Cobble-sized Clasts Research Articles

Trace fossil association related to the Upper Miocene transgression cycle in the Lower Chelif Basin, NW Algeria

The Tortonian transgression is expressed in the Lower Chelif Basin by the deposition of the Blue Marls Formation highlighted at the base by a deposition of sandstone and conglomerate, including ante-Neogene cobble-size clasts, in which the ichnological investigation shows moderately diversified invertebrate ichnofossils preserved mainly in the detrital-dominated deposits. The trace fossil record consists of 18 ichnospecies belonging to 13 ichnogenera, including Arenicolites carbonaria, Diplocraterion parallelum, Gastrochaenolites isp., Gyrolithes isp., Macanopsis isp., Macaronichnus cf. segregatis, Ophiomorpha nodosa, Palaeophycus tubularis, Phycodes circinnatum, Phycodes cf. curvipalmatum, Rhizocorallium isp., Rosselia socialis, Skolithos annulatus, Skolithos linearis, Skolithos verticalis, Thalassinoides horizontalis, Thalassinoides cf. suevicus, and Thalassinoides isp., produced mostly by deposit-, suspension-, and rarely detritus-feeder organisms. Among these traces, Gastrochaenolites isp. is the only specimen being recorded within the conglomerate deposits and represents, therefore, the evidence of the first colonization after the Tortonian marine ingression. The remaining trace fossils are exclusively reported from the sandstone deposits and are typical of shallow-marine settings composed of vertical and horizontal structures, largely dominated ethologically by domichnia, fodinichnia, and combined domichnia/fodinichnia behaviour groups. The lower interval of the sandstone deposits is dominated by mid- to deep-tier, vertical post-depositional dwelling (domichnia) forms (Arenicolites, Diplocraterion, Skolithos, Macanopsis, Ophiomorpha, and Gyrolithes) attributed to opportunistic suspension-feeding organisms, corresponding to the archetypal Skolithos ichnofacies that documents an upper-middle shoreface environment, whereas the upper interval is predominantly represented by shallowest- to shallow-tier horizontal deposit-feeders burrows [Macaronichnus, Ophiomorpha (horizontal components), Palaeophycus, Phycodes, Rhizocorallium, Rosselia (vertical components), and Thalassinoides] displaying intense degree of bioturbation, reflecting a perfect palaeoecological condition reigning in the water column and sea floor. Thus, the association commonly corresponds to the proximal trend of the Cruziana ichnofacies developed within the lower shoreface zone. Consequently, the ichnological investigation reveals that the detrital-dominated deposits show a slight deepening of the environment, shifting from a suspension feeder-dominated to a deposit feeder-dominated association, in which polychaete annelid and crustacean animals are the main benthonic epi- and infaunal trace makers of the recorded ichnofossils. The ichnotaxa Arenicolites carbonaria, Phycodes circinnatum, Phycodes cf. curvipalmatum, Rhizocorallium isp., Rosselia socialis, and Skolithos verticalis are reported herein for the first time from the Cenozoic of Algeria.

Syn-depositional oil seeps in the Late Albian Paddy Member of the Peace River Formation (Early Cretaceous), north-central Alberta

Abstract Controversy exists regarding the timing of emplacement of oil in the giant Athabasca and Peace River oil sands. Bitumen-cemented sandstones are present in the late Albian Paddy Member of the Peace River Formation; some cemented sandstones formed burrowed firmgrounds and reworked intraclasts, showing that oil was reaching surface by about 101–102 Ma. Estuarine and shallow-marine sandstones of the Paddy Member are exposed on the Peace and Heart rivers in the vicinity of the town of Peace River. Bitumen-cemented sandstone occurs in four stratigraphic settings: 1) An in-situ cement forms locally bedding-transgressive, sheet-like masses in cross-bedded estuarine sandstone; 2) A bitumen-cemented and heavily burrowed layer, 30–60 cm thick, lies immediately beneath a marine transgressive surface and shows that oil infiltrated downward into, and cemented, the upper surface of a shoreface sandbody. The firm sand was subsequently exhumed by transgressive erosion, fractured and burrowed by arthropods; 3) Rounded pebble- to cobble-sized clasts of bitumen-cemented sand lie on the floor of a tidal channel, and the channel floor surface is also stained and burrowed. This case suggests floating oil infiltrated the channel floor at low tide, subsequently hardening prior to erosion and burrowing; 4) Rounded cobble-to boulder-size clasts of bitumen-cemented sandstone, up to 1 m wide and 0.5 m thick, lie on a regional marine ravinement surface cut on the uppermost Paddy shoreface sandstone. The sand was permeated with oil, top-down, then oxidized to form a tough bitumen cement. The bituminous sand was subsequently scoured by waves during marine transgression to form a boulder lag that is enclosed in transgressive marine claystone. All four of these examples show that oil was reaching the Earth’s surface during Paddy time where it infiltrated porous sands to a depth of several decimetres, subsequently degrading and oxidizing to form a tough ‘asphalt pavement’ that attracted a burrowing firmground fauna and was eroded into cohesive intraclasts weighing many tens of kilograms. Pyrolysis proved the bitumen cement to be degraded oil, but it was not possible to identify the source-rock using gas chromatography-mass spectrometry. The Paddy ‘asphalt pavements’ are comparable to cements that form in beach sediments in the wake of catastrophic spills from tanker groundings. A direct analogue is provided by Eocene estuarine sediments in Dorset, U.K. There, bitumen-cemented estuarine sands also formed firmgrounds and intraclasts on channel-floor and marine transgressive surfaces.

Localized bank collapse or regional event?

This study presents a detailed synopsis of the sedimentological and structural features displayed within an underdescribed enigmatic facies observed in the basal Lower Jurassic Kayenta Formation of the Colorado Plateau. The facies comprises pebble to cobble-sized clasts of fine to medium-grained crossbedded sandstone with mud-draped and deformed foresets, as well as clasts of parallel-laminated but highly contorted siltstone and mudstone, supported in a silty to sandy matrix. The deposits are internally deformed and show both ductile and brittle structures in close spatial proximity, with a consistent and pervasive westdirected sense of shear. The facies occurs consistently within the same approximate stratigraphic interval, at or near the base of the Kayenta Formation. It is, however, observed only at four localities, distributed in a crudely linear arrangement parallel to the Utah-Idaho trough, despite extensive studies of outcrops of the same stratigraphic interval widely distributed across both Utah and Arizona. This study interprets the depositional processes as that of a partially subaerial debris flow with depositional events perhaps taking place during the waning period after ephemeral stream activity. The clast morphology and composition suggests a local source for the sediment entrained within the flow, and a limited transport distance. All of these observations are difficult to reconcile with the consistency of the stratigraphic interval in which the facies occur, or with the regional distribution of preserved examples. Consequently, this study discusses the potential for a common and time-equivalent triggering mechanism across all examples, which may have regional significance in the Jurassic evolution of the region.

Chemical weathering signatures from Mt. Achernar Moraine, Central Transantarctic Mountains I: Subglacial sediments compared with underlying rock

In order to determine chemical weathering rates on the subglacial land surface of Antarctica, we compare the composition and mineralogy of freshly emerging fine sediments to that of the underlying bedrock, as represented by glacially derived cobble-sized clasts. Samples were collected from Mt. Achernar Moraine, a large blue ice moraine, where subglacial material naturally emerges through sublimation of the surrounding ice. Both rocks and sediments were analyzed for total elemental composition, mineral abundance by X-ray diffraction, and by sequential extractions targeting chemical weathering products. The fine sediment fraction is significantly enriched in chemical weathering products and depleted in primary minerals compared with the cobble clasts. The alteration pathways consist primarily of the development of smectite, kaolinite, carbonate minerals, and amorphous material. Extensive Fe oxidation is evidenced by a decline in magnetic susceptibility and by increases in extractable Fe. If we assume the only input into the subglacial system is the water and ice-trapped gas supplied by basal melt, the net chemical alteration is explained through oxidation of organic matter equal to ∼0.7% of the bedrock mass and subsequent carbonation weathering. The underlying sedimentary rock is sufficiently rich in organic matter for this pathway to be plausible. For the O2 that is oxidizing organic matter to be supplied by basal meltwater, water fluxes would need to be three orders of magnitude larger than sediment fluxes. Independent models of basal melt and sediment transport at our field site confirm that such a difference between water and sediment flux is likely at the study site. The rate of subglacial carbonation weathering inferred from the Mt. Achernar Moraine site may be comparable to that found in high latitude subaerial environments. If Mt. Achernar Moraine is typical of other Antarctic sites, the subglacial land surface of Antarctica does play a role in global geochemical cycling.

Sedimentology, structure, and age of the Wide Bay Canyon submarine landslide on the southeast Australian continental slope

Sedimentological analysis and radiocarbon dating of three marine sediment cores collected from the upper continental slope offshore from Fraser Island, Queensland, Australia, indicate the region is dynamic and provide strong evidence for mass movement processes occurring during the Late Pleistocene and Holocene. Two cores were collected within the Wide Bay Canyon submarine landslide scar with a third collected on the adjacent open slope. All three cores present intraformational, mud-clast conglomerates, interpreted to be debris flow deposits comprised of sub-angular to rounded, pebbled and cobble-sized clasts of the hemipelagic muds that typically accumulate in this area. One within-slide core and the open slope core also present coarse-grained, poorly sorted, unconsolidated sand layers that are interpreted to be grain flow deposits derived from continental shelf sands that were driven over the continental shelf edge by the East Australia Current during the Last Glacial Maximum. Radiocarbon dating of unconformities identified in the Wide Bay Canyon slide scar cores indicates that abrasion and sediment removal during the emplacement of debris flows occurred during several separate events prior to ~35.1 cal. yr. BP. The sand layers interpreted to be grain flow deposits contain Late Pleistocene age material that was probably re-deposited during the sea level low-stand associated with the Last Glacial Maximum. Consideration of the debris flow ages in the context of ages reported for similar materials and other landslide deposits recorded on the South-eastern Australian Continental Margin supports the previously advanced hypothesis that rare, moderate to large, intraplate earthquakes are a likely triggering mechanism for submarine landslides and/or mass failure events on the South-eastern Australian Continental Margin.

A new approach for luminescence dating glaciofluvial deposits - High precision optical dating of cobbles

In recent years luminescence dating has increasingly been applied to date glaciofluvial sediments, but uncertainties about the degree of bleaching of the luminescence signal at deposition make dating of such sediments challenging. Here we test a new approach for luminescence dating of glaciofluvial sediments, based on the analysis of rock cores drilled from granite cobbles, and compare the luminescence ages generated against independent age control.Luminescence measurements from rock slices in cobble-sized clasts can be used to reconstruct the extent of bleaching, thereby giving greater confidence in the ages produced. This study illustrates that another important advantage of using cobbles is that at depths of 2 mm or more below the cobble surface >90% of the total dose rate arises from the cobble itself, making the dose rate insensitive to the water content of the sediment matrix. Ordinarily, uncertainties in estimating water content during burial are one of the largest sources of uncertainty in luminescence dating methods, and hence reducing the reliance upon the dose rate could be particularly advantageous for glacial deposits, where water contents can potentially be large and highly variable.Measurements of cobbles from Orrisdale Head, Isle of Man, demonstrate that the luminescence signal was completely bleached to depths of up to 12 mm into the cobble. Sampling of orientated cobbles from lithofacies diagnostic of bar-top environments was used to maximise the chances of exposure to sunlight. The upper-faces of these orientated cobble surfaces appear to be bleached to a greater depth than the lowermost faces. Data from 45 rock slices from these cobbles were tightly clustered, yielding a mean age of 20.7 ± 0.3 ka that is in agreement with independent age control. One of the well-bleached cobbles shows evidence of two discrete exposure events, potentially recording both the advance at 26.2 ± 0.8 ka, and retreat at 20.7 ± 0.3 ka, of the Irish Sea Ice Stream.

Cluster analysis on a sphere: Application to magnetizations from metasediments of the Jack Hills, Western Australia

Metasediments of the Jack Hills contain the oldest known terrestrial minerals in the form of zircons nearly 4.4 billion years old. Paleointensity data from these zircons provide evidence for a Hadean geodynamo as old as 4.2 billion years old. Given the importance of these zircons for constraining the earliest history of the core, it is vital to understand the fidelity of the zircon record. A fundamental aspect providing context for the preservation of primary magnetic signals is the nature of overprints predicted to have been imparted on rocks of the Jack Hills due to Archean to Proterozoic metamorphic events. To be viable magnetic records of a Hadean geodynamo, zircon magnetization directions should differ from these secondary magnetizations. To evaluate these secondary magnetizations, we report paleomagnetic analyses of a comprehensive sampling of 68 quartzite cobble-sized clasts from the Jack Hills metasediments ∼0.5 to 1.0 km from the Discovery Site (which has yielded the oldest zircons and paleofield estimates). While application of standard paleomagnetic tests suggests that the ensemble of cobble directions cannot be distinguished from those drawn from a random distribution, a new cluster analysis of directions on a sphere and non-parametric resampling approaches reveal significant directions amongst subsets of the data. One, isolated at the lowest temperature analyzed [200 to 300 °C, Declination (Dec.) = 316.8°, Inclination (Inc.) =−51.1°] appears to be dominated by the present day field. Another, isolated at higher (but still relatively low unblocking temperatures that we call “intermediate”, of ∼350–500 °C, Dec. = 243.8°, Inc. = 9.5°) agrees with a magnetic overprint isolated from the secondary Cr–Fe mica fuchsite isolated from the Jack Hills Discovery site, passing a field test at the 80% confidence level. No evidence is found in our data, or in the data of others collected on similar Jack Hills lithologies, for a widespread 1 Ga remagnetization event. Instead, we interpret the most prevalent secondary magnetization of the quartzite (i.e., intermediate unblocking) and the fuchsite characteristic remanent magnetization to be ∼2.65 Ga in age, coincident with peak metamorphism (as high as ca. 475 °C) of the Jack Hills. The presence of this distinct secondary magnetization, its difference from that recorded by Jack Hills zircons at high unblocking temperatures, and the lack of a dominant remagnetization direction at high unblocking temperatures in the cobble data (the expected result for a primary magnetization), lends further support to the fidelity of the Hadean geomagnetic record. The presence of the secondary magnetization also lends support to the conclusion that most of the Jack Hills metasediments were deposited in the Archean, with only minor reworking and potential tectonic interleaving of Proterozoic components. Overall, the application of the new directional cluster analysis presented here has the potential to reveal magnetic directions in highly scattered data sets, typical of weakly magnetized coarse-grained sedimentary rocks

Detrital magnetite and chromite in Jack Hills quartzite cobbles: Further evidence for the preservation of primary magnetizations and new insights into sediment provenance

The magnetization of zircons from sedimentary rocks of the Jack Hills (Yilgarn Craton, Western Australia) provide evidence for a Hadean to Paleoarchean geodynamo, 4.0 to 4.2 billion years old. These magnetizations pass a microconglomerate test, attesting to the fidelity of Jack Hills zircons as recorders of these most ancient magnetic signals. The lack of pervasive remagnetization of the Jack Hills is also documented through a positive conglomerate test conducted on cobble-sized clasts. A key element of the latter test is the preservation of a high unblocking temperature magnetization that can survive peak metamorphic temperatures. Rock magnetic studies suggest the mineral carrier is magnetite. Herein, we investigate the magnetic mineral carriers in cobble samples through scanning electron microscope and microprobe analyses, conduct an inter-laboratory paleomagnetic study to evaluate sensitivities required to evaluate the weak magnetizations carried by the Jack Hills sediments, and assess provenance information constrained by the opaque minerals. These data confirm magnetite as a detrital phase and the presence of high unblocking temperature magnetizations, further supporting the posit that the Jack Hills sediments can preserve primary magnetic signatures. We note that some of these magnetizations are near the measurement resolution of standard cryogenic magnetometers and thus exacting laboratory procedures are required to uncover these signals. In addition to magnetite, the cobbles contain an assemblage of Mg poor Cr–Fe chromites, Ni-sulfides and pyrrhotite that suggest a source in a layered intrusion different from the granitoid source of the zircons. Any Hadean rock fragment in these sediments, if present, remains elusive.

Characteristics of pebble and cobble-sized clasts along the Curiosity rover traverse from sol 100 to 750: Terrain types, potential sources, and transport mechanisms

We combine the results of orbitally-derived morphologic and thermal inertia data with in situ observations of abundance, size, morphologic characteristics, and distribution of pebble- to cobble-sized clasts along the Curiosity rover traverse. Our goals are to characterize rock sources and transport history, and improve our ability to predict upcoming terrain. There are ten clast types, with nine types interpreted as sedimentary rocks. Only Type 3 clasts had morphologies indicative of significant wear through transport; thus, most clast types are indicative of nearby outcrops or prior presence of laterally extensive sedimentary rock layers, consistent with the erosional landscape. A minor component may reflect impact delivery of more distant material. Types 1 and 4 are heavily-cemented sandstones, likely associated with a “caprock” layer. Types 5 and 6 (and possibly 7) are pebble-rich sandstones, with varying amounts of cement leading to varying susceptibility to erosion/wear. Type 3 clasts are rounded pebbles likely transported and deposited alluvially, then worn out of pebbly sandstone/conglomerate. Types 9 and 10 are poorly-sorted sandstones, with Type 9 representing fragments of Square Top-type layers, and Type 10 deriving from basal or other Mt. Sharp layers. Types 2, 8 and 9 are considered exotics.There are few clear links between clast type and terrain surface roughness (particularly in identifying terrain that is challenging for the rover to navigate). Orbital data may provide a reasonable prediction of certain end-member terrains but the complex interplay between variables that contribute to surface characteristics makes discriminating between terrain types from orbital data problematic. Prediction would likely be improved through higher-resolution thermal inertia data.

THE OCCURRENCE OF PALAEOZOIC CONGLOMERATIC ROCKS IN EAST JOHOR, PENINSULAR MALAYSIA

Conglomeratic rocks in East Johor are found in the separately three formations that are the Murau, Tanjung Leman and Linggiu Formations. The Murau Formation is characterized by cobble to boulder grained breccia with very angular to angular and disorganized clasts. It was deposited by fan-delta system in the sub-aerial to shallow marine environment. The Tanjung Leman Formation consists of pebble to cobble grained conglomerate with rounded to subrounded and organized clasts. It was deposited by braided river system in the sub-aerial environment. Both formations outcropped in eastern coastal of Johor. The rudaceous rocks of the Linggiu Formation consist of pebble to cobble-sized clasts with sub angular and disorganized texture. It present as subordinate rocks within sandstone dominant in the central part of East Johor and was deposited by debris flows in the shallow marine environment. All these rudaceous rocks were interpreted as Palaeozoic in age ranging from Late Carboniferous to Late Permian. Those rudaceous rocks indicated that since Late Carboniferous, palaeogeography of East Johor was a continent with subaerial to shallow marine depositional environment. 
 
 Keywords: Conglomerate, Palaeozoic, East Johor, subaerial, shallow marine.

Collection and Application of Outcrop Measurements in Glacial Materials for Geo-Engineering and Hydrogeology along the Vermilion River, East-Central Illinois

Outcrop mapping by close-range photogrammetry was undertaken at three remote sites as part of a study of water resources to improve characterization of glacial sedimentary assemblages and to evaluate the utility of terrestrial remote sensing to supplement routine geologic mapping. Two features were measured from georeferenced stereomodels, clast pavements, and buried channel deposits over a 2-year period along a segment of the Middle Fork of the Vermilion River. Cobble-size clasts, spaced less than 3.28 ft (1 m) apart, form clusters approximately 6.56 to 9.84 ft (2 to 3 m) in length. These clasts compose a semi-continuous pavement between two tills deposited during the Wisconsin Episode. Five of the 26 clusters measured occur within the Tiskilwa Formation. Deposits of coarse- and fine-grained sediments, informally assigned to the Glasford Formation lithostratigraphic unit, fill buried channels that provide an important source of groundwater in east-central Illinois for areas that do not receive water from the Mahomet aquifer. Measurements of buried channels included width, maximum sediment thickness, area, and perimeter. Widths equated to more than half of the outcrop length. Aspect ratios (width: thickness) of the channels are consistent with deltaic distributary systems formed in front of retreating ice margins, and systems having varying interconnectivity. Differences in area: perimeter of the buried channels provide a measure of shape that may partially account for the variation in yields from water wells in this area. Consequently, we postulate that yield could be improved through lateral drilling within the channel or by connecting adjacent channels.

The role of attached kelp (seaweed) in lowering threshold of coarse gravel entrainment in tidal flows

Lag deposits of cobble-sized clasts found within the troughs of fine-gravel dunes on a rock-bed intertidal zone are explained by the process of enhanced fluid transport due to attached seaweed. Field determinations of the competence of the tidal currents demonstrated that the currents were not able to transport weed-free clasts on the tidal flat. Controlled flume experiments were used to demonstrate that the presence of the fucoid alga (Fucus vesiculosus (L.)) attached to clasts reduces the critical velocities and shear stresses to levels that allow the tidal currents to entrain the clasts and move them over the backs of dunes to the lee sides. Subsequent burial of the weedy-clasts in the dune troughs by dune progression kills the weed, leaving the latterly exposed clasts as an accumulating weed-free lag. The weight of attached weed typically equals the submerged weight of the clast and the critical velocity and critical shear stress for initial motion are both roughly halved by the presence of the weed. The drag induced by weed is four-times that experienced by weed-free clasts and mobile clasts exhibit near-equal mobility.

Provenance and sediment characteristics of contemporary gravel deposits at Sellicks Beach, eastern shore of Gulf St Vincent, South Australia

Sellicks Beach, located on the eastern shore of Gulf St Vincent, South Australia, is subject to wave-dominated processes and northward longshore transport. During winter, when wave energy is typically vigorous, gravel deposits are exposed across most of the beach, and three step-like berms are well developed. Sand is restricted to a narrow strip that is exposed only at low tide. In contrast, during summer, when wave energy is generally moderate to low, much of the gravel is covered by a thin veneer of sand and only the high berm, on the landward edge of the beach, remains as an obvious feature. Steeply dipping Neoproterozoic to Cambrian strata that outcrop strongly across Sellicks Hill are the original source rocks for the beach gravel; distinctive sedimentary textures, structures and fossils in the cobble-size clasts can be confidently matched with those of the provenance rocks. Much of the sediment entered the modern beach environment as a consequence of coastal erosion of transitional alluvial fan sediments. The oldest alluvial fan sediments are of late Pliocene to earliest Pleistocene age. Mount Terrible Gully provides a conduit for the input of fluvial sediment at the mouth of Cactus Canyon, where clasts as large as boulders accumulate across the beach. Sellicks Beach gravels are subject to longshore transport northwards. Relatively softer clasts, such as those derived from the Heatherdale Shale, are rare beyond Cactus Canyon. In contrast, quartzite clasts are more abundant towards the north. This lithological differentiation is attributed to preferential survivorship of clasts that are physically harder and chemically less reactive. The change in the shapes of clasts northwards, from predominately shingle-like ‘very platy’ and ‘very bladed’ at Cactus Canyon, to more ‘compact’ towards the boat ramp, is in accord with the more massive fabric of the surviving quartzite clasts. At Sellicks Beach, preservation of uplifted, coarse gravels, with entire and comminuted marine molluscan shells, of last interglacial age, provides evidence of neotectonism. At the landward margin of the beach, imbricated gravels in which pore spaces have been infilled with mud, and which show no evidence of modern coastal erosion, may provide evidence of continuing uplift during the recent Holocene. The geological setting, geomorphic framework and modern sedimentary regime at Sellicks Beach combine to provide an exceptionally useful outdoor laboratory for education in field geology.

El tsunami de 1960 en una planicie de cordones litorales cerca de Maullín, Chile: descenso tierra adentro, surcos renovados, abanicos agradados, múltiples predecesores.

The Chilean tsunami of 22 May 1960 reamed out a breach and built up a fan as it flowed across a sparsely inhabited beach-ridge plain near Maullín, midway along the length of the tsunami source. Eyewitnesses to the flooding, interviewed mainly in 1988 and 1989, identified levels that the tsunami had reached on high ground, trees, and buildings. The maximum levels fell, from about 10 m to 2 m, between the mouth of the tidal Río Maullín and an inundation limit nearly 5 km inland across the plain. Along this profile at Caulle, where the maximum flow depth was a few meters deep, airphotos taken in 1961 show breaches across a road on a sandy beach ridge. Inland from one of these breaches is a fan with branched distributaries. Today its breach holds a pond that has been changing into a marsh. The 1960 fan deposits, as much as 60 cm thick, are traceable inland for 120 m from the breach. They rest on a pasture soil above two additional sand bodies, each atop its own buried soil. The earlier of the pre-1960 sand bodies probably dates to AD 1270-1400, in which case its age is not statistically different from that of a sand sheet previously dated elsewhere near Maullín. The breach likely originated then and has been freshened twice. Evidence that the breach was freshened in 1960 includes a near-basal interval of cobble-size clasts of sediment and soil, most of them probably derived from the organic fill of pre-1960 breach. The cobbly interval is overlain by sand with ripple-drift laminae that record landward flow. The fan of another breach near Maullín, at Chanhué, also provides stratigraphic evidence for recurrent tsunamis, though not necessarily for the repeated use of the breach. These findings were anticipated a half century ago by description of paired breaches and fans that the 1960 Chilean tsunami produced in Japan. Breaches and their fans may provide lasting evidence for tsunami inundation of beach-ridge plains. The breaches might be detectable by remote sensing, and the thickness of the fan deposits might help them outlast an ordinary tsunami sand sheet.

Generation of subsurface injection flow in a carbonate platform

In order to diagnose emplacement of discordant breccia body in carbonate platform, we here describe sedimentary structures and texture as well as bed geometry of two beds of the Chaomidian Formation (Frongian) in Shandong Province, China. Each breccia body consists of granule- to cobble-size clasts of peloidal grainstone and matrices of dolomitic marlstone and/or micritic limestone, showing a tunnel geometry. Marlstone in the matrix is homogeneous, whereas micritic limestone shows irregular and disoriented laminations. It is hypothesized that the breccia body formed by subsurface in situ fluidization and fragmentation and further injection into the ambient deposits. Subsurface sediment remobilization occurred most likely under cyclic loading of large storm waves. The laminated matrix is strongly suggestive of hydroplastic deformation (flowage) of fluidized material during injection. This study suggests that there exists a full spectrum of subsurface deformation from liquefaction/fluidization to injection flow.

Limestone interbedded with submarine volcanics: the Early–Middle Miocene Conejo Volcanics, California

Carbonate sedimentation concurrent with submarine volcanism is very rare in the geologic record but is well displayed in the Early to Middle Miocene Conejo Volcanics of the central Santa Monica Mountains of southern California. Limestone occurs as lenticular deposits on the surface of composite flows units, as matrix within breccia of pebble- to cobble-size volcanic clasts, within primary voids extending down from flow surfaces, as lenses between flows within composite flow units, and as neptunian dikes. The common depositional sequence is of limestone lying on a flow and being overlain by hyaloclastic breccia. Limestone is not deposited on hyaloclastic breccia. Limestone deposition was controlled locally by relief on the sea floor that formed as the volcanic rocks accumulated. The limestone is predominantly skeletal packstone; volcanic clasts ranging in size from silt to boulders are locally common. Major constituent fossils are shallow-water bivalve mollusks, barnacles, serpulids, and regular echinoids; most are epifaunal and hard-substrate taxa in contrast to the soft-substrate and burrowing infaunal biota otherwise dominant in Cenozoic strata along the Pacific Coast of North America. The biota is diagnostic of a non-tropical, warm temperate environment. The limestone was deposited within a local basin that formed along the plate boundary at the western margin of the North American Plate. While volcanic rocks accumulated in the basin at bathyal depth, carbonate sediment accumulated on the outer-shelf margin of the basin and was transported intermittently into the basin by gravity flow. Neogene limestone occurs at only a few other sites in southern California. These have an origin that is similar to those in the Conejo Volcanics but differ in occurring with basin fill of diatomaceous sedimentary rocks rather than of submarine volcanics.

Sea ice-free conditions during the Sturtian glaciation (early Cryogenian), South Australia

In the Neoproterozoic snowball Earth hypothesis, shutdown of the planet9s hydrological system has been attributed to a global ice cover during one or more extreme glaciations. In the central Flinders Ranges, South Australia, the Yudnamutana Subgroup of Sturtian age includes diamictite, sandstone, and siltstone units of glaciomarine origin as much as 5000 m thick, and is overlain by postglacial transgressive siltstone and shale of the Tindelpina Shale Member, Tapley Hill Formation. In the central Flinders Ranges, the Yudnamutana Subgroup consists of (1) the Pualco Tillite (gravity resedimented glacial deposits), (2) the Holowilena Ironstone (glacioturbidites), (3) poorly stratified pebbly diamictite of the Warcowie Dolomite Member, lowermost Wilyerpa Formation (gravity resedimented glacial deposits), succeeded by (4) siltstones and sandstones with abundant hummocky cross-stratification (HCS: storm deposits), and (5) a lonestone-bearing succession with cobble-sized clasts in the upper Wilyerpa Formation (ice-rafted debris interpreted to record a glacial re-advance) in which HCS is absent. Because the action of oscillating waves is required to produce HCS on the seafloor, its presence indicates an interval of significant meltback prior to glacial re-advance. Given that the HCS occurs ∼2 km beneath the Tindelpina Shale Member, it signifies a major ice-free interval during the Sturtian glaciation.

Sediment waves and depositional implications for fine-grained rocks in the Cerro Toro Formation (upper Cretaceous), Silla Syncline, Chile

The Cerro Toro Formation contains a variety of lithofacies including pebble and cobble conglomerate, coarse-grained sandstone, thin-bedded mudstone and sandstone, and slumped to chaotic mudstone. Coarse-grained rocks in the Cerro Toro have been described and well documented, whereas characterization and description of mudstone-prone lithofacies are not well documented. These mudstone-prone rocks exhibit two architectural patterns: 1) broad (>200 m) undulating or wavy-bedded elements that laterally terminate by onlap, truncation (toplap) and downlap patterns, and 2) laterally persistent (>400 m), horizontal, thin-bedded mudstone and sandstone. Adjacent to Channel Complex 3 (Paine C Member), these facies exhibit a stratigraphic transition from horizontal to wavy and curved beds concurrent with pronounced aggradation of laterally equivalent, and possibly coeval, channel facies. Sandstone and mudstone beds within the wavy-bedded facies exhibit turbidite lithofacies that include current-ripple lamination (Tc), planar lamination (Tb) massive, graded intervals (Ta), and laminated to structureless silt- and clay-rich beds (Tde). Typically, these beds are a few centimeters thick, but locally, sandstone beds form bedsets over 1 m thick. These thick sandstone bedsets display inclined bedding or lamination associated with mudstone rip-up clasts and are confined to troughs or swales within large-scale wavy-bedded units. Erosion surfaces within this thin-bedded fine-grained lithofacies are spaced vertically at 10–15 m, commonly associated with the crest of curved bedding and display at least 5 m of relief. Based on map patterns, channel-form architectural elements, and stratal-stacking relationships, the coarse-grained rocks in the Cerro Toro have been interpreted as the fill within deep-water channels. In contrast, the thin-bedded mudstone and sandstone lithofacies located adjacent to and eroded into by the channel facies are interpreted as coeval levee facies. Planar, tabular bedsets located at the base of this fine-grained lithofacies are interpreted as initial overbank deposition associated with the upper part of Channel Complex 3, whereas the curved, wavy beds in the fine-grained lithofacies are interpreted as sediment waves developed on the backside of a levee. The curved bedding, lenticular, medium-grained sandstone bedsets, scattered erosional surfaces, and onlap-downlap stratal geometries in the Cerro Toro bear resemblance to sediment waves associated with Quaternary coarse-grained, channel-levee systems. In contrast, sediment waves generated in other deep-water settings, such as upper slope, continental rise or basinal settings, usually are dominated by silt- and very fine-grained sand, very continuous layering (over 1 km), relatively long wavelength (>1 km) and relatively high amplitudes (>10 m). A possible analog for the coarse-grained and fine-grained rocks in the Cerro Toro is the Quaternary Var Ridge and associated fan valley located in the Mediterranean Sea. Like the Cerro Toro, the Var system has coarse-grained material within the fan valley, pebble- to cobble-size clasts, and thin-bedded mud and sand in levees with documented sand bed thickness up to 40 cm and up to medium-grained sand. Architectural details described from the Var Ridge indicated sediment waves make up a significant portion of the levee material. Beds within the sediment waves terminate laterally by onlap downlap, and toplap, similar to the Cerro Toro. The fine-grained material can be linked to deposition from unconfined turbidity currents that were generated via overbank flows from the Var fan channel. The development of these architectural elements requires out of channel turbidity currents and relatively thick flows. Growth of the sediment waves in the Var system represents active sedimentation and turbidity currents within a coeval channel rather than development during periods of channel inactivity or degradation.

Late Cenozoic oscillations of Antarctic ice sheets revealed by provenance of basement clasts and grain detrital modes in ANDRILL core AND-1B

Petrological investigations of the sand fraction and of granule- to cobble-sized clasts in the Plio-Pleistocene sedimentary cycles of the AND-1B drill core at the NW edge of the Ross Ice Shelf (McMurdo Sound) highlight significant down-core modal and compositional variations. These variations provide: (i) direct information about potential source regions during both glacial maxima and minima; and (ii) evidence of an evolving provenance, documented by long-term shifts in compositional patterns that can be interpreted as reflecting variations in ice volume and ice sheet thermal regimes and changes in paleogeography related to the emergence of several volcanic centres during the deposition of the drill core over the past ca. 3.5Ma.The most significant change in diamictite provenance (identified at 82.7metre below the sea floor, mbsf), coincides with a change in sedimentary cycle architecture from sequences that are dominated by diamictites (Mid-Late Pleistocene, above 82.7mbsf) to sequences characterised by cycles of diamictite (subglacial) and diatomite (open-marine) deposition (Pliocene, below 82.7mbsf). In the Mid-Late Pleistocene glacial/interglacial cycles diamictites show high amounts of Skelton–Mulock sourced clasts, supplied from both basement and overlying Beacon and Ferrar supergroups, and they also include a variable contribution from reworking of glacial sediments that were deposited during earlier glacial activity.In the Pliocene to early Pleistocene diatomite–diamictite cycles basement clast compositions indicate the same provenance (Mulock–Skelton) but the main debris load was picked up from volcanic centres in the McMurdo Sound area. Similarly, associated glacial minima sediments (i.e., diatomite intervals) are dominated by volcanic clasts suggesting calving of glaciers from Ross Island or the Koettlitz Glacier region during interglacials.In agreement with previous glaciological reconstructions and numerical ice sheet models, the provenance of glacially transported material is firmly identified in the region between Ross Island and the Skelton–Mulock glacier area (South Victoria Land). The reconstructed ice directions and ice dynamic model are comparable to the configurations proposed for the grounded ice expansion within the McMurdo Sound during the Last Glacial Maximum, and they are also consistent with ice-flow patterns previously reconstructed for Pliocene and Pleistocene glacial settings in the region.

Provenance signatures of the Antarctic Ice Sheets in the Ross Embayment during the Late Miocene to Early Pliocene: The ANDRILL AND-1B core record

Significant down-core modal and compositional variations are described for granule- to cobble-sized clasts in the Early Pliocene to Middle/Late Miocene sedimentary cycles of the AND-1B drill core at the NW edge of the Ross Ice Shelf (McMurdo Sound). Long-term shifts in compositional patterns outline an evolving provenance which is interpreted as reflecting the combined effects and complex interactions among variations in ice volume, ice flow patterns and paleogeographic changes linked to the local tectonic and volcanic activity. High-frequency variations and the petrological features of the basement clast fraction provide direct information about the potential source regions during both glacial maxima and minima. Provenance of the more distal material is identified in the region between Ross Island and the Skelton–Mulock glacier area (South Victoria Land) (Plio-Late Miocene section) and in the Darwin Glacier catchment (Miocene section). The provenance shifts can be discussed for their implications on ice dynamic models for the glacial evolution recorded in the western Ross Embayment. Reconstructed ice flow directions are consistent with the glaciological models for the Last Glacial Maximum, and the provenance data corroborate the contributions of both the East and West Antarctic Ice Sheets in influencing the modifications of the ice flow pattern of grounded ice in the western Ross Embayment in Miocene to Pleistocene time.