Lag Deposits Research Articles

Luminescence dating of rock surfaces in challenging environments: The case of MIS5e gravelly transgressive lag deposit (Southern Sardinia, West Mediterranean Sea)

Determining the age of precise sea level markers such as marine terraces is often difficult because of the inherent limitation of traditional dating methods. A novel method based on Optical Stimulated Luminescence applicable to rock surfaces has been showing great promise in dating boulder and cobble surfaces from various environments. We performed Optically Stimulated Luminescence Rock Surface Dating (OSL RSD) on five cobbles from a basal transgressive lag deposit sealing a marine terrace referred to as the Last Interglacial (Marine Isotopic Stage 5e). We applied a consistent and highly selective 3-step acceptance criteria on five cobbles and obtained that only one was sufficiently well-bleached prior to burial. The resulting ages of 131 ± 8 ka and 127 ± 8 ka (obtained on 22 analyzed aliquots, n = 22) derived from the post-infrared infrared stimulated signal at 225oC (pIRIR225) and the preceding infrared stimulated signal at 50oC (pIR50/225), respectively, are consistent with each other as well as with the conventional luminescence age of ∼135 ka from the same sequence and with the U/Th age of ∼130 ka obtained from coral fragments. This work demonstrates that the RSD is a promising method for dating gravel veneer deposits overlaying marine terraces, enabling new chronologies for similar Quaternary deposits.

The Formation of Deflation Ridges

The deposition of both pumice and shell is common on beaches during calm and storm wave conditions. This paper describes an investigation of pumice and shell ridges at two sites in Australia, Dark Point in NSW, and in the Younghusband Peninsula in SE South Australia. The formation of lines of wave formed shelly and pumice rich deposits on, and above the backshore is described. The deposits are buried by dune development, but then later exhumed by aeolian deflation as deflation plains and basins evolve. Aeolian erosion of the finer sandy sediments leads to the creation of a ridge form, herein termed a deflation ridge. A new schematic model of ridge formation is also detailed whereby deflation ridges are formed by the aeolian erosion and deflation of shell or pumice concentrations and lag deposits.

Storm deposits and trace fossils of Xinji Formation (Cambrian Series 2) in the southern margin of North China Plate

The Xinji Formation (Cambrian Series 2) in the southern margin of the North China Plate developed typical siliciclastic storm deposits and abundant trace fossils. These deposits are characterized by essential indicators, such as erosional surfaces, lag deposits, blocky beddings, hummocky cross stratifications, horizontal or wavy beddings. Storm deposits in the study area are dominated by the development of hummocky cross stratifications, which is categorized into five storm deposit types based on the arrangement of different sedimentary units in the profile. These five types of storm deposits from I to V show the transition from proximal storm deposits to distal storm deposits, the gradual weakening of storm intensity, and the associated growth of quiescent intervals. The trace fossils are mainly developed in the sandstone-mudstone interbeds between storm events and include 14 ichnospecies in 11 ichnogenera: Didymaulichnus isp., Gordia marina, Helminthopsis tenuis, Oldhamia radiata, Palaeophycus striatus, Palaeophycus tubularis, Phycodes isp., Planolites beverleyensis, Planolites montanus, Planolites punctatus, Skolithos isp., Taenidium isp., Torrowangea rosei, Treptichnus pedum. By analyzing the ichnological index (burrow size, BPBI, ichnodiversity and ichnodisparity) from different storm deposits, it can be seen that from storm deposit type I to storm deposit type V, the ichnological index shows an increasing trend. Combined with the relationship between storm deposits and trace fossils, it is inferred that differences in storm intensity result in different colonization windows, and thus variations in ichnological index of preserved trace fossils. Based on the characteristics of storm deposits and distribution of trace fossils, the preservation forms of trace fossils under storm influence are explored.

Engineered ‘landmarks’ associated with Late Paleolithic engraved shelters

Although more than 2000 shelters with non-figurative carvings ascribed to a Mesolithic style are known within the quartzitic sandstone megaclast lag deposits in the southern Paris area, only three shelters with figurative representations of Late Paleolithic style have been recognized. Upright megaclasts are rare in the regional lag deposits but two upright quartzitic sandstone megaclasts occur close to two of these Late Paleolithic shelters. Detailed morphological surveys of these megaclasts allow us to suggest that they were erected by humans. The fortuitous conjunction of finding an upright megaclast close to two Late Paleolithic shelters is infinitesimally small in term of probabilistic considerations. Thus, the Late Paleolithic engraved shelter – upright megaclast pairs are not likely to be independent but are probably linked by culture, and so the megaclasts are expected to date from the Late Paleolithic. We suggest that they were ‘engineered landmarks’ erected to mark places that had cultural significance for Late Paleolithic hunter-gatherer people. The energy dedicated to erect the megaliths is an indication that shelter - megalith pairs probably had important spiritual and symbolic significance. We suggest that both contributed to locate and anchor stories to landscape features. Indeed, these shelter-megalith pairs were in the hinterland of the large open-air camps settled on fords on game migratory routes crossing the Seine River. These specific markers could have encompassed a storyline about landscapes as the support for learning routes through them. One possibility is that they had a timed relationship with herd migration.

Taphonomic analysis of microfossil bonebeds from Western Gondwana: a case study from the late Permian Corumbataí Formation (Paraná Basin, Brazil)

ABSTRACT Storm deposits are common in geological records and encompass a variety of fossil assemblages affected by taphonomic processes that can biased fossil preservation. Here, we describe a microfossil bonebed to demonstrate the role played by certain filters in the final composition of storm-generated fossil accumulations. These layers are recurrent in the Paraná Basin and possibly in other localities of Gondwana, outcropping over 1000 km in the Permian units of the basin. Thirty samples were systematically collected from the state of São Paulo and their components were analysed. These samples consisted of clay matrix, coprolites, scales, and teeth of lower actinopterygian (Palaeonisciformes) and Xenacanthiformes fish. The taphonomic signatures demonstrated that the layer formed as a lag deposit in a distal aquatic environment. Two major taphonomic filters were recognised. The first filter selected bioclasts according to size and density, whereas the second filter preserved only the original phosphatic elements. The third and minor filter were related to breakage, which occured during reworking and diagenesis, respectively. All of these processes contributed to the generation of an assemblage with limited composition and species richness. Thus, distal tempestites might not be useful for estimating palaeodiversity, but may still offer some valuable information about intraspecies variation.

Timeline of events in the Ordovician–Silurian Transition of the Precordillera (Argentina): Paleoenvironmental, paleoclimatic and paleobiologic implications

This paper aims at analyzing the Ordovician–Silurian (Hirnantian–early Llandoverian) Transition in the Central and Eastern Precordillera of San Juan Province based on stratigraphic relationships, diagnostic deposits and sedimentary, paleobiologic and isotope data. At the Central Precordillera, three sections have been examined and sampled: (a) Los Baños de Talacasto, (b) Poblete Norte in the Talacasto area, and (c) northward the Cerro La Chilca section. At the Eastern Precordillera, only the Don Braulio section at the Villicum Range has been considered. These four sections have been selected because they include significant brachiopod assemblages related to the three Transitional Benthic Faunas (TBF 1–3), Hirnantian and Rhuddanian graptolites zones and palynomorph assemblages, in addition to isotope data as a useful tool for dating and correlations. The analysis of these sections has allowed recognizing the timeline of four events that span early Hirnantian–early Llandoverian. Event 1 is linked to the early Hirnantian postglacial transgression, in accordance with TBF 1 and Metabolograptus extraordinarius Zone correlation. Event 2 developed in the middle-upper Hirnantian, is witnessed in a transgressive shallow-water succession, coeval with the Hirnantia Fauna and Metabolograptus persculptus Zone. Event 3 is widely represented by the basal cherty pebbly conglomerate of La Chilca Formation and its correlatives Los Bretes and lower Tambolar formations in the Central Precordillera. This conglomerate is thought to be a lag deposit, which erosively overlies Ordovician strata, as a result of the Hirnantian postglacial transgression, but it is absent in the Eastern Precordillera. Finally, Event 4, at the Villicum section, spans the late Hirnantian–early Llandoverian, standing out a shallow-water, bioturbated mudstone succession, of 12 m thick with no diagnostic fossils, indicating upwelling processes and probably including the Ordovician–Silurian Boundary. However, in the Los Baños de Talacasto and Poblete Norte sections, this succession is composed of a graptolite-rich pelite succession 12 m thick, exhibiting at the base Fe-phosphate strata, and bearing graptolite assemblages of the Hirnantian Metabolograptus persculptus Zone, and the Rhuddanian Parakidograptus acuminatus Zone, indicating the Ordovician–Silurian boundary. It should be noted that the A. ascensus Zone has not been registered to date in the Precordillera.

Sands on Meridiani Planum, Mars

AbstractThe analyses of 184 sediment targets and more than 70,000 individual grains revealed that along the Opportunity rover traverse there are four distinct fractions of deposits related to different geomorphological settings: (a) dust mixed with very fine sand is common behind topographical obstacles, (b) fine sands are deposited in depressions, (c) very coarse sands occur in coarse‐grained ripple fields, and (d) gravel dominates at the rims of craters and on bedrock as lag deposits. Medium size sands were not observed on the plains, but they can be trapped in relatively large craters, where they form dunes and are within coarse‐grained ripples and transverse aeolian ridges (TARs). The fine sands show no regional variations in chemical composition and granulometry, as these sands are easily transported by wind. The very coarse sands vary in composition and shape between the plains and the Endeavour crater rim as their sources are local and their transport distances are short. On the plains, the gravel and the coarse sands are enriched in iron and characterized by higher roundness than the grains from the Endeavour crater rim. The source of iron‐rich, rounded grains on the plains are hematite spherules that are eroded out of Burns formation rocks. The smallest, the best sorted, and the least rounded coarse sand samples are found on coarse‐grained ripple crests. They are mainly composed of spherule fragments and their low roundness indicates shorter transport path lengths than those of grains transported in the past when coarse‐grained ripples migrated.

Changes in Vessel Traffic Disrupt Tidal Flats and Saltmarshes in the Tagus Estuary, Portugal

The Tagus Estuary is one of the largest in Europe with 320 km2, and it has been, for centuries, a gateway to Lisbon. This study focuses on the Moita-Montijo Bay and on the recent dynamics of its tidal flats and saltmarshes. Aerial photographs, orthomosaics, and very high-resolution satellite imagery were used to analyze landcover and shoreline changes. Tidal flats have shown expansion from 1977 to 1995, but since then, contraction dominated, with a change of position of the tidal flat edge of −2.8 m/year in the north bank and −4.2 m/year in the south bank of the Montijo channel. Most contraction occurred along the route of the fast transport catamarans that started operating in 1995, while in the sector without catamaran navigation, expansion was observed. Saltmarshes have been suffering contraction since 1958, with increased rates after 1995 (−0.38 to −0.44 m/year), especially along the catamaran route (−0.57 to −1.27 m/year). The analysis of the wake generated by different vessel types shows a wake increase with the catamarans, in agreement with the increase in contraction along the Montijo channel. Inside abandoned salt pans, saltmarshes expanded. Since 1995, major changes are also observed along the tidal flat margin, with the formation of coarse lag deposits of coarse sands and shells. Given the contraction increase associated with catamaran traffic and the resulting degradation of the tidal flat and the saltmarshes, it is important to introduce measures for containing contraction.

Regolith of the Crater Floor Units, Jezero Crater, Mars: Textures, Composition, and Implications for Provenance

AbstractA multi‐instrument study of the regolith of Jezero crater floor units by the Perseverance rover has identified three types of regolith: fine‐grained, coarse‐grained, and mixed‐type. Mastcam‐Z, Wide Angle Topographic Sensor for Operations and eNgineering, and SuperCam Remote Micro Imager were used to characterize the regolith texture, particle size, and roundedness where possible. Mastcam‐Z multispectral and SuperCam laser‐induced breakdown spectroscopy data were used to constrain the composition of the regolith types. Fine‐grained regolith is found surrounding bedrock and boulders, comprising bedforms, and accumulating on top of rocks in erosional depressions. Spectral and chemical data show it is compositionally consistent with pyroxene and a ferric‐oxide phase. Coarse‐grained regolith consists of 1–2 mm well‐sorted gray grains that are found concentrated around the base of boulders and bedrock, and armoring bedforms. Its chemistry and spectra indicate it is olivine‐bearing, and its spatial distribution and roundedness indicate it has been transported, likely by saltation‐induced creep. Coarse grains share similarities with the olivine grains observed in the Séítah formation bedrock, making that unit a possible source for these grains. Mixed‐type regolith contains fine‐ and coarse‐grained regolith components and larger rock fragments. The rock fragments are texturally and spectrally similar to bedrock within the Máaz and Séítah formations, indicating origins by erosion from those units, although they could also be a lag deposit from erosion of an overlying unit. The fine‐ and coarse‐grained types are compared to their counterparts at other landing sites to inform global, regional, and local inputs to regolith formation within Jezero crater. The regolith characterization presented here informs the regolith sampling efforts underway by Perseverance.

Sedimentary characteristics of Chang 2 and its relationship with sand body superimposition in Resiwan oil region

On the basis of core observation, logging interpretation and previous research results, the sedimentary microfacies characteristics of each sub layer of the Chang 2 oil-bearing formation in the Resiwan oil area of Zichang Oilfield and the superimposed relationship of sand bodies are studied. The results show that the Chang 2 member in the Resiwan oil area of Zichang oilfield belongs to braided river deposits, and the study area mainly develops sedimentary microfacies such as river island, channel lag deposits and floodplain. The vertical superimposition types of sand bodies in Chang 22 oil-bearing formation are multilayer superimposition sand bodies and massive thick sand bodies, which are also the dominant sand body combination of oil-bearing reservoirs in the study area. Chang 21 oil-bearing formation is dominated by thin interbeds. Flood plains are developed at the top of Chang 23 oil-bearing formation, and massive thick sandstone is developed at the middle and lower parts, but the horizontal continuity is poor. Horizontally, the sand bodies of Chang 22 oil-bearing formation are mainly overlapped horizontally and cut laterally, which improves the connectivity of the sand bodies to a certain extent. Chang 21 and Chang 22 are mostly isolated sand bodies, and the sand bodies are wrapped in mud, resulting in poor oil and gas display. The contact pattern of sand bodies in the study area is mainly affected by sedimentary microfacies and river water volume.

Geomorphological significance of the Atacama Pediplain as a marker for the climatic and tectonic evolution of the Andean forearc, between 26° to 28°S

Pediplains are classically identified as flat landscape surfaces in arid regions linked to tectonic quiescence, whereas deep incision of a pediplain is attributed to tectonic uplift. In the Atacama Desert, pediplains are generally used as morphotectonic markers to define the chronology of episodes of Late Cenozoic Andean uplift from their erosion and incision patterns and timings. The Atacama Pediplain (AP) extends over >12,000 km2 (26° to 28°S Lat) through the Central Depression and Precordillera of the southern Atacama Desert. In this study we perform geomorphologic and stratigraphic observations on the AP in the Salado Canyon area, combining new geochronological results derived from 40Ar/39Ar biotite ages from volcanic layers interbedded within the alluvial deposits, and 21Ne exposure ages on quartz-clasts on alluvial plains, to determine the chronology of the AP evolution. Results show that the evolution of the AP is a long-term and continuous process (from >20 to ~2.3 Ma) of alluvial deposition and subsequent alluvial plain formation developed by interplay between the climate variability of the Atacama Desert and Andean uplift. The AP alluvial deposition occurred in two spatially and temporally separated episodes of alluvial backfilling: 1) shortly before ~20.14 Ma and to prior to ~9.4 Ma, a timespan that allows for the drainage capture of the eastern Precordillera, and considerable landscape rearrangement; 2) post ~9.4 Ma, with a re-positioning of alluvial backfilling from the Precordillera towards the Central Depression. This occurs after the Mid Miocene onset of the hyperarid conditions in the Central Depression and is more likely due to late Miocene surface uplift. Exposure ages reveal the cessation of basin-scale deposition and the abandonment of the alluvial plains during ~5.24 to ~3.8 Ma linked to the incision of Salado Canyon. Subsequent climate conditions modulated the surfaces by the development of lag deposits until ~2.69 to ~2.3 Ma when hyperarid conditions reach a threshold that limits surficial activity. Additionally, the drainage capture of the Precordillera by headwards erosion of the Salado Canyon explains marked deep incision depth of this canyon without the need for an increase in surface uplift or a change in climate conditions. The AP is not a general marker of a single climatic or tectonic event/period but a composite paleosurface formed by a complex concatenation of extrinsic and intrinsic geomorphic processes over more than ~17 myr.

Surface Roughness Variation Across Polar Ice Deposit Boundaries on Mercury

AbstractWe analyze the surface roughness of five north polar craters on Mercury using 125 m/pixel Mercury Laser Altimeter data. Each crater hosts a polar deposit (PD) with a low‐reflectance surface. The PDs have distinct geologic contacts, enabling an analysis of roughness across deposit boundaries onto ice‐free portions of the host craters' floors. The low‐reflectance surfaces in Angelou, Despréz, and Ensor collocate with radar‐bright signatures consistent with the presence of several‐meters‐thick water‐ice deposits beneath their low‐reflectance surfaces. Subdued roughness in these three craters is consistent with the superposition of several meters of ice. The difference between roughness on and off their low‐reflectance surfaces is within one standard deviation (SD) of the results, but is found to be statistically significant, as indicated by K‐S and Mann‐U Whitney tests, and meaningful, as indicated by Cohen's d tests. There is no meaningful difference in Jiménez and Josetsu, two craters that lack strong radar‐bright signatures, consistent with the hypothesis that they do not host substantial water ice beneath their low‐reflectance surfaces and thus not all of Mercury's available cold traps are occupied by water ice. Analyzing the roughness of polar craters may provide insight into the presence/absence of thick volatile deposits, but higher‐resolution topography would be helpful given the high SD associated with roughness. Finally, we identify subdued roughness along three PDs that may be related to thick lag deposit margins or enhanced diffusive mixing along deposit edges. Roughness may provide new insight into surface characteristics of the ices and inform surface evolution models.

Teredolites driftwood from the Arkadelphia Formation–Midway Group Contact (K–Pg), Malvern, Arkansas, USA

Partially carbonized driftwood recovered from a lag deposit at the Arkadelphia Formation–Midway Group Contact (K–Pg) near Malvern, Arkansas contains an abundance of macrobioerosion. Macroborings are oriented perpendicular and oblique to the wood grain, straight to sinuous in shape, ≤8 cm long, may have calcitic linings, and belong to Teredolites clavatus (Kelly & Bromley, 1984). The abundance and tightly spaced position of borings on all driftwood surfaces suggests the wood was afloat and/or exposed on or near the seafloor in a shallow marine setting for an extended period of time before becoming buried. Transverse, radial, and tangential thin section analysis of the driftwood identifies distinct growth rings, tracheid and ray cells, ray pits, and resin canals of the bald cypress genus, Taxodium. The presence of Taxodium wood in addition to a diverse and abundant assemblage of transitional to shallow marine vertebrates in the Gulf Coastal Plain of southwestern Arkansas indicate that brackish water swamps were in proximity to the ancestral shoreline around the K–Pg boundary and that these remains were concentrated into a lag deposit as the result of storm events, sea-level cyclicity, and possibly impact generated tsunamis.

Postcranial description of Wendiceratops pinhornensis and a taphonomic analysis of the oldest monodominant ceratopsid bonebed.

The early centrosaurine ceratopsid, Wendiceratops pinhornensis, was discovered in Alberta, Canada in a medium density monodominant bonebed from the Oldman Formation (mid-Campanian, ~79 Ma). The bonebed contains abundant, well-preserved, adult-sized and some juvenile-sized postcranial material, allowing for the first description of a number of elements of the postcrania of this basal centrosaurine ceratopsid. The postcranial elements described are generally consistent with postcrania described for more derived centrosaurine taxa. However the rectangular-shaped distal terminus of the ischium previously considered to be an apomorphy of Wendiceratops. is shown to also be present in Medusaceratops, and thus may be a synapomorphy of basal centrsaurines. The bonebed represents a lag deposit within a mudstone-bearing overbank facies and contains individuals from multiple age classes. It contains over 95% ceratopsid remains, with all identifiable elements referable to Wendiceratops. The elements are completely disarticulated, but have undergone little weathering or abrasion (both Stage 0), although the ends of long bones and processes capped by cartilage in life frequently exhibit evidence of wet rot and breakage by hydrological reworking after decomposition. The taphonomy of the bonebed is consist with other monodominant centrosaurine bonebeds that have been interpreted as mass death assemblages preserving evidence of gregarious (herding) behavior. At approximately 79 million years old, the Wendiceratops bonebed is approximately two million years older than other ceratopsid bonebeds indicating that this bonebed is the oldest documented evidence of herding behavior in a ceratopsid.

New insights for origin of fine-grained sediments from the early neogene Qaidam Basin: Wind and fluvial–lacustrine source-to-sink processes

Fine-grained sediment deposition is widely distributed in lacustrine and marine facies and significant for petroleum geology. However, the causes of fine-grained sediment deposition remain controversial. In this study, we examine the cause of fine-grained sedimentation in the Neogene saline lacustrine Qaidam Basin, and propose a new insight into the sedimentary process for fine-grained sediments. Various sedimentary structures identified from the fine-grained sediments such as erosional structures with lag deposits, hummocky cross-stratification (HCS)-like structures, asymmetrical ripples, or sandy ball-and-pillow structures, are generated by winter storms. These winter storms are related to the winter winds that cause eolian sedimentation in Asia. Abundant detrital material may be rapidly transported into lakes by winter storms, favoring extensive terrigenous detrital deposition in the lacustrine basin, hardly controlled by the division of sedimentary facies and topographical differentiation. The genetic features and shallow-water origin with wind and fluvial–lacustrine source-to-sink processes, may be the main reason for the poor hydrocarbon potential of the Neogene fine-grained sediments in the Qaidam Basin.

Orbital Observations of a Marker Horizon at Gale Crater

AbstractA dark‐toned, indurated, smooth horizon, and (or) bed (herein called “marker horizon”) is exposed across much of the northwestern, southwestern, and southeastern portions of Mount Sharp in Gale crater. Compact Reconnaissance Imaging Spectrometer for Mars spectra taken from the marker horizon exhibit evidence for the presence of high calcium pyroxene and other basaltic minerals in contrast to the hydrated sulfate signatures associated with strata above and below it. Mean dips for the marker horizon and sulfate‐bearing strata are 1–5° with almost all azimuths radially away from the center of Mount Sharp. The marker horizon thickness varies between <1 and 3 m and its elevation changes by 1.6 km across the mound. Surface slopes along the sulfate‐bearing strata above and below the marker horizon are typically 20°–40° compared to <5° surface slopes on the marker horizon. Features observed on the marker horizon include ridges, fractures, faults, layering, oval depressions, small craters, and possible yardangs/indurated bedforms. We interpret the marker horizon to be a single unit that formed contemporaneously across Mount Sharp during the same depositional sequence that created the sulfate‐bearing strata. Plausible formation mechanisms for the marker horizon include (1) emplacement of a more indurated sulfate unit, either from (a) primary deposition or (b) secondary diagenesis; (2) deposition of a more resistant sandstone unit during a brief drier period; (3) emplacement of a volcanic ash deposit laid down in the midst of the sulfate formation period; or (4) a lag deposit created during a drier period. Based on our observations, origins (1a) and (3) are the most plausible.

Morphology and Late Pleistocene–Holocene sedimentation of the Strait of Istanbul (Bosphorus): a review

Abstract The Bosphorus (Istanbul) Strait is a natural strait that connects the Black Sea with the Aegean Sea via the Sea of Marmara and the Dardanelles Strait. It is a 31 km-long and 3.5 km-wide winding channel, with an irregular bottom morphology. It has depressions up to 110 m deep, and two sills with depths of 35 and 58 m in the south and north, respectively. Presently, a two-layer water exchange exists through the strait, with the Mediterranean and Black Sea waters forming the lower and upper layers, respectively. The Bosphorus channel extends as shelf valleys on the Black Sea and Sea of Marmara shelves. However, it operated as a river valley or an estuary during the stadial lowstand periods. The infill sedimentary succession of the Bosphorus channel is up to 100 m thick above the Paleozoic–Cretaceous basement with an irregular topography. The oldest sediments are sandy to muddy fluvial–lacustrine facies of late Pleistocene age, which are preserved only in up to 160 m-deep scoured depressions of the basement. They are overlain by mid–late Holocene estuarine–marine shelly sandy to muddy sediments with patches of bioherms and shelly lag deposits. The Bosphorus outlet areas of the Black Sea and Sea of Marmara are characterized by a submarine fan and a shelf valley, respectively. The fan system in the Black Sea started depositing c. 900 years after the initial vigorous marine water incursion at c. 8.4 14 C ka BP. On the Marmara shelf, extension of the Bosphorus channel is a sinuous shelf valley with a channel–levee complex that was deposited by the Black Sea outflow during 11–10 14 C ka BP. Catastrophic floodings of the Sea of Marmara by torrential Black Sea outflows during the Greenland Interstadial melt-water pulses, as well as the strong Mediterranean current towards the Black Sea during the interglacial periods, were responsible for carving out the Bosphorus channel and the shelf valleys, as well as removing the sediments belonging to the earlier periods.

Investigating Sources of Spectral Olivine Enrichments in Martian Bedrock Plains Using Diurnal Emissivity Changes in THEMIS Multispectral Images

AbstractWe investigate spectral olivine detections in the Noachian cratered highlands that are spatially associated with flat bedrock exposures. Strong olivine detections were previously attributed to relatively high (∼10%–15%) modal abundances of olivine in the bedrock, and then later hypothesized to instead originate from a patchy cover of olivine‐rich sediment formed from erosional lag deposits and mineral‐specific comminution/transport. We test these hypotheses by developing a new technique to partially separate the spectral signals of bedrock and unconsolidated surficial sediment. It uses the differing thermophysical properties of sand and indurated materials, which change the relative contributions of these materials to the emitted spectrum over the diurnal cycle. We use multispectral Mars Odyssey Thermal Emission Imaging System (THEMIS) observations collected in the mid‐afternoon and early evening to apply this technique to eight Terra Sabaea bedrock plains and the Nili Fossae olivine‐carbonate plain. Significant changes in spectral shape arising from compositional differences between bedrock and sediment cover are observed in four bedrock plains. Where these differences are observed, the spectral changes are consistent decreased olivine abundance in the sediment fraction, rather than increased as previously hypothesized. Our results indicate that bedrock materials in the Terra Sabaea region are olivine‐rich in composition, likely indicating a pyroclastic origin for these materials.

Tectonic and eustatic control on channel formation, erosion, and deposition along a strike-slip margin, San Diego, California, USA

Active margins are characterized by small mountainous streams, which contribute significantly to global sediment transport from land to sea. The response of active margin streams to base-level change due to eustatic sea-level cycles is less well understood than passive margin streams, which form the basis of most sequence stratigraphic models. The southern California margin offers an opportunity to investigate the evolution of small mountainous streams along a strike-slip margin over a sea-level cycle. High-resolution Chirp subbottom data were examined from the Silver Strand littoral cell in San Diego, California, which is located in a region of localized transtension associated with a step in the Newport Inglewood-Rose Canyon fault zone. Chirp data reveal active faulting on multiple fault strands, a subaerial surface likely formed from MIS 4 to MIS 2, fluvial incision associated with MIS 2, and the transgressive surface of erosion formed during sea-level rise from MIS 2 to MIS 1. Interpreted acoustic units observed in the data include the Cretaceous outcropping Rosario Group, Miocene to Pliocene prograding clinoforms, subaerial deposits associated with MIS 4 to MIS 3, channel fill and transgressive lag deposits formed during the most recent sea-level transgression, and Holocene marine sediment. The Chirp data examined in this study build upon findings from other seismic studies, sediment cores, trenching, and outcrops that have been examined on- and offshore San Diego. Combined, these data suggest that the fault-controlled pull-apart basin formed in this region influences sediment transport and deposition throughout the last glacial cycle. During highstand, the fault-controlled wide shelf, subsiding basin, and circulation patterns result in trapping of sediment in San Diego Bay and nearshore, cutting off sediment delivery to the deep-sea. During lowstand, the fault-controlled morphology creates a drainage divide that deflects streams to the south which lengthens and reduces slope of the drainage pathways. These findings are in contrast to observations in other fault controlled littoral cells along the southern California margin where sediment delivery to the deep-sea remains active during highstands, highlighting the variability of stream response to base-level change along active strike-slip dominated margins.

Impacts of in‐stream aggregate mining on the distributary channels of the alluvial fans of Tafí valley (Northwest Argentina)

AbstractIn‐stream mining is a common aggregate mining practice around the world. However, the impacts of such practice are not always taken into account when the mines are established, and the environmental cost of in‐stream aggregate mining is generally not assessed. The purpose of this study was to evaluate the impacts of aggregate mining conducted in La Puerta River (Tafí valley, Northwest Argentina), a dry‐pit in‐stream mine, by considering the geomorphological, geoarchaeological, and human effects. Multitemporal analysis of the area using remote sensors (aerial photographs and satellite images 1970–2020), high‐resolution drone digital elevation models and orthomosaics (2018–2019), and an intensive survey demonstrated that the mining area grew exponentially between 2002 and 2020 under unregulated mining. As a result, this practice exerts great environmental impact, including channel section alterations and destabilization of riverbanks, soil loss, river profile changes, and the formation of lag deposits of discarded materials, thus increasing environmental hazard under unpredictable flows. Finally, this is a highly touristic area that has suffered substantial landscape degradation and irreversible archaeological damage.