Infilling Of Cavities Research Articles

Applications of Micro-CT Imaging in Age-At-Death Estimates of Maya Dogs

ABSTRACT While there are many techniques for estimating age-at-death in archaeological dogs, the pulp cavity/tooth width ratio is considered one of the most accurate methods. This study adapts this technique for application to MicroCT imaging, a non-destructive methodology that is rapidly gaining ground in faunal analysis. Mandibular first molar and upper and lower canine teeth recovered from two Late Classic-Early Postclassic period sites in the Maya highlands, Moxviquil and Tenam Puente, were imaged using a SCANCO μCT35 MicroCT scanner. The widths of tooth roots, pulp cavities, cuspal enamel thicknesses, and enamel attrition measurements were then taken using both the scanner’s post-processing system and 3D Slicer, an open-access programme designed for imaging biomedical scans and other 3D files, and pulp cavity infilling ratios were calculated to obtain an age-at-death estimates in months for each specimen. Based on these, this study presents preliminary interpretations of canid age-at-death patterns at Moxviquil and Tenam Puente, including a range of juvenile specimens from a funerary cave context at Moxviquil.

Contemporary formation of ice-wedge pseudomorphs during the expansion of a thermokarst lake in Old Crow Flats, Yukon, Canada

Ice-wedge pseudomorphs (IWPs) are thermokarst structures that form by the secondary infilling of cavities left by the slow melting of ice wedges. Contemporary IWP formation in periglacial environments informs our understanding of past processes and dynamics implied by their presence in the stratigraphic record. However, contemporary IWPs are seldom studied as they are difficult to locate and observe due to the general lack of surface expression and because they can deform with time or be confused with sediment wedges. This study presents detailed information about the morphology and soil composition of IWPs that formed beneath a thermokarst lake in Old Crow Flats, northern Yukon (Canada), and provides the first conceptual model of lacustrine IWP development. Seven IWPs were unearthed along a transect perpendicular to a former eroding lakeshore in a recently (2019), partially drained lake basin. The IWPs were between 76 cm and 122 cm high and had varying morphologies, generally forming triangles pointing down or T shapes. They had a much higher organic matter content and were composed of slightly coarser sediment than their host sediment, characteristics that were shared by basin floor surface sediment found in the near-shore zone (<15 m) of the former lake. Within five meters of the former shoreline, some troughs showed pronounced subsidence due to the melting of remnant wedge ice following lake drainage. Based on ice-wedge dimensions, bank height, shore erosion rates, and field measurements of sublake active-layer thickness prior to lake drainage, it is likely that ice wedges were entirely melted by the development of the sublake active layer, prior to talik development. Together, the results indicate that the IWPs formed within 5 to 10 m of the eroding shore bank, 9 to 18 years after lake submergence, and were filled with the organic-rich lacustrine sediment that accumulated in the near-shore zone due to sediment sorting by wave action following the erosion of peaty polygonal tundra. This study highlights that IWPs can form during the natural evolution of thermokarst lakes in regions with stable permafrost and are not necessarily indicators of conditions leading to generalized permafrost degradation.

Reconstructing depositional environments through cave interior facies: The case of Galería Complex (Sierra de Atapuerca, Spain)

This work showed that cave sediments are useful for geomorphologic studies and for reconstructing depositional environments. While the cave entrance facies have been extensively studied for their relationship with the fossil and archaeological record, the cave interior facies have received much less attention, although they can provide much information on the geomorphological evolution of the karst. This work presents the stratigraphic and sedimentological study of a section >6 m thick and 10 m long of cave interior sediments of Galería Complex (Sierra de Atapuerca, Spain). Galería Complex is a cavity infill of the Sierra de Atapuerca (Spain), composed of three sections filled by at least 30 m of Pleistocene sediments. This sequence is divided into 5 lithostratigraphic units named from bottom to top: GI – GV. GI unit is 19 m thick of interior facies in the base of the Galería Complex, divided into two sub-unit, GIa and GIb, by the Matuyama-Bruhnes paleomagnetic boundary. GI unit shows an issue with the chronology since has uncoherent between TT-OSL and ESR/U-series and paleomagnetism dates. This work has been done by combining field observation with laboratory sedimentary analysis to characterize the texture and structure of the sediments. Based on these studies, 12 layers and 9 sedimentary facies have been identified. The facies associations indicate a clear separation between GIa and GIb sub-units. GIa sub-unit is dominated by epiphreatic conditions and represents continuous relativity sedimentation during the Early Pleistocene; meanwhile, GIb shows important erosion events and facies with reworked materials that indicate vadose conditions during the Middle Pleistocene. This environmental change is related to the geomorphological evolution of the Arlanzón River. In addition, soft-sediment deformation structures have been described, including faults and low-angle folds. An important leaching process has been identified by the presence of phosphates that could explain the underestimated ages obtained in other works.

Age estimation of archaeological dogs using pulp cavity closure ratios

This study describes a non-destructive method that can be used to estimate the age of archaeological dog remains involving tooth pulp cavity closure ratios. This technique was first developed in wildlife management and zoology for wild carnivores. For the first time, we develop this technique for dogs by utilizing a modern sample of 751 teeth roots from 106 animals with known life histories. The method involves measuring widths of teeth roots and their pulp cavities by means of X-ray images, and then calculating the ratios of pulp cavity infilling for each tooth. The method is developed for upper and lower canines and the distal and mesial roots of the mandibular M1. Our assessments demonstrate that upper and lower canines are most strongly correlated with age, followed by the mesial and distal roots of the mandibular M1. The method is useful for assigning specimens to relative age categories, and proved most reliable with younger individuals. We illustrate the applicability of the method by analyzing 369 teeth roots from 77 archaeological dogs from the Iron Age Ust’-Polui site in Arctic Siberia. Previously, ageing of the dog remains at this site involved only assessments of dental eruption patterns and long bone epiphysis fusion. The overall age estimation results from Ust’-Polui produced a mortuary profile dominated by juveniles and prime age adults, suggesting a regional preference for dogs of these age groups in sacrifices.

Fracture, Dissolution, and Cementation Events in Ordovician Carbonate Reservoirs, Tarim Basin, NW China

Ordovician carbonate rocks of the Yijianfang Formation in the Tabei Uplift, Tarim Basin, contain deeply buried (&gt;6000 m), highly productive oil and gas reservoirs associated with large cavities (&gt;10 m). Previous workers inferred that large cavities are paleocaves (paleokarst) formed near the surface and subsequently buried. Alternately, caves may have formed by dissolution at depth along faults. Using 227 samples from 16 cores, we document textures and cement compositions bearing on cavity histories with petrographic, high-resolution scanning electron microscopy (SEM), isotopic, and fluid inclusion microthermometric observations. Results show that dissolution occurred at depth and was caused by (1) acidic fluids derived from Middle-Late Silurian and/or Devonian-Permian hydrocarbon generation and maturation, (2) high-temperature fluids, of which some were associated with Late Permian igneous activity, and (3) Mg-rich fluids that accompanied Jurassic-Cretaceous deformation and the formation of partially open fractures and stylobreccias (fault breccias). The relative paragenetic sequence of the structure-related diagenesis suggests seven stages of fracturing, dissolution, and cementation. Mottle fabrics in the Yijianfang Formation contain argillaceous carbonate-rich silt and are bioturbation features formed within the marine environment. Those mottled fabrics differ from clearly karstic features in the overlying Lianglitage Formation, which formed by near-surface dissolution and subsequent infilling of cavities by allochthonous sediment. Mottle fabrics are crosscut by compacted fractures filled with phreatic-vadose marine cements and followed by subsequent generations of cement-filled fractures and vugs indicating that some fractures and vugs became cement filled prior to later dissolution events. Calcite cements in fractures and vugs show progressively depleted values of δ18O documenting cement precipitation within the shallow (~220 m), intermediate (~625 m), and deep (~2000 m) diagenetic environments. Deep (mesogenetic) dissolution associated with fractures is therefore the principal source of the high porosity-permeability in the reservoir, consistent with other pieces of evidence for cavities localized near faults.

Submarginal debris transport and till formation in active temperate glacier systems: The southeast Iceland type locality

Exhaustive sedimentological analysis of freshly exposed subglacial surfaces and moraines in southern Iceland provides diagnostic sedimentological signatures of: a) debris transport pathways through active temperate glacier snouts; and b) till production in subglacial traction zones dominated by deforming layers. Three till end members are recognised based on stratigraphic architecture: 1) thin and patchy tills over eroded bedrock; 2) single push moraines and complexes; and 3) overridden moraines or outwash fans. Typical till thicknesses are 0.10–1.40 m, with each till relating to a deformation event driven by the seasonally tuned processes of glacier sub-marginal shearing, freeze-on, squeezing and bulldozing. Clast form trends demonstrate progressive modification towards mature forms in subglacial traction zones with till being clearly differentiated from scree and glacifluvial deposits. Clast macrofabric strengths are variable, rarely matching those of laboratory shearing experiments, except where obviously lodged clasts are abundant. They also consistently record former glacier flow directions. But localized variability is introduced by bedrock protuberances, cavity infill, clast interference and freshly imported plucked clasts. Within tills, macrofabrics strengthen from lower (B horizon) to upper (A horizon) tills but at the outer edges of sub-marginally thickening till wedges or push moraines, seasonally-driven cycles of squeezing/flowage, freeze-on/melt-out and bulldozing give rise to a range of clast macrofabric strengths as well as superimposed deformation signatures. This reflects two extremes of till emplacement including the more mobile, flowing and often liquefied matrixes in push/squeeze moraines and, in contrast, the lodgement, deformation and ploughing at the thin end of sub-marginal till wedges.

Using UAV acquired photography and structure from motion techniques for studying glacier landforms: application to the glacial flutes at Isfallsglaciären

AbstractGlacier and ice sheet retreat exposes freshly deglaciated terrain which often contains small‐scale fragile geomorphological features which could provide insight into subglacial or submarginal processes. Subaerial exposure results in potentially rapid landscape modification or even disappearance of the minor‐relief landforms as wind, weather, water and vegetation impact on the newly exposed surface. Ongoing retreat of many ice masses means there is a growing opportunity to obtain high resolution geospatial data from glacier forelands to aid in the understanding of recent subglacial and submarginal processes. Here we used an unmanned aerial vehicle to capture close‐range aerial photography of the foreland of Isfallsglaciären, a small polythermal glacier situated in Swedish Lapland. An orthophoto and a digital elevation model with ~2 cm horizontal resolution were created from this photography using structure from motion software. These geospatial data was used to create a geomorphological map of the foreland, documenting moraines, fans, channels and flutes. The unprecedented resolution of the data enabled us to derive morphological metrics (length, width and relief) of the smallest flutes, which is not possible with other data products normally used for glacial landform metrics mapping. The map and flute metrics compare well with previous studies, highlighting the potential of this technique for rapidly documenting glacier foreland geomorphology at an unprecedented scale and resolution. The vast majority of flutes were found to have an associated stoss‐side boulder, with the remainder having a likely explanation for boulder absence (burial or erosion). Furthermore, the size of this boulder was found to strongly correlate with the width and relief of the lee‐side flute. This is consistent with the lee‐side cavity infill model of flute formation. Whether this model is applicable to all flutes, or multiple mechanisms are required, awaits further study. © 2016 The Authors. Earth Surface Processes and Landforms published by John Wiley &amp; Sons Ltd.

The Unicorn Cave, Southern Harz Mountains, Germany: From known passages to unknown extensions with the help of geophysical surveys

In soluble rocks (limestone, dolomite, anhydrite, gypsum, …), fissures and bedding partings can be enlarged with time by both physical and chemical dissolution of the host rock. With time, larger cavities evolve, and a network of cave passages can evolve. If the enlarged cave voids are not too deep under the surface, geophysical measurements can be used to detect, identify and trace these karst structures, e.g.: (i) gravity revealing air- and sediment-filled cave voids through negative Bouguer anomalies, (ii) electrical resistivity imaging (ERI) mapping different infillings of cavities either as high resistivities from air-filled voids or dry soft sediments, or low resistivities from saturated sediments, and (iii) groundwater flow through electrical potential differences (SP) arising from dislocated ionic charges from the walls of the underground flow paths.We have used gravity, ERI, and SP methods both in and above the Unicorn Cave located in the southern Harz Mountains in Germany. The Unicorn Cave is a show cave developed in the Werra dolomite formation of the Permian Zechstein sequence, characterised by large trunk passages interrupted by larger rooms. The overburden of the cave is only around 15m, and passages are filled with sediments reaching infill thicknesses up to 40m.We present results from our geophysical surveys above the known cave and its northern and southern extension, and from the cave interior. We identify the cave geometry and its infill from gravity and ERI measurements, predict previously unknown parts of the cave, and subsequently confirm the existence of these new passages through drilling. From the wealth of geophysical data acquired we derive a three-dimensional structural model of the Unicorn Cave and its surrounding, especially the cave infill.

Pleistocene sedimentary facies of the Gran Dolina archaeo-paleoanthropological site (Sierra de Atapuerca, Burgos, Spain)

Abstract Gran Dolina is a cavity infill of the Sierra de Atapuerca, containing three important Early and Middle Pleistocene archaeo-paleontological layers, including hominid bones, fauna and lithic remains. Due to the relevance of this site to understand human evolution in Europe, it is essential to define in detail the sedimentary processes and environments associated with the archaeological remains. Gran Dolina has a 19 m thick sedimentary infill divided into 11 lithostratigraphic units. In this work, we describe the sedimentary facies of the Early and Middle Pleistocene units and we update its stratigraphy. For that purpose, we have studied the stratigraphic excavation profiles available, where we have combined field observations with laboratory sedimentary analysis (sieving, laser diffraction, and image analysis) to characterize the texture and structure of the sediments. Through these studies, 19 sedimentary facies have been distinguished, grouped as sediment gravity flow facies, fluvial facies and autochthonous facies. The facies associations indicate two main trends in the allochthonous sequence. During the Early Pleistocene (TD4–TD7), the cavity acted as a stream sink, where channel and floodplain facies migrated along the sequence, and were interbedded with lateral gravity sediment flows. On the other hand, the Middle Pleistocene sequence between TD7 and TD10 is dominated almost exclusively by gravity flows. At least three main entrances have been inferred from input directions of the sediments, which changed over time. Sediment characteristics have allowed us to preliminarily infer environmental conditions around the cavity.

Occurrence of diagenetic alunites within karst cavity infill of the Dammam Formation, Ahmadi, Kuwait: an indicator of hydrocarbon gas seeps

Alunite minerals occur as white powdery lumps and laminated coloured deposits within cavity and solution channel infill of the palaeokarst zone of the Upper Eocene Dammam Formation. This formation is exposed in a quarry located on the Al Ahmadi ridge within the Greater Burgan oil field in southern Kuwait. Field occurrences and sedimentary structures of the alunite deposits were described. Collected samples were petrographically described, and their mineralogy and geochemistry were determined using X-ray diffraction and X-ray fluorescence, respectively. Microfabrics were investigated using SEM, revealing that they are primarily composed of fibrous alunogen (hydrous aluminium sulphate) and pseudo-cubical K-alunite (hydrous potassium aluminium sulphate). Their mode of occurrence suggests a hypogenetic origin, where sulphide gases associated with hydrocarbon gases reacted with an Al-rich solution leached from clay minerals and feldspars of the cavity-fill muddy sand sediments. The hydrocarbon gases may have seeped from sub-surface petroliferous formations within the Greater Burgan oil field along vertical fractures. This study suggests that these acidic seeps may have played a role in the development of the palaeokarst zone of the Dammam Formation.

The Janda cavity at Fruška Gora, the first cave assemblage from the southeast Pannonian lowland (Vojvodina, Serbia)

The recently discovered Janda cavity yielded the first large mammal fossil assemblage of the Late Pleistocene age in the southeast Pannonian lowland (Vojvodina), outside the karst region of Serbia. The cavity is formed in the Badenian reef limestone on the northern hillside of the Fruška Gora mountain. The paper provides a paleontological description and taphonomic data on mammal remains collected from the loose sediments or scree. Herbivores are represented by Bison priscus (Bojanus, 1827), which is the most abundant species, followed by Megaloceros giganteus (Blumenbach, 1803) and Equus germanicus Nehring, 1884, with several remains of Mammuthus sp. and Coelodonta antiquitatis (Blumenbach, 1799). Carnivores are represented by Ursus spelaeus ingressus (Rabeder et al., 2004), Crocuta crocuta spelaea (Goldfuss, 1823), Panthera leo spelaea (Goldfuss, 1810), as well as Canis lupus Linnaeus, 1758. Wolf remains are characterized by their large size, almost reaching the size of hyenas, which is in contrast with the finds of rather small Pleistocene wolves from the caves in the mountainous part of Serbia. The faunal composition and the presence of mostly grazing forms among the herbivores indicate glacial conditions and an open landscape in the surroundings of the site during the formation of the Janda cavity infill. Taphonomic observations indicate the cave hyena as a dominant bone accumulating agent. In contrast to other cave hyena sites in Serbia, where mostly small and medium sized prey remains were accumulated, prey preference for large and extra large mammals is noted. It is supposed that this is the consequence of the differences between mountainous and lowland landscapes and environments, climatic conditions, different composition of animal and plant associations, and also in different modes of competitions between predators.

Carbon and oxygen isotopic composition of the Middle Miocene Badenian gypsum-associated limestones of West Ukraine

The Middle Miocene Badenian basin of the Carpathian Foredeep is characterized by complex sedimentary and diagenetic carbonate-evaporite transitions. Six locations have been selected to evaluate the controls on the carbon and oxygen isotopic composition of the Badenian gypsum-associated limestones of the Tyras Formation in West Ukraine. At three locations marine limestones overlie the gypsum, at one location (Anadoly) the gypsum-associated limestones are polygenic, and at two locations (Pyshchatyntsi and Lozyna) gypsum deposits are lacking. The studied limestones have originated as primary, mostly peloidal carbonates as well as secondary carbonates formed by hypogene sulphate calcitization. They show a wide range of δ13C (from from -0.9‰ to -39.8‰) and δ18O values (from 0.9‰ to -12.2‰). The Badenian limestones formed in marine environments (either as deposits accumulated at the bottom of the sea or forming the infillings of solution cavities within gypsum) have less negative δ18O values compared to predominantly diagenetic formations. Wide ranges and usually very negative δ13C values and low δ18O values of those limestones indicate that they suffered important meteoric diagenesis as supported by common sparitic fabrics. In addition, a large range of δ13C values even in the group of samples characterized by less-negative δ18O values shows that bacterial sulphate reduction and methane oxidation were active processes in the pore fluids of the Tyras Formation. Very low carbon isotopic compositions (δ13C values from -22 to -40‰) of some sparitic limestones in the studied sections indicate the occurrence of oxidized methane within the diagenetic environment. Accordingly, the isotopic signatures of the studied limestones are a combination of both primary and secondary processes, the latter having a primordial importance. The common occurrence of similar negative δ13C and δ18O values in evaporite-related carbonates in other Miocene evaporite basins suggest that extensive dissolution- reprecipitation in diagenetic or vadose-phreatic environments was common in evaporite-related carbonates.

Secondary quartzites and their relation to gold mineralization at the Svetloe deposit, Ul’ya Trough, Okhotsk-Chukotka volcanic belt

Hydrothermally altered rocks and their relation to gold mineralization at the Svetloe deposit in the Ul’ya Trough, western sector of the Okhotsk-Chukotka volcanic belt, are studied. The rock composition and style of mineralization are typical of acid-sulfate type deposits. The hydrothermally altered rocks of the deposit are medium-temperature secondary quartzites with developed alunite facies. The upper structural level in the massif of secondary quartzites is made up of a sheetlike body 30 km2 in area and up to 300 m thick with gentle (5–10°) periclinal bedding relative to the arcuate axis of the volcanic range. Below the sheet, there are a number of local steep mostly carinate zones and small (a few hundreds of square meters) multilayered trough-shaped structures, the foot of which is located 100–150 m under the secondary quartzite sheet. All structures of the lower level are restricted to the volcanic range pericline, marked by elevated thicknesses of monoquartzites, and have a gradual transition to the sheetlike part of the massif. The massif of secondary quartzites shows symmetrical zoning. The central zone of the sheetlike body is composed of alunite quartzites framed by dickite varieties. Down the section, the latter grade into hydromica and hydromica-montmorillonite argillic rocks. Monoquartzites form axial zones in the structures of the lower level. In the blanketlike part of the deposit, monoquartzites occur as rare thin lenses and interlayers spatially related to local structures of the lower level. Gold mineralization at the deposit associates with porous monoquartzites. Gold precipitated after quartzite formation during infilling of pores and cavities with colloform silica with barite, and hypogene jarosite.

Atypical Gold Mineralization in an Orogenic Setting--The Bohaun Deposit, Western Irish Caledonides

The Bohaun gold deposit in County Galway, western Ireland, is located in the Caledonian orogenic belt, proximal to a major accretionary boundary. The mineralization is hosted in a normal extensional fault, representing a zone of repeated extension and vein infilling of open cavities, within clastic sedimentary rocks that have only reached the late diagenetic zone of low-grade metamorphism. The mineralization consists of undeformed veins, stockworks, and breccias. Quartz dominates all the veins and commonly shows multiple growth stages and a variety of classic epithermal textures, including comb, plumose, and banded, indicative of low confining pressures. The veins have a simple mineralogy made up of quartz-sericite-chlorite with minor sulfides, hematite, rare visible gold, with a high silver content (up to 41 wt %), and late barite and dolomite. Vein quartz is associated with two fluids compositions: type I with moderate to high salinity (8.0–23.6 wt % NaCl equiv) and type II with low to moderate salinity (0.0–7.8 wt % NaCl equiv). CO2 contents in both fluid types are <3.5 wt percent. The majority of fluid inclusion homogenization temperatures range from 126° to 257°C and geologically constrained depth estimates (≤5 km) indicate maximum fluid inclusion trapping temperatures between 175° and 245°C. The geodynamic setting of Bohaun, which is located in a deformed continental margin of an allochthonous terrane, is typical of orogenic gold deposits. However, an orogenic association for the Bohaun mineralization is discounted on the basis of fluid inclusion evidence, atypical vein textures, gold grain composition, and undeformed veins. While vein textures locally resemble those found in epithermal quartz-adularia mineralization, there is no obvious magmatic association and alteration is restricted to the veins and immediate host rocks. The origin of the gold at Bohaun remains enigmatic. However, we favor an association with widespread Carboniferous or later mineralizing fluids and a model which involves transport and deposition of gold from a basinal brine-type fluid, in a similar fashion to red-bed Au–Pd mineralization. Bohaun provides further evidence that low temperature brines may be capable of generating economic-grade gold mineralization, independent of other fluids.

Reconstruction of light conditions within a submarine cave during the past 7000 years based on the temporal and spatial distribution of algal symbiont-bearing large benthic foraminifers

Coral reef cavities, crevasses, and caves are inhabited by cryptic communities. The sediments within the submarine Daidokutsu Cave (29 m water depth) on the fore-reef slope of Ie Island, Okinawa, Japan, preserve a fossil record of cryptic bivalve species over the past 7000 years. The record suggests that infilling of cavities caused a progressive decrease in the exchange of water between the interior and exterior of the cave, resulting in a decline in food supply to the cave. To test this hypothesis, the light conditions within the cave were reconstructed from the spatial and temporal distributions of algal symbiont-bearing large benthic foraminifers, based on the assumption that infilling of reef cavities would have resulted in reduced light intensity within the cave. The results show that progressive darkening of the cave occurred by about 5130 yr BP, synchronous with a weakening in water flow within the cave. This synchronicity indicates that infilling of cavities and narrowing of the cave entrance might cause declines in the exchange of water between the interior and exterior of the cave, as well as in light intensity within the cave. These resulted in a deficiency in food in the cave, thereby affecting the species composition of cavernicolous bivalves.

Early Tithonian serpulid-dominated cavity-dwelling fauna, and the recruitment pattern of the serpulid larvae

A Lower Tithonian cavity-dwelling community from pelagic carbonate platform deposits of the Czorsztyn Unit, Western Carpathians, represents a succession of mostly solitary coelobite organisms, dominated by scleractinian corals and small-sized serpulids during the initial recruitment stage, and by serpulids during the following recruitment stages. These bioconstructors were accompanied with other suspension feeders: thecideidine brachiopods, oysters, bryozoans, sponges, crinoids and sessile foraminifers. The boundary between the first and the second recruitment stage represents an interval of aggregate growth interruption, when a thin sheet of cyclostome bryozoans developed. Corals and serpulidsNeovermiliaandVermiliopsisare primary bioconstructors; all other associated organisms profited from the free spaces between the serpulid tubes. The aggregates were already bioeroded, mineralized and encrusted during their growth. Serpulid larvae show a special recruitment pattern. Their tubes were observed attached on the inner surfaces of adult serpulid tubes only. Possible causes of such a larval behaviour involve several physical, biological or chemical factors. Except for the first recruitment stage, the rest of the succession seems to be physically controlled by the gradual infilling of cavities.

Ribbed moraine formed by subglacial folding, thrust stacking and lee‐side cavity infill

Transverse‐to‐iceflow ribbed moraine occurs in abundance in the coastal zone of northern Sweden, particularly in areas below the highest shoreline (200–230 m a.s.l.), but occasionally also slightly above. Based on detailed sedimentological and structural investigations of machine‐dug sections across five ribbed moraine ridges, it is concluded that these vertically and distally prograding moraine ridges were formed as a result of subglacial folding/thrust stacking and lee‐side cavity deposition. The proximal part of the moraines (Proximal Element) was formed by subglacial folding and thrust stacking of sequences of pre‐existing sediments, whereas the distal part (Distal Element) was formed by glaciofluvial and gravity‐flow deposition in lee‐side cavities. The initial thrusting and folding is suggested to be a result of differences in bed rheology at the ice‐marginal zone during the early or late melt season, and that generated a compressive zone transverse to ice flow as a result of a more mobile bed up‐glacier compared to a less mobile bed down‐glacier. It is considered that the lee‐side cavities were formed as a result of ice‐bed separation on the distal slope of the thrust/fold‐created obstruction. The lee‐side cavities formed an integral part of a subglacial linked‐cavity drainage network regulated in their degree of interconnection, size and shape by fluctuations in basal meltwater pressure/discharge and basal iceflow velocity. The proximal and distal elements of the ribbed moraine ridges are erosively cut and/or draped with a consistently more homogeneous deforming bed till (Draping Element) marking the final phase of ribbed moraine formation considered to be contemporaneous with De Geer moraine formation further down‐flow at the receding ice‐sheet margin.

Geology and Geochemistry of the Reocin Zinc-Lead Deposit, Basque-Cantabrian Basin, Northern Spain

The Reocin Zn-Pb deposit, 30 km southwest of Santander, Spain, occurs within Lower Cretaceous dolomitized Urgonian limestones on the southern flank of the Santillana syncline. The Reocin deposit is one of the largest known strata-bound, carbonate-hosted, zinc-lead deposits in Europe. The total metal endowment of the deposit, including past production and remaining reserves, is 62 Mt of ore grading 8.7 percent Zn and 1.0 percent Pb. The epigenetic mineralization consists of sphalerite and galena, with lesser marcasite and trace pyrite with dolomite as gangue. Microprobe analyses of different generations of dolomite revealed nonstoichiometric compositions with various amounts of iron (up to 14 mol % of FeCO3). Replacement of host dolomite, open-space filling of fractures, and cementation of breccias derived from dissolution collapse are the principal types of ore occurrence. Detailed cross-section mapping indicates a stratigraphic and structural control on the deposit. A stratiform morphology is present in the western part of the orebody (Capa Sur), whereas mineralization in the eastern part is highly discordant but strata bound (Barrendera). Stratigraphic studies demonstrate that synsedimentary tectonic activity, related to the rifting of the North Atlantic (Bay of Biscay), was responsible for variation in sedimentation, presence of unconformities (including paleokarsts), local platform emergence and dolomitization along the N60 fault trend. In the Reocin area, two stages of dolomitization are recognized. The first stage is a pervasive dolomitization of the limestone country rocks that was controlled by faulting and locally affected the upper part of the Aptian and the complete Albian sequence. The second dolomitization event occurred after erosion and was controlled by karstic cavities. This later dolomitization was accompanied by ore deposition and, locally, filling of dolomite sands and clastic sediments in karstic cavities. The circulation of hydrothermal fluids responsible for sulfide deposition and the infilling of karst cavities were broadly contemporaneous, indicating a post-Albian age. Vitrinite reflectance data are consistent with previously measured fluid inclusion temperatures and indicate temperatures of ore deposition that were less than 100°C. Carbon and oxygen isotopic data from samples of regional limestone, host-rock dolostone and ore-stage dolomite suggest an early hydrothermal alteration of limestone to dolostone. This initial dolomitization was followed by a second period of dolomite formation produced by the mixing of basinal metal-rich fluids with local modified seawater. Both dolomitization events occurred under similar conditions from fluids exhibiting characteristics of basinal brines. The δ 34S values of sulfides are between –1.8 and +8.5 per mil, which is consistent with thermochemical sulfate reduction involving organic matter as the main source of reduced sulfur. Galena lead isotope compositions are among the most radiogenic values reported for Zn-Pb occurrences in Europe, and they are distinct from values reported for galena from other Basque-Cantabrian deposits. This suggests that a significant part of the lead was scavenged from the local underlying Asturian sediments. The stratigraphic and structural setting, timing of epigenetic mineralization, mineralogy, and isotopic geochemistry of sulfide and gangue minerals of the Reocin deposit are consistent with the features of most of Mississippi Valley-type ore deposits.

North Polar Region Craterforms on Mars: Geometric Characteristics from the Mars Orbiter Laser Altimeter

This study focuses on topographic characterization of ice-associated north polar region impact craters and several enigmatic polar craterforms using new information from the Mars Orbiter Laser Altimeter (MOLA), an instrument aboard the Mars Global Surveyor (MGS) orbiter. We find that, for ice-associated craters, the topography reveals several surprising results not previously apparent from orbital images alone. First, geometric properties for several impact craters associated with ice and frost deposits suggest that cavity infill is as high as 80% of reconstructed, preerosion levels. Second, craters associated with ice also demonstrate unique cavity geometries relative to their nonpolar counterparts. Finally, in some cases, ice-associated impact features are anomalously deep, on the basis of depths modeled from the best available scaling laws. We suggest that burial of these impact features by either episodic advance of the polar cap margin or by continuous deposition at the highest rates previously estimated for the north polar region of Mars has occurred. Subsequent stripping has exhumed the features, leaving behind cavity infill deposits, and a few of these display topographic levels above the surrounding preimpact surface. In at least one case, cavity interior deposits show layering with a typical thickness of ∼10 m, suggesting episodic deposition and ablation of materials as in the polar layered terrain. In addition to ice-associated craters, we investigated the topography of several enigmatic polar craterforms. In particular, a few craterforms within ∼150 km of the permanent north polar cap appear to resemble simple, effusive lava shield volcanoes found on Earth. Their geometric properties cannot be reconciled with previous suggestions that they were manifestations of martian hydromagmatic processes (i.e., maar volcanism). MOLA's initial measurements of impact craters and other craterforms in the north polar latitudes of Mars support the concept of a geologically recent surface, with evidence of effusive volcanism and enhanced sedimentation. In addition, the measured impact crater depths for ice-associated craters suggest at least a few cases of enhanced excavation and thus a possibly weaker target relative to the typical northern hemisphere plains.

Eocene and younger biostratigraphy and lithofacies of the Cascade Seamount, East Tasman Plateau, southwest Pacific Ocean

Six dredgings, five taken during fishing operations, have recovered sedimentary rocks from the volcanic framework of Cascade Seamount, which is built on thinned continental crust on the East Tasman Plateau off eastern Tasmania. The top of the original volcano is now about 600 m below sea‐level. Planktonic foraminifers and supplementary calcareous nannofossils yielded Late Eocene‐Early Oligocene, mid‐Late Oligocene, Early/Middle Miocene, approximately Late Miocene and Quaternary ages. The history of the seamount is more complex than for most seamounts built on oceanic crust and may involve several intervals of volcanism, burrowing or dissolution and cavity infilling. Some of the volcanism was subaerial and some submarine. Older rocks include conglomerate, volcaniclastic sandstone, and a variety of interstitial sparry calcite, formed under a shallow‐water, high‐energy regime, whereas later sediments are mainly oozes. The oldest sedimentary rocks are Late Eocene, shallow water, fully marine with calcareous algae and abundant echinoid debris. Two phases, Chiloguembelina‐dominated and Globigerinatheka‐dominated, can be differentiated. Warm waters prevailed in the Late Eocene and in the Middle Miocene, cooler conditions between and since, consistent with other indications around the southern Australian margin. Post‐depositional alteration consists of phosphatisation and development of ferromanganese crusts.