation. These heavy minerals are kyanite, staurolite, rutile, tourmaline, zircon, black opaques (primarily ilmenite and magnetite), and white opaques (primarily leucoxene). The suite of heavy minerals present at Red Bluff belongs to the east Gulf province. This metamorphic assemblage of heavy minerals implies the source area of the sediments at Red Bluff to be the southern Appalachians. The silicified pebble-size Devonian-Mississippian fossils were derived most likely from formations flanking the southern Appalachians in northern Alabama. End_of_Article - Last_Page 1472------------

Sedimentation in Mississippi Trough, Gulf of Mexico: ABSTRACT

Interpretation of high-resolution seismic data indicates that the Mississippi Trough was eroded, and then partially filled, by submarine gravity flows. These flows occurred primarily during late Quaternary stages of lowered sea level. Present-day mass transport appears to be a combination of seafloor creep and low-velocity turbid-layer flow. An acoustically chaotic seismic facies characterizes the deeper parts of the trough fill. Visible reflectors are discontinuous, wavy, and subparallel; they commonly disappear into almost reflectorless, seismically homogeneous units. Sediments deposited lateral to the chaotic facies are commonly composed of continuous, parallel reflectors. These strong reflecting units were deposited as onlapping fill and ponded sediments, and many are confined to semienclosed depressions within the trough walls. These various seismic-facies units represent the freezing-in stage of submarine-canyon sedimentation, and may result from separate submarine debris flows. The deposits of the Mississippi Trough debris flows are fine-grained sediments generated by mass failures of oversteepened deposits which occurred at the mouth of the ancestral Mississippi River. High-energy gravity flows thoroughly mixed the depositional material, leaving little bedding to produce coherent seismic-reflector patterns. Debris flows generated lower velocity gravity flows, which moved independently and lateral to the main flows. The onlapping fill deposits and sediment ponding were deposited by lower velocity flows. Recent sediments, which were sampled by piston coring along the axis of the trough, are rapidly deposited, hemipelagic, olive-gray silty clays. Sedimentary structures are limited to scattered, very thin laminae and thin beds. Clay sedimentation has been continuous during the late Quaternary, as is revealed by the clays containing mixtures of indigenous planktonic Foraminifera, displaced shallow-water microfauna, and terrigenous mineral grains. Large diapirs controlled the position and flow direction of the main erosional channel. Small feeder channels were eroded into the walls of the trough. Trough-wall sediments cover a steep erosional escarpment; they have moved downslope by slump and creep failure. An isopach map of the canyon fill, above erosional surfaces, outlines a linear channel-fill deposit. End_of_Article - Last_Page 449------------

Uranium Deposits of Gas Hills, Wyoming: ABSTRACT

The Gas Hills uranium district is in central Wyoming along the southeastern margin of the Wind River basin. The initial discovery was made by Neil E. McNeice in the fall of 1953. Earliest development of the district's ore reserves was quite slow, but accelerated when the larger, more experienced mining firms became active in the area. Surface drilling proved to be the most effective tool for finding and developing the uranium ore reserves. New drilling techniques were needed to obtain samples of the below-water-table ores. Many methods were tried, but frozen core drilling and bucket augering proved to be the most reliable for obtaining accurate samples. Since the initial discovery, the Gas Hills uranium district has produced about 12% of the United States total production. The present land surface is characterized by barren, subdued, rolling hills. These are traversed locally by steeply dipping hogback ridges of older, more resistant rocks, which are the flanks of truncated, northward plunging folds formed by crustal disturbances prior to the deposition of the Wind River Formation. A steep erosional escarpment that rises abruptly above the north sloping basin floor bisects the region and divides the surface drainage between tributaries of the Wind River on the north and tributaries of the Sweetwater River on the south. Volcanism occurred during late Eocene time, as evidenced by relic vents at the southern end of the Rattlesnake Hills, and by local volcanic debris in the middle and upper Eocene rocks. Sedimentary rocks exposed in the Gas Hills uranium district include sandstone, limestone, dolomite, shale, and tuffaceous sandstone, mudstone, and shale. They range in age from Cambrian to Miocene and have a composite thickness of over 14,000 ft. The source beds for the uranium deposits are arkosic sandstones interstratified with lenticular mudstones and shales. Two distinct types of sandstone are present in the Wind River Formation. The youngest is yellowish-orange to yellowish-gray arkose, derived primarily from Precambrian gneissoid and granitoid rocks; it contains little clay, abundant calcium carbonate, and limonite cement, and is host for all uranium deposits of the district. The second type of sandstone is pale yellowish-gray to pale olive, derived from areas of schists of Precambrian age; it contains abundant clay matrix. There are four types of uranium deposits in the district, the most important being the solution-front deposits. They were formed along the margins of highly altered, tabular sand beds that are enclosed by overlying and underlying fine-grained siltstone, claystone, and carbonaceous mudstone beds. Solution fronts can be followed for long distances and individual ore bodies along them may reach thousands of feet in length. The solution fronts are ideally crescentic or C-shaped when viewed in cross section, with thin mineralization forming the tips of the crescents. The uranium minerals occur as earthy brown to black coating on, and interstitial fillings between, the quartz-sand grains. The primary uranium ore minerals are coffinite and uraninite. The three other types of deposits include transitional-bedded, oxidized, and residual-remnant deposits. Several quite large transitional bedded deposits have been mined, but the oxidized and residual-remnant deposits are commonly small and difficult to mine. Ground waters, trapped by the southward tilting of the Tertiary rocks during late Miocene time, became stagnant. These waters dissolved uranium and other elements from the enclosing rocks, and after erosion had exposed the highest beds of the Wind River Formation, the mineral-rich solutions gained egress from the enclosing sand aquifers on the north and the solution-front ore deposits began to form. End_of_Article - Last_Page 905------------

Tectonic Development of the Triassic Newark-Gettysburg Basin in Pennsylvania

Research Article| March 01, 1973 Tectonic Development of the Triassic Newark-Gettysburg Basin in Pennsylvania RODGER T. FAILL RODGER T. FAILL 13407 Rutherford St., Harrisburg, Pennsylvania 17111 Search for other works by this author on: GSW Google Scholar Author and Article Information RODGER T. FAILL 13407 Rutherford St., Harrisburg, Pennsylvania 17111 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1973) 84 (3): 725–740. https://doi.org/10.1130/0016-7606(1973)84<725:TDOTTN>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation RODGER T. FAILL; Tectonic Development of the Triassic Newark-Gettysburg Basin in Pennsylvania. GSA Bulletin 1973;; 84 (3): 725–740. doi: https://doi.org/10.1130/0016-7606(1973)84<725:TDOTTN>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The Newark-Gettysburg basin, one of several Triassic basins in eastern North America which have long been described as fault-troughs or grabens, is herein interpreted to have developed by simple downwarp, filled with sediment from all sides. Igneous intrusion occurred late in basin filling. Following deposition, the basin was tilted north and northwestward with simultaneous faulting and folding.The strongest evidence against a graben origin is the absence of a continuous system of faults along the north and northwest margin, and the lack of syndepositional faulting either within the basin or at the basin margins. Aeromagnetic and gravity data do not support the presence of buried faults under overlapping sediment. The coarse, poorly sorted fanglomerates at the basin margins are not sufficient evidence of syndepositional faulting. Erosional escarpments at the basin margins were of sufficient relief to produce these fanglomerates during the arid climate extant during deposition.The absence of large syndepositional faults along the basin margins, the limited range of igneous compositions (tholeiitic basalt), and the mimetic arrangement of the Triassic basins along the Appalachian Mountain system contrasts markedly with the well-substantiated margin faults, the wide range of igneous compositions (generally alkaline), and the transection of pre-existing regional structures in other continental graben systems, such as the East African rift system and the Rhine graben. The Newark-Gettysburg basin is most similar to the early stage of the Baikal rift, which initiated as a downwarp with crustal thinning and basaltic effusions. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

REGIONALS AND RESIDUALS IN SEISMIC PROSPECTING FOR STRATIGRAPHIC FEATURES

Many geological features associated with oil accumulation show up on seismic maps as interruptions of regional trends rather than as true structural closures. Among such features are reefs, which are often best detected by draping of overlying formations; erosional escarpments which truncate porous limestone beds on their updip sides; and buried ridges which cause productive stratigraphic buildups in overlying beds. In the presence of regional tilting, seismic indications from such features can be so obscured that special data‐processing techniques are required to make them readily recognizable on seismic maps. The problem here is very similar to that of separating gravity and magnetic effects of features having economic interest from regional background. Residual techniques developed for accomplishing this type of separation can be applied advantageously to seismic data where regional structure obscures significant anomalies. Both contour‐smoothing or grid methods can be used depending on the nature of the problem and the preference of the interpreter. As with gravity or magnetics, the grid methods are particularly adaptable for high‐speed electronic computation. Some examples are shown where regional effects are removed from seismic maps over known reefs and productive erosional escarpments by techniques using electronic computation. A somewhat different approach is necessary when it is desired to remove the effect of velocity variation from time maps by treating the velocity function as a regional effect. Here the regional is multiplicative rather than additive and cross‐product terms must be taken into account. By relating the time maps and velocity maps using this approach, the principal hazards of using time maps for interpretation can be avoided.

Regionals and Residuals in Seismic Prospecting for Stratigraphic Features: ABSTRACT

Many geological features associated with oil accumulation show up on seismic maps as interruptions of regional trends rather than as true structural closures. Among such features are reefs, which are often best detected by draping of overlying formations; erosional escarpments which truncate porous limestone beds on their updip sides; and buried ridges which cause productive stratigraphic build-ups in overlying beds. In the presence of regional tilting, seismic indications from such features can be so obscured that special data processing techniques are required to make them readily recognizable on seismic maps. The problem here is very similar to that of separating gravity and magnetic effects of features having economic interest from regional background. The techniques developed for accomplishing this type of separation, such as residual and second derivative determinations and orthogonal polynomials, can be applied advantageously to seismic data where regional structure obscures significant anomalies. Both contour-smoothing or grid methods can be used depending on the nature of the problem and the preference of the interpreter. As with gravity or magnetics, the grid methods are particularly adaptable for high-speed electronic computation. Some examples are shown where regional effects are removed from seismic maps over known reefs and productive erosional escarpments by derivative and residual techniques using electronic computation. A somewhat different approach is necessary when it is desired to remove the effect of velocity variation from time maps by treating the velocity function as a regional effect. Here the regional is multiplicative rather than additive and cross-product terms must be taken into account. By relating the time maps and velocity maps using this approach, the principal hazards of using time maps for interpretation can be avoided. End_of_Article - Last_Page 1769------------

Ensaio sobre a Tectonica moderna do Brasil

Ensaio sobre a Tectonica moderna do Brasil