Foresets Research Articles

The gravel fans of upper Glen Roy, Lochaber, Scotland: their importance for understanding glacial, proglacial and glaciolacustrine dynamics during the Younger Dryas cold period in an Atlantic margin setting

Over the past 200 years significant research effort has gone into explaining the origin of the obvious former shorelines in Glen Roy (the so-called “Parallel Roads”). The large gravel deposit at the mouth of Glen Turret has attracted similar interest, but a solution to its origin and age remains contested: the same applies to the associated gravel fans in upper Glen Roy. This paper presents the results of systematic mapping and instrumental levelling of these features and new evidence from two previously unrecorded gravel fans in the nearby Allt Chonnal valley. Interpretation of altitudinal and lithofacies data indicates that all the fans (including the one at the mouth of Glen Turret) were deposited in a series of ice-dammed lakes during the Loch Lomond Stadial (Younger Dryas). The largest gravel fans were deposited in the shallow lake heads of the 260m, 325m and 350m lakes in upper Glen Roy, infilling these areas to the extent that deposition was in part subaerial. The absence of foreset bedding from the deposits is explained by the relatively shallow depth of lake waters, which inhibited development of classic Gilbert-type deltas and encouraged Hjulström-type fans. The previous assumption that gravel deposition into the 325m and 350m lakes was relatively limited is shown to be erroneous. The Allt Chonnal gravel fans, deposited into these lakes, have an estimated combined volume of 6,000,000m3 deposited in about 200 years. Significant gravel deposition into these former lakes continued at least until glaciers started to retreat in Glen Roy. When glacier retreat began, gravel deposition was reduced by over 85%. This research also concludes that the glacier which deposited the Turret fan emanated from Glen Gloy, supplied not only by ice from the south but also from ice that spilled into Glen Gloy from the adjacent Great Glen. The Loch Lomond Stadial (LLS) ice cap mapped in the Monadhliath Mountains was connected to the west Highland ice cap, which at the time filled the Great Glen. Regional equilibrium line altitude (ELA) estimates, based solely on the Monadhliath LLS ice cap evidence, may therefore be altitudinally too high.

Ichnology of Deltaic Mouth-Bar Systems of the Lajas Formation (Middle Jurassic) in the Sierra de la Vaca Muerta, Neuquén Basin, Argentina

Fil: Canale, Nerina. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - Patagonia Norte. Instituto de Investigacion en Paleobiologia y Geologia; Argentina

Grounding-zone wedges on Antarctic continental shelves

The concept of sediment accumulation at high rates in ice-stream grounding-zone settings originates with Alley et al. (1987) who envisioned ‘grounding zone deltas’ composed of sediment transported in a thin deformation-till layer beneath ice streams. This led to the discovery of sediment accumulations on Antarctic continental shelves that, in some instances, revealed foreset beds in seismic sections (e.g. Bart & Anderson 1995). Subsequently, with the advent of multibeam swath bathymetry, these wedges were identified in a number of shelf areas and the term grounding-zone wedge (GZW) was adopted (Shipp & Anderson 1997). Seismic images of GZWs show that they are wedge-shaped in a direction parallel to cross-shelf trough axes, tapering in a landward direction (Fig. 1) and lens-shaped in a transverse direction (Shipp et al. 1999; Howat & Domack 2003). GZWs typically range in thickness from a few metres to a few tens of metres and are occasionally stacked in a back-stepping …

The response of deltaic systems to climatic and hydrological changes in Daihai Lake rift basin, Inner Mongolia, northern China

Delta systems are ubiquitous around lacustrine rift basins. Its peripheral geometry, progradation structures and sedimentary successions were controlled by both tectonic settings and climatic changes. Peripheral geometry of a delta was strongly influenced by depositional gradients which formed the fan-shape delta on the steep slopes and developed the lobe-shape delta on the gentle slopes. Due to the discharge feed rivers can change rapidly driven by climatic variations, and the nearshore area of deltas display considerable facies variability. The rise of annual rainfall, which suggests the rivers feeding deltas are continuous, and result in distributary mouth bars that are prevalent in the front of deltas since the down-slope flows are greater than the along-slope currents. On the contrary, when the annual rainfall decreases and evaporation increases, the rivers only can feed deltas ephemerally. If the along-slope currents were in a dominant position, the distal bars were deposited. Progradation structure and sedimentary successions of deltas were controlled by the gradients of slopes. On gentle depositional slopes, shingle foreset beds predominate with fine sediments and small-scale sedimentary structures or vice versa.

New insights on the Sorbas Basin (SE Spain): The onshore reference of the Messinian Salinity Crisis

The Sorbas Basin is the land reference of the Messinian Salinity Crisis (MSC) that affected the Mediterranean Sea in the latest Miocene. Its stratigraphy has been re-visited using calcareous nannofossils and planktonic foraminifers, which provide a reliable biostratigraphic frame and lead to particularly specify the relationships between the Sorbas and Zorreras members with Yesares evaporites.The evaporites overlie a shallowing upward sequence ending with the deposition of the Reef Unit and Terminal Carbonate Complex (TCC) on the periphery of the basin. The reefal carbonates of the TCC are overlain by clastic deposits that are foreset beds of post-MSC Gilbert-type fan deltas developed on the northern edge of the basin. These sedimentary structures are separated from reefal carbonates and the Reef Unit by the Messinian Erosional Surface (MES). The various facies of the Sorbas Member have been correlated with the bottomset beds of the Gilbert-type fan deltas despite some differences in palaeobathymetry. In the southeastern periphery of the basin, the MES separates the Sorbas Member from the Yesares gypsums. In the central part of the basin, a hiatus characterizes the contact between these members. The Zorreras Member postdates the MSC and entirely belongs to Zanclean. Its white “Lago Mare” layers are lagoonal deposits, the fauna of which is confirmed to result from Mediterranean–Paratethys high sea-level exchange after the post-MSC marine reflooding of the Mediterranean Basin.This study allows to re-assert the two-step scenario of the MSC (Clauzon et al., 1996) with the following events:- at 5.971–5.600 Ma, minor sea-level fall resulting in the desiccation of this peripheral basin with secondary fluctuations;- at 5.600–5.460 Ma, significant subaerial erosion (or lack of sedimentation) caused by the almost complete desiccation of the Mediterranean Sea;- instantaneous marine reflooding, accepted at 5.460 Ma, followed by continuing sea-level rise.

The Umiak field discovery, Northwest Territories, Canada

This paper examines the discovery process in terms of changing exploration paradigms and describes a field discovered because of this change in mindset. Prior to the Umiak discovery all wells in the Mackenzie Delta had been drilled on structural highs. Reevaluation of two 30-yr-old dry holes along with existing two-dimensional (2-D) seismic data resulted in discovery of the Umiak gas field. This reappraisal led to recognition that a stratigraphic trap might exist between these two wells drilled in the early 1970s. The Kilagmiotak M-16 well contained 290 m (951 ft) of porous sandstone in the Eocene Taglu Formation, whereas the Umiak J-37 well, up dip and 11 km (6.8 mi) to the west, had no sandstone in the same interval. Examination of 2-D seismic lines found evidence of an updip sandstone pinchout beneath an angular unconformity on a tilted fault block. Strata in the tilted fault block below the unconformity contain strong amplitudes and flat spots. Interpretation of a subsequent three-dimensional (3-D) survey supported the play. A partnership of Alberta Energy Company (operator, now Encana), Anadarko, and Gulf Canada (now ConocoPhillips Canada) drilled the Umiak N-16 discovery well during 2004. Gas and some oil was found in gently dipping Eocene Taglu shoreface and delta front sandstones, and within gently folded foreset beds of the Eocene Richards Formation above a mid-Eocene unconformity. The Umiak N-05 appraisal well drilled a year later confirmed the discovery. Together these two wells delineate the fourth largest onshore gas accumulation on the Mackenzie Delta.

TRACKWAYS OF A GREGARIOUS, DUNEFIELD-DWELLING, EARLY JURASSIC THERAPSID IN THE AZTEC SANDSTONE OF SOUTHERN NEVADA

ABSTRACT We describe and interpret a tracksite in the Lower Jurassic Aztec Sandstone in Valley of Fire State Park, southern Nevada. The site contains approximately one hundred tracks of the ichnogenus Brasilichnium, arranged in twelve, subparallel trackways, all on the same foreset bedding plane. The Brasilichnium trackmaker was most probably a fossorial, tritylodontid therapsid. Sedimentologicial analyses indicate that the trackway surface is a wind-ripple horizon with a primary dip of about 25°, and that the animals climbed straight up the slip face of the dune. A combination of features leads us to conclude that the footprints were impressed into a crust of moist, cohesive sand, leaving two modes of preserved tracks: (1) shallow, well-defined tracks without associated sand crescents, and (2) deeper, less well defined tracks with associated sand crescents. We interpret this assemblage of tracks to record gregarious behavior in a mixed-age group of tritylodontid therapsids. In the correlative Navajo Sand...

Pillow lava and spasmodic submarine fire fountaining in the middle Miocene marginal basin, Sado Island, Japan

AbstractThe Sawasaki basalt unit of the Ogi Basalt Member is a middle Miocene submarine volcano formed in a marginal basin, cropping out on the coast of the Ogi Peninsula, Sado Island, Japan. It is approx. 100 m thick, more than 4 km wide, and is composed of pillow lavas (Sawasaki pillow lava) and overlying fire fountain deposits (Sawasaki pyroclastic rocks). The Sawasaki pillow lava is a pile of nearly‐horizontal lava flows. Massive sheet lavas and lava pods grade into pillow lavas. The flat surfaces of the sheet lavas suggest that they spread over a summit plain. The margin of the Sawasaki pillow lava is dominated by thin elongated pillow lobes (20–30 cm across) dipping at 45–60°, contrast with the large pillows (40–100 cm across) in the core. The proximal lithofacies of the Sawasaki pyroclastic rocks is scoria agglomerate which is composed of tens of stratified beds. Individual beds have a range of clast sizes and abundance of subaqueous bombs (sometimes up to >70%), which suggests that they were formed by submarine spasmodic fire fountain eruptions. Progressive aggradation from a turbulent flow allowed settling of well‐preserved fluidal‐shaped subaqueous bombs in a fines‐depleted matrix. The basal layer shows lamina, scour and imbrication of scoria clasts. The thick scoria agglomerate beds with large subaqueous bombs, derived from energetic fire fountains, were emplaced during transition to higher degrees of hydrovolcanic fragmentation. Alternating scoria lapilli tuff and tuff beds in the distal area may represent the coupled topset and foreset beds formed by deposition from dilute, turbulent flows, and cascading of volcanic debris from the crest of the volcanic hill.

Geomorphic relations among glacial Lake Algonquin and the Munising and Grand Marais moraines in eastern Upper Michigan, USA

Data obtained from ground-penetrating radar (GPR), optically stimulated luminescence (OSL) dating, and geostatistical reconstructions of Lake Algonquin water planes indicate that high-standing outwash aprons flanking the Munising moraine's southern margin in eastern Upper Michigan terminate as large, coalescing Gilbert-type ice-contact deltas graded to the Main level of Lake Algonquin. Nearly 30km of GPR imagery provides clear evidence of topset and foreset beds diagnostic of deltaic deposition. Landform assemblages suggest that the deltas likely evolved from subaqueous grounding line fans formed along a retreating ice margin characterized by widespread stagnation. In time, these deltas aggraded to the Main Lake Algonquin level as the ice margin stabilized. An OSL date of 12.5±1.1ka on a nearby beach ridge associated with the Main Algonquin shoreline is consistent with this interpretation. These new data, coupled with the presence of nested fans at the downstream end of meltwater channels incised into the delta, indicate that Lake Algonquin was falling as delta deposition waned, eventually reaching a level concordant with the Lower Orillia level before final abandonment. Large, regionally extensive heads of outwash marking the crest of the Munising moraine are graded to the upper margins of these deltas and, by inference, must also correlate with Main Lake Algonquin and the associated Two Rivers deglaciation ca. 13–12.5ka. The deltas, in turn, are conspicuously crosscut by outwash aprons that are graded to levels much lower than any recognized upper group Lake Algonquin shoreline. Because these aforementioned heads of outwash, part of Drexler and others' Grand Marais moraine, correlate with the Marquette moraine farther west, they likely date to the Marquette advance ca. 11.5ka. Thus, landforms related to Two Rivers and to Marquette glaciations appear to comingle across the study area. In the absence of extensive exposures and datable materials, our work highlights the fact that crosscutting relationships and distinct differences in base level can provide an important first step for untangling the complicated landform relationships in this part of Michigan.

Facies analysis and depositional architecture of shelf-type deltas in the Crati Basin (Calabrian Arc, south Italy)

This study focuses on facies analysis and depositional architecture of the Arente shelf-type delta, located at the southernmost margin of the Pleistocene Crati Basin (northern Calabria, southern Italy). Tectonically-controlled Gilbert deltas are extensively discussed in the literature, whereas few works document shelf-type deltas and factors controlling sedimentation. This paper contributes to these issues, with a special focus on evaluating eustatic and tectonic control on depositional architecture also by considering the occurrence of coeval Gilbert-delta systems.The Arente delta complex represents part of the coarse-grained sediments presently exposed along the eastern margin of the Crati Basin. It was sourced from the rising Sila Massif to the E and shows an overall westerly progradation.Conventional sedimentological analysis allowed to distinguish seven different facies associations. Well-exposed sections, at the basin margin, generally show an overall upward transition, locally basinward, from mouth-bar conglomerates and sandstones, to deltafront sandstone, to offshore mudstones. Facies distribution and arrangement are interpreted as the record of a shoal-water fan delta.Depositional processes were clearly influenced by the high sediment input, with a dominance of hyperpycnal deposits.Based on the occurrence of a series of bounding surfaces indicating abrupt drowning stages, the Arente deltaic succession can be divided into 5 distinct transgressive-regressive sequences.The fining and the thinning upward trend of the whole succession and the stacked landward-stepping architecture of the transgressiveregressive sequences, i.e. the E-directed retrogradational behavior of the Arente delta complex indicate that.Tectonic subsidence provided the accommodation space for the deposition of the delta complex, for the marked local backstepping of the system, and for the progressive drowning of bedrock. Highfrequency sea level fluctuations led to the repeated arrangement of the facies associations; in particular, the stacking of the individual deltaic sequences separated by transgressive surfaces suggests amplitudes of eustatic sea-level changes of a few tens of metres, thus representative of a higher-order cyclicity. The differing internal architecture, if compared to the steeply-dipping foreset beds of the Gilbert-type delta cropping out to the N of the study area, is interpreted as due to a decrease in displacement toward the southern tip of the Crati system fault.

Deltaic deposits at Aeolis Dorsa: Sedimentary evidence for a standing body of water on the northern plains of Mars

AbstractA fundamental long‐standing question regarding Mars history is whether the flat and low‐lying northern plains ever hosted an ocean. The best opportunity to solve this problem is provided by stratigraphic observations of sedimentary deposits onlapping the crustal dichotomy. Here, we use high‐resolution imagery and topography to analyze a branching network of inverted channel and channel lobe deposits in the Aeolis Dorsa region, just north of the dichotomy boundary. Observations of stacked, cross‐cutting channel bodies and stratal geometries indicate that these landforms represent exhumed distributary channel deposits. Observations of depositional trunk feeder channel bodies, a lack of evidence for past topographic confinement, channel avulsions at similar elevations, and the presence of a strong break in dip slope between topset and foreset beds suggest that this distributary system was most likely a delta, rather than an alluvial fan or submarine fan. Sediment transport calculations using both measured and derived channel geometries indicate a minimum delta deposition time on the order of 400 years. The location of this delta within a thick and widespread clastic wedge abutting the crustal dichotomy boundary, unconfined by any observable craters, suggests a standing body of water potentially 105 km2in extent or greater and is spatially consistent with hypotheses for a northern ocean.

Temporal and spatial evolution of coastline and subaqueous geomorphology in muddy coast of the Yellow River Delta

Based on measured data of coastline and bathometry, processed by softwares of Surfer and Mapinfo, and combined with sediment loads in different phases at Lijin gauging station, temporal and spatial evolution of coastline and subaqueous geomorphology in muddy coast of the Yellow River Delta is analyzed. The results show that ∼68% of sediments were delivered by the Yellow River deposited around the river mouth and in the littoral area from 1953 to 2000. Coastline in different coasts had distinctive changes in response to shifts of river course. Coastline was stable in the west of the Diaokou river mouth. Coastline from the east of the Diaokou river mouth to the north of the Gudong oilfield had experienced siltation, then serious erosion, and finally kept stable with sea walls conservation. Generally, coastline of the survived river mouth of the Qingshuigou river course stretched seaward, whereas the south side of sand spit at the Qingshuigou old river mouth was eroded after the Yellow River inpouring near the position at the Qing 8. The subaqueous geomorphology off the survived river mouth exhibited siltation from 1976 to 1996, with flat topset beds and steeper foreset beds. From 1996 to 2005, the subaqueous geomorphology off the Qingshuigou old river mouth was eroded in the topset and foreset beds, but silted in the bottomset beds. The subaqueous geomorphology off the new river mouth sequentially performed siltation with small degree compared to that of 1976–1996.

Coastal geomorphic conditions and styles of storm surge washover deposits from Southern Thailand

The characteristics of tropical storm washover deposits laid down during the years 2007 to 2011 along the southern peninsular coast of the Gulf of Thailand (GOT) were described in relation to their different geomorphic conditions, including perched fan, washover terrace and sheetwash lineations preserved behind the beach zone within 100m of the shoreline. As a result, washover terrace and sheetwash lineations were found where the beach configuration was uniform and promoted an unconfined flow. Non-uniform beach configurations that promoted a confined flow resulted in a perched fan deposit. Washover sediments were differentiated into two types based on sedimentary characteristics, including (i) a thick-bedded sand of multiple reverse grading layers and (ii) a medium-bedded sand of multiple normal grading layers. In the case of thick-bedded washover deposits, the internal sedimentary structures were characterized by the presence of sub-horizontal bedding, reverse grading, lamination, foreset bedding and wavy bedding, whereas, horizontal bedding, normal grading, and dunes were the dominant structures in the medium-bedded washover sand. Rip-up clasts were rare and recognized only in the washover deposits in the bottom unit, which reflects the condition when a mud supply was available. All washover successions were found in the landward inclined-bedding with a basal sharp contact. A high elevated beach ridge associated with a large swale at the backshore proved suitable for a thick-bedded washover type, whereas a small beach ridge with uniformly flat backshore topography promoted a medium-bedded washover sediment.

The Hydraulic Autogenesis of Distinct Cyclicity In Delta Foreset Bedding: Flume Experiments

Abstract We report the experimental discovery of autogenic cyclicity in delta foreset bedding that arises simply from steady water and sediment input under particular hydraulic characteristics. This autogenesis is intrinsic to Froude-supercritical flow associated with a train of upstream-migrating hydraulic jumps that delineate cyclic steps. Upstream migration of hydraulic jumps is associated with discrete packages of sediment accumulation on the foreset slope. This synchronism originates from the periodic alternation between supercritical and subcritical flow states on the delta topset just upstream of the shoreline, i.e., the topset–foreset break of the delta. The alternation in turn depends on the distance from the topset–foreset break to the nearest hydraulic jump immediately upstream. When the hydraulic jump is still in close proximity to the river mouth (delta shore), it reworks existing topset deposits (and perhaps upper foreset deposits as well) as it migrates upstream. Reworked sediment is then entrained into the subcritical flow between the shore and the hydraulic jump, which then emplaces this fine suspended sediment onto the foreset slope. As the hydraulic jump moves sufficiently far upstream of the mouth, the topset flow becomes supercritical, with a flow velocity sufficient to transport fine sediment offshore beyond the foreset. Under these conditions, the sediment emplaced on the foreset tends to be coarser material deposited via avalanching or proximal deposition from suspension. The details of this selective sedimentation vary according to the material in the topset bed near the mouth that is available for entrainment into the subcritical flow. Regardless of the details, however, distinct cyclic sedimentation of the delta foreset can be sustained as long as the inflow conditions of water and sediment dictate the formation of cyclic steps on the alluvial bed of the topset. Such hydraulic autogenesis accounts for a set of stratigraphic features that are common in ancient Gilbert deltas.

PRESERVATION OF TRACE FOSSILS AND MOLDS OF TERRESTRIAL BIOTA BY INTENSE STORMS IN MID-LAST INTERGLACIAL (MIS 5c) DUNES ON BERMUDA, WITH A MODEL FOR DEVELOPMENT OF HYDROLOGICAL CONDUITS

Abstract We document massive deposition of carbonate sands along the south shore of Bermuda that were emplaced during one or two great storms during the last interglacial. As determined by their stratigraphic position and geochronological data, these deposits formed during marine isotope substage (MIS) 5c ca. 100 ka ago. Within a leeward set of eolian beds, evidence of a living landscape was preserved that includes delicate footprints of a shorebird (Scolopacidae, Catoptrophorus) preserved in frothy dune foreset beds. In the same stratigraphic unit, outlines of a standing forest of palm trees (Sabal bermudana), some evidently with fronds in place, were molded in the fine carbonate dune sand. All available evidence points to an MIS 5c sea level positioned several meters below the present datum, which would require great intensity of storms to transport such voluminous deposits well above present sea level. Waves and storm currents transported loose sediments from the shallow shelf onto the shore, where hur...

Facies characteristics and architecture related to palaeodepth of Holocene fjord–delta sediments

AbstractLithofacies characteristics and depositional geometry of a sandy, prograding delta deposited as part of the Holocene valley‐fill stratigraphy in the Målselv valley, northern Norway, were examined using morpho‐sedimentary mapping, facies analysis of sediments in exposed sections, auger drilling and ground penetrating radar survey. Various lithofacies types record a broad range of depositional processes within an overall coarsening‐upward succession comprising a lowermost prodelta/bottomset unit, an intermediate delta slope/foreset unit containing steeply dipping clinoforms and an uppermost delta plain/topset unit. Bottomset lithofacies typically comprise sand‐silt couplets (tidal rhythmites), bioturbated sands and silts, and flaser and lenticular bedding. These sediments were deposited from suspension fall‐out, partly controlled by tidal currents and fluvial effluent processes. Delta foreset lithofacies comprise massive, inverse graded and normal graded beds deposited by gravity‐driven processes (mainly cohesionless debris flows and turbidity currents) and suspension fall‐out. In places, delta foreset beds show tidal rhythmicity and individual beds can be followed downslope into bottomset beds. Delta plain facies show an upward‐fining succession with trough cross‐beds at the base, followed by planar, laminated and massive beds indicative of a bedload dominated river/distributary system. This study presents a model of deltaic development that can be described with reference to three styles within a continuum related primarily to water depth within a basin of variable geometry: (i) bypass; (ii) shoal‐water; and (iii) deep‐water deltas. Bypass and deep‐water deltas can be considered as end members, whereas shoal‐water deltas are an intermediate type. The bypass delta is characterized by rapid progradation and an absence of delta slope sediments and low basin floor aggradation due to low accommodation space. The shoal‐water delta is characterized by rapid progradation, a short delta slope dominated by gravity‐flow processes and a prodelta area characterized by rapid sea‐floor aggradation due to intense suspension fallout of sandy material. Using tidal rhythmites as time‐markers, a progradation rate of up to 11 m year−1 has been recorded. The deep‐water delta is characterized by a relatively long delta slope dominated by gravity flows, moderate suspension fall‐out and slow sea‐floor aggradation in the prodelta area.

Sedimentary facies and evolution of aeolianites on Shidao Island, Xisha Islands

The aeolianite deposits on Shidao Island of the Xisha Islands, the South China Sea, contain five stages of aeolian biocalcarenites and four paleosols. The aeolian biocalcarenites consist of two sedimentary facies: dune and interdune deposits. In the dunes, large-scale festoon cross-bedding, humped cross-bedding and high-angle foreset bedding are well developed, and in the interdunes, large-scale flat-bedding and low-angle wedge shaped cross-bedding are well developed. The sedimentary structures and lamella features indicate that the aeolian deposits are driven mainly by the northeast monsoon. The aeolian biocalcarenite and paleosols may reflect the arid and humid climates of the East Asian monsoon, respectively. By comparison with the stalagmite oxygen isotope climosequence of Hulu Cave, Nanjing, we inferred that the aeolianite formed in the last glacial stage, and the paleosols were formed during relatively long-term warm events.

Seismostratigraphic analysis with centennial to decadal time resolution of the sediment sink in the Ganges–Brahmaputra subaqueous delta

The deltas of large rivers have become the focus of many research studies due to their vulnerability in times of expected sea level rise. One major delta in peril is the Ganges–Brahmaputra Delta (Bangladesh) due to its low elevation and high subsidence rate. In this study, high-resolution seismic and sediment echosounder data of the subaqueous part of the delta are analyzed by an integrated seismo-acoustic stratigraphic approach. Transparent Units (TUs) are the most prominent, continuously traceable architectural elements within the sigmoidal clinoform. TUs consist of homogenized sediments and are interpreted to be formed by liquefaction flows generated by subduction-related earthquakes of the nearby plate-boundary. The uppermost TUs are related to the historical well known major earthquakes which occurred in 1762, 1897 and 1950. Using the TUs as time marks the mean annual storage rates for the intervals could be assessed. A direct comparison of echosounder data gathered in 1994, 1997 and 2006 is used as another method to estimate the last decade mean annual storage rate. In general, the averaged decadal to centennial mean annual storage rates in the foreset beds have significantly decreased within the last ∼300 years from 22% to 18% and down to 13.8% of the present total annual fluviatile suspension supply to the delta (10 9 t a −1). This decrease, however, is not evenly distributed along the foreset beds. In the western clinoform mean annual storage rates slightly decreased during the last ∼300 years, whereas in the eastern clinoform the mean annual storage rate of the last decade is one-third of the initial value in the late eighteenth century. The variation of clinoform slope angles generally coincides with the local accumulation rates but a variable degree of asymmetry gives some new insights in the future trend of the subaqueous delta development. The sink of sediment in the western clinoform depocentre landward of the inflection point, where shape changes from convex to concave, is producing an increase in concavity. The decline in monsoon precipitation over the last centuries is assumed to have significantly contributed to the decreased mean annual storage rate in the eastern clinoform where the convexity of the clinoform increases seaward of the inflection point. It is very likely that the whole depocentre of the fluvial sediment load is shifting toward the western subaqueous delta and to the Swatch of No Ground Canyon and, thereby, increasing the export to the deep-sea fan.

Liquefaction Probability Curves for Surficial Geologic Deposits

Liquefaction probability curves that predict the probability of surface manifestations of earthquake-induced liquefaction are developed for 14 different types of surficial geologic units. The units consist of alluvial fan, beach ridge, river delta topset and foreset beds, eolian dune, point bar, flood basin, natural river and alluvial fan levees, abandoned river channel, deep-water lake, lagoonal, sandy artificial fill, and valley train deposits. Probability is conditioned on earthquake magnitude and peak ground acceleration. Curves are developed for water table depths of 1.5 and 5.0 m. Probabilities are derived from complementary cumulative frequency distributions of the liquefaction potential index (LPI) that were computed from 927 cone penetration tests. For natural deposits with a water table at 1.5 m and subjected to a M7.5 earthquake with peak ground acceleration (PGA) = 0.25 g , probabilities range from 0.5 for beach ridge, point bar, and deltaic deposits. The curves also were used to assign ranges of liquefaction probabilities to the susceptibility categories proposed previously for different geologic deposits. For the earthquake described here, probabilities for susceptibility categories have ranges of 0–0.08 for low, 0.09–0.30 for moderate, 0.31–0.62 for high, and 0.63–1.00 for very high. Retrospective predictions of liquefaction during historical earthquakes based on the curves compare favorably to observations.

Stratigraphy, mineralogy, and origin of layered deposits inside Terby crater, Mars

The 174 km diameter Terby impact crater (28.0°S–74.1°E) located on the northern rim of the Hellas basin displays anomalous inner morphology, including a flat floor and light-toned layered deposits. An analysis of these deposits was performed using multiple datasets from Mars Global Surveyor, Mars Odyssey, Mars Express and Mars Reconnaissance Orbiter missions, with visible images for interpretation, near-infrared data for mineralogical mapping, and topography for geometry. The geometry of layered deposits was consistent with that of sediments that settled mainly in a sub-aqueous environment, during the Noachian period as determined by crater counts. To the north, the thickest sediments displayed sequences for fan deltas, as identified by 100 m to 1 km long clinoforms, as defined by horizontal beds passing to foreset beds dipping by 6–10° toward the center of the Terby crater. The identification of distinct sub-aqueous fan sequences, separated by unconformities and local wedges, showed the accumulation of sediments from prograding/onlapping depositional sequences, due to lake level and sediment supply variations. The mineralogy of several layers with hydrated minerals, including Fe/Mg phyllosilicates, supports this type of sedimentary environment. The volume of fan sediments was estimated as >5000 km 3 (a large amount considering classical martian fan deltas such as Eberswalde (6 km 3)) and requires sustained liquid water activity. Such a large sedimentary deposition in Terby crater is characteristic of the Noachian/Phyllosian period during which the environment favored the formation of phyllosilicates. The latter were detected by spectral data in the layered deposits of Terby crater in three distinct layer sequences. During the Hesperian period, the sediments experienced strong erosion, possibly enhanced by more acidic conditions, forming the current morphology with three mesas and closed depressions. Small fluvial valleys and alluvial fans formed subsequently, attesting to late fluvial processes dated as late Early to early Late Hesperian. After this late fluvial episode, the Terby impact crater was submitted to aeolian processes and permanent cold conditions with viscous flow features. Therefore, the Terby crater displays, in a single location, geologic features that characterize the three main periods of time on Mars, with the presence of one of the thickest sub-aqueous fan deposits reported on Mars. The filling of Terby impact crater is thus one potential “reference geologic cross-section” for Mars stratigraphy.