Erosional Escarpment Research Articles

Anatomical changes in dwarf shrub roots provide insight into aeolian erosion rates in northeastern Iceland

Iceland is known for having strong aeolian erosion events, as evidenced by erosional escarpments on the soil surface; these are known locally as rofabards. The aim of the study was to estimate aeolian erosion rates at two research plots affected by severe erosion, using anatomic features of the roots of Arctic, woody, dwarf shrubs. Ours is the first study to utilize dendrochronological analyses of exposed roots of dwarf willows (Salix herbacea L.) as an indicator of aeolian erosion. When dwarf shrub roots are exposed to the atmosphere by erosion, cell sizes are reduced by >50 %, with maximum changes in individual plants exceeding 150–200 %. We detected the erosion signal by analyzing changes in cell-size and width of growth rings in the roots. Using this relationship, were able to estimate erosion rates (the retreat of escarpments) since the 1970s for the study area. During the 1970s and 1980s, erosion rates were fairly constant, with retreat ranging from 1.0 to 2.5 cm yr−1. Rapid increases in erosion rates were observed after the late 1990s, occasionally as much as 4.4–5.4 cm yr−1. Increased aeolian erosion rates may be linked to continuous heavy grazing by sheep and climate change, as this period coincides with increased numbers of hot and warm, and dusty days. Our results indicate that dendrochronological methods can be highly useful in determining past geomorphic activity, due to modern environmental changes observed in the Arctic. The methods employed here can be applied even in treeless areas.

Subglacial evolution from distributed to channelized drainage: Evidence from the Lake Murtoo area in SW Finland

AbstractDuring the last millennia of the Weichselian glaciation (c. 11 700–14 000 cal BP) and the first millennium of the Holocene interglacial (c. 10 500–11 700 cal BP), melting of the ice sheet and subsequent subglacial drainage resulted in the formation of murtoos and murtoo‐related landforms in the Fennoscandian Ice Sheet area in Finland and Sweden. Murtoos are composed of clay‐poor diamictons produced by sediment‐concentrated creep and material sorting in a subglacial environment under effective pressure close to zero, and potentially represent a transition form from non‐channelized to channelized subglacial drainage networks. Here, we explore the geomorphology and substratum characteristics of the Lake Murtoo area, one of the locations where murtoos were initially found and described in SW Finland. Our interpretation suggests an interconnected development from inefficient distributed or semi‐distributed cavities (murtoos) to efficient channelized drainage (esker) via erosional escarpments, all displayed in the same region. In addition, our excavation into a murtoo‐related erosional escarpment (MRE) revealed subglacial erosional channels alongside MREs that were filled with fine‐grained finely laminated sediments. We suggest that these channel fill sediments were deposited subglacially, because they are covered by flow till and show spatial and lithological characteristics that deviate significantly from the proglacial basin fill clay (varve) sediments in the area. The appearance of rhythmically laminated silt and clay sediments indicates sedimentation in a suspension‐load‐dominated environment in shallow subglacial channels with relatively low (but repetitiously changing) water flow velocities, possibly reflecting disconnected spill‐over routes of subglacial drainage in small tributaries that were only periodically used due to water pressure changes in the up‐ice direction.

A New Look at Landslides of the Vermilion and Echo Cliffs, Northern Arizona

ABSTRACT The Vermilion and Echo Cliffs form a nearly continuous escarpment more than 160 km long within the Colorado Plateau physiographic province of North America. The cliffs overlie the Marble Platform in northern Arizona and are located along the Colorado River, just upstream of the Grand Canyon. Large rotational block landslides mantle the erosional escarpment along most of its extent. Although these landslides have been noted for over 100 years, their likely origin has never been explained. Landslide failure surfaces appear to be influenced by the Petrified Forest Member of the Triassic Chinle Formation, a shale layer containing smectite clay weathered from volcanic ash. Although landslides are common along the majority of escarpments comprising the Colorado Plateau where the Petrified Forest Member and other shales outcrop, most appear to have been inactive since the early Holocene. Multiple generations of landslides and remnants of previous slides exist up to 3 km from the present cliff face. Multiple working hypotheses explaining these landslides are explored in this article, including past landslides and/or lava dams along the Colorado River within the Grand Canyon, periods of wetter climate with higher groundwater levels, and earthquakes related to nearby faulting and volcanism. Various sliding modes along these cliffs are described along with potential triggering mechanisms. Back-analysis of these landslides has been conducted using mechanical properties of the formations involved as well as varying groundwater levels. Calculated factors of safety for existing slides under present conditions are greater than unity, consistent with their apparent stability.

Rates of Historical Anthropogenic Soil Erosion in the Midwestern United States

AbstractErosion degrades soils, reduces crop yields, and diminishes ecosystem services. The total amount of soil that has been eroded since the initiation of farming is unknown in most agricultural landscapes, which hinders assessment of soil erosion trends. In the Midwestern U.S., erosion has caused native prairie remnants to become perched above surrounding farmland, providing an opportunity to measure historical soil erosion rates. We use high‐resolution topographic surveys conducted across erosional escarpments at the boundary between 20 prairies and adjacent agricultural fields and show the median reduction in soil thickness ranges from 0.04 to 0.69 m, corresponding to erosion rates of 0.2–4.3 mm year−1, with a median value of 1.9 mm year−1. We used an association between the measured reduction in soil thickness and topographic curvature to predict regional soil erosion integrated since the beginning of farming to the present. We estimate a median historical erosion rate of 1.8 ± 1.2 mm year−1, which is nearly double the rate considered tolerable by the U.S. Department of Agriculture (USDA). Current soil loss predictions from the USDA National Resources Inventory (NRI) and the Daily Erosion Project (DEP) are lower than our historically averaged erosion rate by a factor of 3 and 8, respectively. We suggest that the NRI and the DEP underpredict soil loss rates because they do not include tillage erosion, a process shown to be important throughout the Midwestern U.S. Our findings indicate that further implementation of conservation practices is needed to reduce the high centennial‐averaged soil erosion rates that we measure to sustainable levels.

How a New Cenozoic Geology and Glacial History Paradigm Explains Topographic Map Drainage System and Erosional Landform Evidence: Elbert and Lincoln Counties, Colorado, USA

Detailed topographic map drainage system and erosional landform evidence such as drainage route orientations, drainage divides, divide crossings (low points on drainage divides), erosional escarpments, and similar features in the east central Colorado Elbert and Lincoln County region are considered as pieces of a complex but solvable drainage history puzzle. A satisfactory solution to date has eluded investigators who have worked from the accepted Cenozoic geology and glacial history paradigm (accepted paradigm) perspective in which climatic and tectonic factors operating over long time periods lead to what might be considered to be a randomly determined regional drainage history. A new and fundamentally different Cenozoic geology and glacial history paradigm (new paradigm) in which immense and prolonged south-oriented continental ice sheet meltwater floods flowed across the Elbert and Lincoln County area which at that time was near the rising rim surrounding a thick continental icesheet created and occupied deep “hole”. Map evidence documents how northeast-oriented Republican River headwaters valleys eroded headward across must have been large southeast-oriented floods probably moving toward what at that time would have been an actively eroding and deep east-oriented Arkansas River valley head and how those massive southeast-oriented floods subsequently lowered the Colorado Piedmont surface before being beheaded and reversed when the deep northeast- and east-oriented South Platte River valley eroded headward to create in an identifiable sequence (from east to west) what are now long north-oriented South Platte River tributaries. New paradigm predicted massive and prolonged south-oriented meltwater floods flowing across what must have been a rising region explains much, if not all of the Elbert and Lincoln County detailed topographic map drainage system and erosional landform evidence.

Post Gondwana breakup evolution of the SE Australia rifted margin revisited

AbstractLow‐temperature thermochronology (LTT) is commonly used to investigate onshore records of continental rifting and geomorphic evolution of passive continental margins. The SE Australian passive margin, like many others, has an elevated plateau separated from the coastal plain by an erosional escarpment, presumed to originate through Cretaceous rifting prior to Tasman Sea seafloor spreading. Previous LTT studies have focused on reconciling thermal histories with development of the present‐day topography. New apatite LTT data along an escarpment‐to‐coast transect define a classic “boomerang” (mean track length vs. fission‐track age), indicating variable overprinting of late‐Palaeozoic cooling ages by a younger, mid‐Cretaceous cooling event. Regionally, however, the boomerang trend diverges NNW away from the coast and crosses the escarpment, implying the underlying thermal history pre‐dates escarpment formation and is largely independent from post‐breakup landscape evolution. We suggest that Cretaceous cooling might relate to erosion of Permo‐Triassic sedimentary cover from a formerly more extensive Sydney Basin.

How a Fundamentally Different and New Glacial History Paradigm Explains North America Glaciated Prairie Region Erosional Escarpments and Drainage Patterns

Scientific paradigms are frameworks of ideas governing how a discipline conducts its research. Paradigms by themselves are neither correct nor incorrect, but are judged on their ability to explain evidence and to open up research opportunities. The commonly accepted glacial history paradigm requires North American glaciated prairie region erosional landform features, such as erosional escarpments and abandoned valleys associated with the north-oriented Bell River drainage system, to be pre-glacial in origin. While considerable literature is based on such interpretations those escarpments and abandoned valleys are formed in easily eroded bedrock and should not have survived continental ice sheet erosion. In addition to defying common sense logic the pre-glacial origin of those erosional escarpments and abandoned valleys is not well understood. A new paradigm requiring at least one continental ice sheet to have occupied a deep North American “hole” (formed by deep ice sheet erosion and ice sheet caused crustal warping) offers geomorphologists an opportunity to explain the erosional escarpments as remnants of canyon walls originally formed when supra-glacial rivers sliced ice-walled and bedrock-floored canyons into a decaying continental ice sheet’s surface and the abandoned north-oriented Bell River drainage system valleys to have been eroded as the ice-walled and bedrock-floored canyon network captured and diverted massive melt water floods onto and then across the decaying ice sheet’s floor and then in northeast and north directions between detached and semi-detached ice sheet remnants. The diversion of immense melt water floods from the Gulf of Mexico to the North Atlantic Ocean triggered climatic change that ended the first ice sheet’s melting. Water in the newly formed north-oriented drainage systems then froze between the detached and semi-detached (and greatly thinned) ice sheet remnants to create a second and much thinner ice sheet and to complete creation of the glaciated prairie region glacial features seen today.

Belle Fourche River-Cheyenne River Drainage Divide Area in the Wyoming Powder River Basin Analyzed by Topographic Map Interpretation Methods, USA

The dearth of scientific literature in which specific erosional landform origins are determined is an example of what Thomas Kuhn considered a scientific crisis. Scientific crises arise when scientists following their discipline’s established paradigm’s rules, or doing what Kuhn calls normal science, cannot explain observed evidence. Scientific crises are resolved in one of three ways. Normal science may eventually explain the evidence and normal science returns, the unsolved problems may be identified and labeled and left for future scientists to solve, or a new paradigm may emerge with an ensuing battle over its acceptance. To succeed any new paradigm must demonstrate its ability to explain the previously unexplained evidence and also open up new research opportunities. During the 20th century’s first half regional geomorphologists abiding by their discipline’s paradigm rules unsuccessfully tried to explain origins of numerous erosional landforms, such as drainage divides and erosional escarpments. Their failures eventually caused the regional geomorphology discipline, at least that part of the discipline concerned with determining specific erosional landform origins, to almost completely disappear. A new and fundamentally different geomorphology paradigm that requires massive southeast-oriented continental ice sheet melt-water floods to have flowed across the Powder River Basin has the ability to explain specific erosional landform origins and is demonstrated here by using detailed topographic map evidence to show how large southeast-oriented floods eroded the Powder River Basin’s Belle Fourche River-Cheyenne River drainage divide segment, eroded through valleys now crossing that drainage divide segment, eroded the Powder River Basin’s Belle Fourche River valley, established Belle Fourche and Cheyenne River Powder River Basin tributary valley orientations, and eroded the north-facing Pine Ridge Escarpment. The success of this and other similar new paradigm demonstrations suggest many if not all specific erosional landform origins can be determined.

Inventory and analysis of geological and topographic distribution of “Balze” crags in the Upper Valdarno basin (Tuscany region, Italy)

In this work we provide an analysis and an updated inventory map of the main outcropping crags, known as Balze in Italy, in the Upper Valdarno basin (Tuscany, Italy). These erosional forms develop in the alluvial-fan deposits that characterize this area. The study area extends up to 30 km2 within the local ANPIL (Natural Protected Area of Local Interest) and can be ascribed as a geomorphosite. Detection and mapping of Balze exposures were outlined on a pre-existing broad raster database, and then improved by means of photo-interpretation of optical imagery and analysis of Digital Terrain Model (DTM) and of topographic layers at 1:2,000 and 1:10,000 scale, supported by field work. A total of 151 Balze exposures were mapped and classified according to their progressive erosional stages, i.e.linear or indented escarpments, spire rocks and pinnacles or residual pillars. Furthermore, the spatial distribution of Balze scarps was investigated with respect to the geological and geomorphological setting and to the landscape zonation of the study area. Results of the inventory and analysis of geological and topographic distribution of the Balze crags can be useful for the characterization of the geomorphosite in the framework of its activity and management.

History, Biogeography, Biodiversity and Bioprospecting: Scenario from the Western Ghats

The history of the Western Ghats goes back to the event of breaking‐up of the super continent of Gondwana, during the continental drift, some 150 million years ago. The origin is a debatable subject among geologists with several views and theories based on different kinds of evidences. Fault escarpment theory suggest that mountains of the Western Ghats are results of a gigantic fault due to an upward distorting of a large rigid shield of the Indian peninsula during the volcanic period. Erosional escarpment hypothesis advocates the origin of the Western Ghats due to coastal planation whereas Oldham proposed that the Western Ghats as a dead cliff. Biogeographically, the Western Ghats represents 4% of India's land region with high torrential rainfall which experiences monsoon and tropical climate with high variation in wind speed makes it one of the ten bio‐geographic zones in India. With nearly 40% of the total number of species being endemic, the Western Ghats is considered to be one among the hotspots in the world but facing severe threats. Bio prospecting may help to ensure the participation of indigenous ethnic people in biodiversity protection and sustainable resource utilization. A National policy has to be designed in such a way that local ethnic communities get benefits from the utilization of natural resources by marketing natural resources.

Location and Associated Carbon Storage of Erosional Escarpments of Seagrass Posidonia Mats

Seagrasses of the genus Posidonia can form an irregular seascape due to erosional processes exposing thick walls of organic matter-rich soils. However, little is known about the location and characteristics of these particular formations. Here we provide comprehensive estimates of organic carbon (Corg) storage in P. oceanica and P. australis meadows, while providing insight into their location and mechanisms of formation, and highlighting future research directions. Erosional reef escarpments are restricted to shallow highly productive P. oceanica meadows from the Mediterranean Sea and P. australis meadows from the Indian Ocean, and sustain the existence of Corg-rich deposits in surrounding meadows. The thickness of the mat escarpments can reach up to 3 m and their length can vary from few to hundreds meters. Mechanisms of formation appear to differ among sites, from naturally-induced escarpments by wave action and/or tidal flow to human-induced escarpments by dredging activities. The inter-twined remains of seagrass shoots within the sediment matrix consolidate the sandy substrate and hold the exposed Posidonia mat escarpments together, maintaining a semi-rigid structure. This phenomenon is unusual but of exceptional importance in marine biogeochemical cycles, revealing the largest Corg sinks among seagrasses worldwide (ranging from 15-176 kg Corg m-2 in 2 m-thick mats accumulated at 2-249 g Corg m-2 yr-1 over 300 to 3000 yr).

Late Devonian Carbonate Margins and Foreslopes of the Lennard Shelf, Canning Basin, Western Australia, Part A: Development During Backstepping and the Aggradation-To-Progradation Transition

Abstract Carbonate reefal margin and foreslope settings are characteristically heterogeneous and difficult to predict due to a spectrum of sediment source factories, resedimentation processes, resultant deposit types, and controlling parameters. In particular, the effects of changes in long-term accommodation on the composition, architecture, and sediment distribution patterns of carbonate margin–foreslope–basin systems are poorly understood. Upper Devonian (Frasnian) outcrop exposures along the Lennard Shelf, northeast Canning Basin, Western Australia, were investigated to assess the development of reefal margin and foreslope settings during long-term 1) platform backstepping with aggradational pulses, and 2) across the transition from platform aggradation to progradation. Measured sections tied to interpreted photomosaics and detailed mapping using aerial photographs were collected from the South Lawford Range and Windjana Gorge areas. The exposures reveal distinctive differences in foreslope grain composition, deposit characteristics and proportions, margin morphology, and stratigraphic expression 1) during platform evolution between backstepping events, and 2) depending on position within the long-term accommodation setting. Between backstepping events, aggradational margins can be classified as “growth escarpments” with associated grain-dominated, onlapping foreslope deposits. Margins across the long-term transition from aggradation to progradation evolved from erosional escarpments with onlapping debris deposits to accretionary, interfingering configurations. Development of growth escarpments between backstepping events was a function of vertical reefal growth from sustained high accommodation conditions during the long-term transgressive systems tract, coupled with a Frasnian reefal assemblage that responded to light and tracked relative sea level. This net vertical reefal growth also resulted in relative margin stability and the deposition of grain-dominated foreslopes. Conversely, margins were highly unstable and underwent repeated failure across the long-term aggradation-to-progradation transition, reflecting a lack of underlying substrate to support basinward advance, and resulting in debris-dominated foreslopes. These observations provide relationships that predict margin and foreslope associations of facies-scale heterogeneity and seismic-scale geometry within a low-frequency sequence stratigraphic framework.

From dust to dust: Quaternary wind erosion of the Mu Us Desert and Loess Plateau, China

The Ordos Basin of China encompasses the Mu Us Desert in the northwest and the Chinese Loess Plateau to the south and east. The boundary between the mostly internally drained Mu Us Desert and fluvially incised Loess Plateau is an erosional escarpment, up to 400 m in relief, composed of Quaternary loess. Linear ridges, with lengths of ∼10 2 –10 3 m, are formed in Cretaceous- Quaternary strata throughout the basin. Ridge orientations are generally parallel to near-surface wind vectors in the Ordos Basin during modern winter and spring dust storms. Our observations suggest that the Loess Plateau previously extended farther to the north and west of its modern windward escarpment margin and has been partially reworked by eolian processes. The linear topography, Mu Us Desert internal drainage, and escarpment retreat are all attributed to wind erosion, the aerial extent of which expanded southeastward in China in response to Quaternary amplification of Northern Hemisphere glaciation.

Differentiating between gravitational and tectonic faults by means of geomorphological mapping, trenching and geophysical surveys. The case of the Zenzano Fault (Iberian Chain, N Spain)

The Zenzano Fault scarp has been developed on the top of a 475m-high erosional escarpment underlain by an upper brittle sequence and a lower halite-bearing evaporitic formation. The 1850m-long fault, with a maximum displacement of 45m and parallel to the escarpment, has offset the back of a dip slope, generating an antislope scarp with prominent triangular facets up to 38m high and obstructing consequent transverse drainages (defeated streams). Both gravitational and tectonic interpretations were previously proposed for this active surface rupture. The fault scarp has been investigated by detailed geological and geomorphological mapping, geophysical surveys (GPR and ERT) and two trenches excavated across the fault in disrupted drainages. The geophysical profiles clearly imaged the fault, helping to site the trenches and roughly estimate the thickness of the deposits accumulated in the downthrown block. Although the maximum displacement to length ratio (Dmax/L) of the fault fall within the range of tectonic faults of similar length, according to the world database, the geological and geomorphological context, together with several parameters reveal that this structure corresponds to a gravitational failure: (1) anomalously high vertical displacement per event (>1.4m); (2) high apparent vertical slip rate (~0.6mm/year); and (3) short recurrence interval of faulting events. The negligible horizontal displacement component on the dip-slip fault, as well as the presence of very large caprock collapse sinkholes and saline springs, strongly suggest that interstratal karstification of the halite-bearing evaporites has played a primary role in the development of the gravitational failure. The formation of this morphostructure has been favoured by pre-existing tectonic faults, erosional unloading related to the removal of an overburden ca. 800m thick, and enhanced groundwater recharge due to the blockage of the drainage. This work illustrates that the proposed multidisciplinary approach allows the identification of features and calculation of parameters that may help to elucidate whether an active failure has a gravitational or a tectonic origin.

An overfilled lacustrine system and progradational delta in Jezero crater, Mars: Implications for Noachian climate

The presence of valley networks and open-basin lakes in the late Noachian is cited as evidence for overland flow of liquid water and thus a climate on early Mars that might have supported precipitation and runoff. Outstanding questions center on the nature of such a climate, its duration and variability, and its cause. Open basin lakes, their interior morphology, and their associated channels provide evidence to address these questions. We synthesize the extensive knowledge of terrestrial open basin lakes, deltaic environments, and fluvial systems to assess these questions with evidence from Jezero crater, a 45km diameter open basin lake and its 15,000km2 catchment area, ∼645-km long drainage network, interior sedimentary facies, and ∼50km long outlet channel system. We document the presence of extensive scroll bars and epsilon cross-bedding, both indicative of meandering distributary channels that are not observed on alluvial fans but are typical of fluvial-deltaic depositional environments. A fluvial-deltaic environment is further supported by the post-formational erosion of the deltaic complex: the present-day “delta front” is actually an erosional escarpment truncating delta plain features with the clay-rich prodelta environment, predicted from facies models to make up the outer third of the complex, having been largely removed by eolian erosion. The extensive development via lateral accretion of scroll bars and epsilon cross-bedding, and the reconstructed sedimentary architecture suggest a stable baselevel, in contrast to an environment of constantly rising and falling baselevel related to variable input and evaporation that would favor incision during lowstands. The development of the outlet channel is interpreted to have provided baselevel control in the Jezero open-basin lake. The maturity of the outlet channel, in contrast to the catastrophically scoured landscapes typical of dam-breach channels, favors a consistent overfilled hydrology for the paleolacustrine environment. Sediment transport modeling studies of other valley network and related deposits on Mars have suggested durations in the decades to centuries range. We review meander migration rates in terrestrial fluvial environments to provide a comparison for considering the temporal stability implied by the evolution of scroll bars; values of 20–40 years are not uncommon for the structures and migration implied by observations in Jezero. Taking sediment accumulation rates from a variety of terrestrial fluvial-lacustrine environments in conjunction with our estimates of the sedimentary basin-fill thickness suggest timescales of the order of 106–107 years, far longer than implied by some sediment transport models, but still a short period of time geologically. The presence of significant residual accommodation space (space available for potential sediment accumulation) in Jezero indicates that sediment transport into the lake terminated before the basin was completely filled. Climate conditions sufficient for sustained overland flow of water in the valley networks are required to fill Jezero crater, to cause its breaching in a non-catastrophic manner, and to form the significant fluvial-deltaic environment of laterally migrating fluvial channels and scroll bars formed with an apparently stable baselevel. The lack of late-stage channel downcutting suggests that the conditions producing overland flow of water into the basin may have ended abruptly. Our estimates of the duration of fluvial activity (of order 106–107 years) suggest longer times than previously suggested (years to centuries) by sediment transport models, but generally relatively short durations from a geologic perspective.

Low post-Cenomanian denudation depths across the Brazilian Northeast: Implications for long-term landscape evolution at a transform continental margin

The Brazilian Northeast affords good opportunities for obtaining reliable timings and rates of landscape evolution based on stratigraphic correlations across a vast region. The landscape formed in the context of an episodically fluctuating but continuously falling base level since the Cenomanian. After formation of the transform passive margin in Aptian times, landscape development was further driven by a swell-like uplift with its crest situated ∼ 300 km from the coastline. The seaward flank of this swell or broad monocline between the interior Araripe and coastal Potiguar basins was eroded, and currently forms a deeply embayed plain bordered by a semi-circular, north-facing erosional escarpment. The post-Cenomanian uplift caused an inversion of the Cretaceous basins and generated a landscape in which the most elevated landforms correspond either to resistant Mesozoic sedimentary caprock, or to eroded stumps of syn-rift Cretaceous footwall uplands. Denudation in the last 90 My never exceeded mean rates of 10 m·My − 1 and exhumed a number of Cretaceous stratigraphic unconformities. As a result, some topographic surfaces at low elevations are effectively Mesozoic land surfaces that became re-exposed in Cenozoic times. The Neogene Barreiras Formation forms a continuous and mostly clastic apron near the coast. It testifies to the last peak of erosion in the hinterland and coincided with the onset of more arid climates at ∼ 13 Ma or earlier. The semi-circular escarpment is not directly related to the initial breakup rift flanks, which had been mostly eroded before the end of the Mesozoic, but the cause and exact timing of post-Cenomanian crustal upwarping are poorly constrained. It could perhaps have been a flexural response of the low-rigidity lithosphere to sediment loads on the margin, and thus a slowly ongoing process since the late Cretaceous. Uplift could instead be the consequence of a more discrete dynamic event related either to Oligocene magmatism in the region, or to continental-scale far-field stresses determined by Andean convergence.

Origin of the Blue Ridge escarpment along the passive margin of Eastern North America

AbstractThe Blue Ridge escarpment is a rugged landform situated within the ancient Appalachian orogen. While similar in some respects to the great escarpments along other passive margins, which have evolved by erosion following rifting, its youthful topographic expression has inspired proposals of Cenozoic tectonic rejuvenation in eastern North America. To better understand the post‐orogenic and post‐rift geomorphic evolution of passive margins, we have examined the origin of this landform using low‐temperature thermochronometry and manipulation of topographic indices. Apatite (U–Th)/He and fission‐track analyses along transects across the escarpment reveal a younging trend towards the coast. This pattern is consistent with other great escarpments and fits with an interpretation of having evolved by prolonged erosion, without the requirement of tectonic rejuvenation. Measured ages are also comparable specifically to those measured along other great escarpments that are as much as 100 Myr younger. This suggests that erosional mechanisms that maintain rugged escarpments in the early post‐rift stages may remain active on ancient passive margins for prolonged periods. The precise erosional evolution of the escarpment is less clear, however, and several end‐member models can explain the data. Our preferred model, which fits with all data, involves a significant degree of erosional escarpment retreat in the Cenozoic. Although this suggests that early onset of topographic stability is not required of passive margin evolution, more data are required to better constrain the details of the escarpment's development.

Landsliding in an urban area

The city of Wollongong is nestled within a narrow coastal plain approximately 70 km south of Sydney in the state of New South Wales (NSW), Australia (Fig. 1 ). Over the last 150 years of settlement the population of the Wollongong area, extending from Helensburgh in the north to Windang in the south has increased to about 200 000 people. The coastal plain is triangular in shape with a coastal length of 25 km. It is widest in the south at Yallah (14 km) and narrows sharply towards the north, disappearing north of Thirroul. The coastal plain is bounded to the north, west and south by an erosional escarpment ranging in height from 300 m up to 500 m. Fig. 1 Location plan. The escarpment consists of slopes with moderate to steep inclinations with several intermediate benches and cliff lines. Spectacular cliffs of Hawkesbury Sandstone (Middle Triassic) cap the escarpment and there is dense vegetation over most of the escarpment below these cliffs (Fig. 2 ). Fig. 2 Escarpment with large rock fall debris flow near Dombarton railway siding, 1974. Photo by Young. The main road link to Sydney is the F6 freeway that traverses the escarpment via Mount Ousley Road. There are …

Controls on facies architecture of a large Triassic carbonate platform; the Great Bank of Guizhou, Nanpanjiang Basin, South China

ABSTRACT The Great Bank of Guizhou (GBG) is an exceptionally well exposed isolated Triassic platform in the Nanpanjiang Basin of South China. The platform is exhumed with its depositional profile preserved and is dissected by a faulted syncline that exposes a complete and uncomplicated cross section providing a unique opportunity to evaluate mechanisms involved in its birth, evolution, and demise. The GBG formed near the southern margin of the Yangtze Platform during a deepening event that expanded the Nanpanjiang Basin and drowned the region surrounding the GBG in the latest Permian. Initial accumulation of the GBG began in the latest Permian with small reef mounds and open-marine skeletal packstones. Following the end-Permian extinction, cyanobacterial boundstones grew over the bank top. During the Early Triassic the GBG developed a low-relief bank profile with mobile oolite shoals at the margin, shallow-subtidal and peritidal deposits in the interior, and gentle slopes dominated by pelagics, debris-flow deposits, and turbidites at the basin margin. In the Middle Triassic (Anisian) the GBG developed a progressively steepening profile rimmed with massive Tubiphytes reefs. The platform was flat topped with tidal-flat deposits across the interior. Basin-margin deposition was dominated by turbidites and debris-flow deposits but eventually shifted to avalanche and rock-fall deposits as the slopes steepened to the angle of repose. In the Middle Triassic (Ladinian) an erosional escarpment up to 1700 m high developed at the margin. Platform-margin strata are bedded packstones similar to interior strata, whereas breccias at the basin margin contain coral-boundstone clasts suggesting erosion of reefs from the escarpment. A restricted subtidal lagoon formed in the interior, producing an atoll-like morphology. Later, a flat-topped profile was restored as tidal flats spread across the interior. In the beginning of the Late Triassic deepening contributed to termination of the GBG before siliciclastic turbidites and shales were deposited over the platform. In contrast with the well known platforms of the Dolomites of northern Italy, the GBG contains abundant muddy carbonates and a progressively steepening bank to reef-rimmed and escarpment architecture. The Italian platforms contain little mud and have angle-of-repose, pinnacle geometries. The GBG's larger size increased mud production and protected it from extensive winnowing of mud, which in turn resulted in off-bank shedding of muddy sediments that were stable on relatively gentle, basin-margin slopes which progressively steepened and ultimately led to avalanche deposits and a high-relief erosional escarpment. In contrast, the lesser mud content of the Dolomites platforms forced avalanche and talus deposition to dominate throughout deposition of basin margins, which in turn produced their angle-of-repose geometries.

Influence of rock strength properties on escarpment retreat across passive continental margins

We examine the influence of bedrock strength properties, imparted by jointing and fracturing, on the retreat of erosional escarpments across passive continental margins. Two types of escarpment are identified, and focus is placed on the gorge head–knickpoint form of escarpment, where upland streams drain across the escarpment. Along the Macleay River system of eastern Australia, major knickpoints associated with escarpment retreat are currently located about 200 km upstream from the river mouth, despite differences in bedrock lithology and order-of-magnitude differences in drainage areas upstream of the gorge heads, implying an average escarpment retreat rate of 2 km/m.y. since the margin formed 85 m.y. ago. Field observations from Apsley River gorge, a Macleay River tributary, indicate that bedrock jointing profoundly affects the evolution of the gorge headwall and sidewalls. Where dominant jointing dips into hillslopes, failure occurs by block and column toppling. Conversely, where jointing dips out of hillslopes, mass wasting occurs by joint-parallel rock sliding, allowing slopes to remain steep. As material is shed from the gorge sidewalls, the toppling slopes develop lower overall gradients, the gorge becomes asymmetric in cross section, and the channel remains nearer to the steeper sidewall affected by joint-parallel sliding. Coupling between fluvial and hillslope processes is essential to maintain headward propagation of the knickpoint–gorge head and to facilitate mass wasting from the gorge sidewalls. Peak stream discharge across the gorge head must exceed a threshold sufficient to transport debris away from the gorge head to keep the knickpoint rock face free of talus, to incise the stream channel, and to erode the toes of the sidewalls. If these conditions on fluvial processes are met, then hillslope processes, including rockslope failure mechanisms, are the rate controlling processes for knickpoint propagation and therefore determine the rate of escarpment retreat across the passive margin.