Periods Of Subaerial Exposure Research Articles

Climate, duration, and mineralogy controls on meteoric diagenesis, La Molata, southeast Spain

We isolated the impact of climate, duration, and mineralogy on porosity enhancement and cementation associated with seven surfaces of subaerial exposure in Miocene carbonate strata of southeast Spain. We integrated new and published petrographic and geochemical studies on the same strata to quantify the approximate impact of each event of subaerial exposure. The results quantified that only minor dissolution and/or cementation occurred when subaerial exposure was short-lived ([Formula: see text] thousand years) and in an arid climate, or when carbonate sediments were composed primarily of calcite. For such exposure surfaces, alteration was confined to the uppermost 0.5–2 m with 2%–5% porosity from dissolution and 3% cementation. After deposition of the last Miocene carbonate sequence, dolomitization and dissolution occurred during the initial stages of sea-level fall, associated with mixing during influx of meteoric water. This resulted in dissolution to create 10%–20% porosity over 83% of the platform system, and this indicated that even incipient subaerial exposure can lead to major porosity enhancement during times of high freshwater recharge and hydrogeology that promotes mixing. During a long-lived period of subaerial exposure (greater than 5.3 million years) associated with wet climate, major amounts of cementation reduced porosity in the phreatic zone and some porosity was enhanced in the vadose zone. This included 25% freshwater calcite cement affecting 53% of the platform. Dissolution during and after calcite cementation was approximately 8% throughout the platform. These observations predict that duration and climate combine to have the most significant impacts on porosity associated with subaerial exposure.

Middle Devonian invertebrate trace fossils from the marginal marine carbonates of the Zachełmie tetrapod tracksite, Holy Cross Mountains, Poland

The dolomitic deposits of the Middle Devonian Wojciechowice Formation exposed at the tetrapod tracksite in the Zachelmie Quarry in the Holy Cross Mountains (Poland) are characterised by a low diversity of invertebrate trace fossilassociation. Four ichnoassemblages can be identified in the track-bearing, lower part of the succession. The most conspicuous are trace fossils produced by arthropods (probably crustaceans), which can form distinctive and large horizontal burrows. The described ichnotaxa (cf. Skolithos isp., cf. Balanoglossites isp., Alcyonidiopsis isp., Spongeliomorpha isp., Gordia isp., and Rhizocorallium isp.) are well known from typical marginal-marine and shallow-marine deposits. Nevertheless, the studied assemblages were found in sparsely distributed horizons and are dominated by a single or a few ichnotaxa with locally high trace-densities. Distribution and composition of the trace fossil assemblages probably reflects occurrence of the impoverished, stressed Cruziana ichnofacies. It was affected by changes in water depth with intermittent periods of subaerial exposure connected with salinity fluctuations. The invertebrate trace fossil assemblage, tetrapod tracks and associated sedimentological features point to deposition in a marginal-marine, mostly peritidal and lagoonal environment with minor terrestrial influences.

Abrupt weakening of the Indian summer monsoon at 8.2 kyr B.P.

An oxygen isotope record of biogenic carbonate from paleolake Riwasa in northwestern (NW) India provides a history of the Indian Summer Monsoon (ISM) from ∼11 to 6 kyr B.P. The lake was dry throughout the Late Glacial period when aeolian sands were deposited. Lacustrine sedimentation commenced in the early Holocene and the lake deepened significantly at ∼9.4 kyr B.P., indicating a strengthening of the ISM in response to summer insolation forcing. This high lake stand was interrupted by an abrupt desiccation, which is marked by a 12-cm limestone hardground that formed during a period of sub-aerial exposure after ∼8.3 kyr B.P. The base of the hardground surface coincides with the beginning of the ‘8.2-kyr B.P. cooling event’ in the North Atlantic that has been associated with a glacial outburst flood and slowdown of Atlantic meridional overturning circulation. The hardground provides robust evidence of a weakening of the ISM on the Indian subcontinent at ∼8.2 kyr B.P., and supports previous results of a strong teleconnection between monsoon Asia and North Atlantic climate. Lacustrine sedimentation resumed at ∼7.9 kyr B.P. suggesting the 8.2-kyr desiccation of paleolake Riwasa represented an abrupt response of the ISM to forcing from the North Atlantic.

Trilobites in early Cambrian tidal flats and the landward expansion of the Cambrian explosion

Research Article| February 01, 2014 Trilobites in early Cambrian tidal flats and the landward expansion of the Cambrian explosion M. Gabriela Mángano; M. Gabriela Mángano * 1Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, Saskatchewan S7N 5E2, Canada *E-mails: gabriela.mangano@usask.ca; luis.buatois@usask.ca; raastini@efn.uncor.edu; ARindsberg@uwa.edu. Search for other works by this author on: GSW Google Scholar Luis A. Buatois; Luis A. Buatois * 1Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, Saskatchewan S7N 5E2, Canada *E-mails: gabriela.mangano@usask.ca; luis.buatois@usask.ca; raastini@efn.uncor.edu; ARindsberg@uwa.edu. Search for other works by this author on: GSW Google Scholar Ricardo Astini; Ricardo Astini * 2Laboratorio de Análisis de Cuencas, Centro de Investigaciones en Ciencias de la Tierra, Universidad Nacional de Córdoba, Av. Velez Sarsfield 1611, X5016GCA Córdoba, Argentina *E-mails: gabriela.mangano@usask.ca; luis.buatois@usask.ca; raastini@efn.uncor.edu; ARindsberg@uwa.edu. Search for other works by this author on: GSW Google Scholar Andrew K. Rindsberg Andrew K. Rindsberg * 3Department of Biological and Environmental Sciences, Station 7, University of West Alabama, Livingston, Alabama 35470, USA *E-mails: gabriela.mangano@usask.ca; luis.buatois@usask.ca; raastini@efn.uncor.edu; ARindsberg@uwa.edu. Search for other works by this author on: GSW Google Scholar Author and Article Information M. Gabriela Mángano * 1Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, Saskatchewan S7N 5E2, Canada Luis A. Buatois * 1Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, Saskatchewan S7N 5E2, Canada Ricardo Astini * 2Laboratorio de Análisis de Cuencas, Centro de Investigaciones en Ciencias de la Tierra, Universidad Nacional de Córdoba, Av. Velez Sarsfield 1611, X5016GCA Córdoba, Argentina Andrew K. Rindsberg * 3Department of Biological and Environmental Sciences, Station 7, University of West Alabama, Livingston, Alabama 35470, USA *E-mails: gabriela.mangano@usask.ca; luis.buatois@usask.ca; raastini@efn.uncor.edu; ARindsberg@uwa.edu. Publisher: Geological Society of America Received: 29 Jul 2013 Revision Received: 29 Oct 2013 Accepted: 03 Nov 2013 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2014 Geological Society of America Geology (2014) 42 (2): 143–146. https://doi.org/10.1130/G34980.1 Article history Received: 29 Jul 2013 Revision Received: 29 Oct 2013 Accepted: 03 Nov 2013 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation M. Gabriela Mángano, Luis A. Buatois, Ricardo Astini, Andrew K. Rindsberg; Trilobites in early Cambrian tidal flats and the landward expansion of the Cambrian explosion. Geology 2014;; 42 (2): 143–146. doi: https://doi.org/10.1130/G34980.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The timing of the early invasion of the continents, the routes to the land, and the environmental breadth of the Cambrian explosion are important topics because they are at the core of our understanding of early evolutionary breakthroughs. Illuminating some aspects of these problems are trilobite trace fossils in tidal-flat deposits from the lower Cambrian Rome Formation in the southern Appalachian Mountains of Tennessee (USA). Morphologic details and size range of the trace fossils suggest production by olenellid trilobites, which occur as body fossils in the same unit. The occurrence of this ichnofauna, together with physical structures indicative of periodic subaerial exposure (desiccation cracks) and deposition within the intertidal zone (flat-topped ripples), shows that trilobites forayed into the upper intertidal zone during the Cambrian. Our finding supports the migration of subtidal organisms into marginal-marine, intertidal settings at the dawn of the Phanerozoic, suggesting that trilobites contributed to the establishment of the intertidal ecosystem during the Cambrian. The sequence of events involved in the colonization of early Paleozoic tidal flats is consistent with the idea that most terrestrial taxa originated from marine rather than freshwater ancestors, and that direct routes to the land from marginal-marine ecosystems were involved in the colonization of continental environments early in the Phanerozoic. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Análise estratigráfica da Formação Brejo Santo, Bacia do Araripe, Nordeste do Brasil: implicações paleogeográficas

The Araripe Basin has been the subject of several publications in the recent years, notably with respect to the rich paleontological collection of Brejo Santo, Crato and Romualdo Formations. However, papers detailing stratigraphic aspects of Brejo Santo Formation are scarce. The formation (Mesozoic era - Dom Joao Stage) is represented by a predominantly pelithic succession (up to 450 m thick) outcropping in eastern portion of the basin. The Brejo Santo Formation unconformably overlies the Cariri Formation, presumably of Paleozoic age, and makes gradational contact with the overlying Missao Velha Formation. It consists of pelithes, such as massive to laminated reddish-brown chalks and calciferous shales with decimetric intercalations of white to greenish-grey siltstones and centimetric layers of reddish mottled, speckled and striped shales, with calcareous nodules, and subordinately, muddy limestones and highly fossiliferous calciferous sandstones and fine sandstones with planar cross-stratifications. The measured paleocurrents have a consistent dispersion pattern for SE, SW and S, suggesting the installation of an open wide basin toward the south. The facies and facies associations described were interpreted as generated by (i) lake systems in which periodically the level of the lakes and/or ponds suffered sudden variations, causing changes in coloration related to seasonal fluctuations in the level of the lake (periods of subaerial exposure) and (ii) ephemeral river systems which fed these lakes. The widespread occurrence of fossil organisms, such as non-marine ostracods and conchostracheans suggest that the sediments of Brejo Santo Formation were possibly deposited in lacustrine-fluviatile systems favorable to the formation of red beds layers, under hot climatic conditions with well-defined dry seasons, corroborating the depositional system interpretation.

Paleoclimate and tectonic controls on the depositional and diagenetic history of the Cenomanian–early Turonian carbonate reservoirs, Dezful Embayment, SW Iran

Integrated facies and diagenetic analyses within a sequence stratigraphic framework were carried out on mid-Cretaceous Sarvak carbonate reservoirs in five giant and supergiant oilfields in the central and southern parts of the Dezful Embayment, SW Iran. Results of facies analysis indicate a homoclinal ramp-type carbonate platform for this formation with the frequencies of different facies associations in six wells reflecting their approximate position in the sedimentary model. Diagenetic studies indicate periods of subaerial exposure with different intensities and durations in the upper Sarvak carbonates producing karstified profiles, dissolution-collapse breccias, and thick bauxitic-lateritic horizons. Sequence stratigraphic interpretations show that the tectonic evolution of the NE margin of the Arabian Plate (Zagros Basin) during Cenomanian–Turonian times shaped the facies characteristics, diagenetic features, and strongly influenced reservoir formation. Reactivation of basement-block faults and halokinetic movements (related to the Hormoz salt series) in the middle Cretaceous, resulted in the development of several paleohighs and troughs in the Dezful Embayment hydrocarbon province. Movements on these structures generated two and locally three disconformities in the upper parts of Sarvak Formation in this region. The paleohighs played an important role in reservoir evolution within the Sarvak Formation in three giant-supergiant oilfields (including Gachsaran, Rag-e-Safid, and Abteymour oilfields) but where these structures are absent reservoir quality is low.

Coastal-plain origin of trace-fossil bearing red beds in the Early Permian of Southern New Mexico, U.S.A.

The Early Permian Robledo Mountains Formation of the Hueco Group in the Robledo Mountains, south-central New Mexico, U.S.A., is a world-class ichnofossil Lagerstätte of supposed peritidal origin. Abundant and diverse invertebrate and vertebrate trace fossils occur in several thin horizons of siliciclastic red beds that are intercalated with thick successions of shallow marine limestone and shale. The depositional environment of the red beds has been discussed for decades but without any detailed sedimentary facies analysis. A recently completed systematic excavation shows that the majority of trace fossils is related to mud-draped surfaces within distal crevasse-splay siltstone to fine-grained sandstone. Interbedded mudstone with well-preserved macrofloral remains and a lungfish aestivation burrow represents a hitherto disregarded facies in overbank fines. Both environmental subzones were at least loosely covered with low-growing plants and experienced common periods of subaerial exposure. None of the physical and biological structures recorded in the measured section indicates tidal influence, but instead they support the reconstruction of a freshwater ecosystem dominated by arthropods and tetrapods. Given their proximity to carbonates of unambiguous subtidal origin, we propose that the trace-fossil-bearing red beds of the Robledo Mountains Formation formed in distal parts of an extensive coastal floodplain during alternating wet and dry conditions. If this interpretation is correct, it provides an impetus to continue ichnological and sedimentological field research in the study area, because the true tidal-flat ichnoguilds still await elucidation.

Characterization of a deeply buried paleokarst terrain in the Loppa High using core data and multiattribute seismic facies classification

The recognition of paleokarst in subsurface carbonate reservoirs is not straightforward because conventional seismic interpretation alone is generally not sufficient to discriminate karstified areas from their surroundings. In the Loppa High (Norwegian Barents Sea), a protracted episode of subaerial exposure occurring between the late Paleozoic and mid-Triassic—Late Permian to Anisian—resulted in a significant overprinting of the previously deposited carbonate units. Here, we map the extension of the karstified areas using an integrated approach consisting of (1) a core study of critical paleokarst intervals, (2) a three-dimensional (3-D) seismic stratigraphic analysis, and (3) a 3-D multiattribute seismic facies (SF) classification. A core retrieved in the flat-topped Loppa High revealed breccia deposits at least 50 m (164 ft) thick, which probably resulted from cave collapses following the burial of the karst terrain. The SF classification was tested on a 3-D cube to (1) discriminate the respective SF related to the breccia deposits compared with other SF and (2) to estimate their spatial extent. Seismic-facies analysis suggests that breccias occupied the topmost area of the structural high, extending up to 12 km (7 mi) in width, 46 km (29 mi) in length, and tens of meters in thickness. The inference of such a large amount of breccia suggests that a significant part of this terrain was derived from the amalgamation of successive cave-development events—including periods of subaerial exposure and subsequent burial and collapse—resulting in a coalesced collapsed paleocave system. Previous observations from the Loppa High revealed the presence of karst plains associated with sinkholes, caves, and other dissolution phenomena associated with the breccia facies, further suggesting that a large volume of carbonate rocks in this area was affected by subaerial exposure and karstification. Our integrated approach and proposed karstification model could be applied to similar sedimentary basins that accommodate deeply buried carbonate successions affected by protracted episodes of subaerial exposure, where only few wells as well as 3-D seismic data are available.

First record of an arthropod from the Passaic Formation (Late Triassic), near Milford, New Jersey

AbstractMudstones of the Triassic Passaic Formation, near Milford, New Jersey, have yielded the first evidence of an arthropod impression in that formation. Associated trace fossils include Helminthoidichnites, Lockeia, Scoyenia, Spongeliomorpha, and the reptile footprint Rhynchosauroides, representing the Scoyenia ichnofacies. Associated sedimentary structures include desiccation cracks and raindrop impressions. The Passaic sediments were deposited under shallow water lacustrine shoreline conditions subject to periodic subaerial exposure.

Ichnological constraints on the depositional environment of the Sawahlunto Formation, Kandi, northwest Ombilin Basin, west Sumatra, Indonesia

A low diversity trace fossil assemblage is described from the Oligocene Sawahlunto Formation near Kandi, in the northwestern part of the Ombilin Basin in western Sumatra, Indonesia. This trace fossil assemblage includes six ichnogenera attributed to invertebrate infaunal and epifaunal activities ( Arenicolites, Diplocraterion, Planolites, Monocraterion/ Skolithos and Coenobichnus) and two ichnotaxa attributed to vertebrate activity (avian footprints: two species of Aquatilavipes). Arenicolites, Diplocraterion and Monocraterion/ Skolithos record the suspension feeding activities of either arthropods (most likely amphipods) or vermiform organisms. Planolites reflects the presence of an infaunal deposit feeder. Coenobichnus records the walking activities of hermit crabs. Both the Coenobichnus and the avian footprints record the surficial detritus scavenging of epifaunal organisms within a subaerial setting. These traces occur within a fine-grained sandstone succession characterized by planar laminae and low-relief, asymmetrical, commonly mud-draped (locally bidirectional) ripples. The presence of traces attributable to suspension feeders implies deposition in a subaqueous setting. Their occurrence (particularly the presence of Arenicolites and Diplocraterion) in a sandstone bed characterized by mud-draped and bidirectional ripples implies emplacement in a tidally-influenced marine to marginal marine setting. Co-occurrence of these traces with well-preserved avian footprints ( Aquatilavipes) further implies periodic subaerial exposure. Thus, it is most likely that the Sawahlunto Formation near Kandi records deposition within an intertidal flat setting. Definitive evidence of marine influences in the Oligocene interval of the Ombilin Basin implies a more complex tectono-stratigraphic history than has previously been implied.

Hydrogeology, sequence stratigraphy and diagenesis in the Paleoproterozoic western Thelon Basin: Influences on unconformity-related uranium mineralization

The Paleoproterozoic (Statherian) Thelon Formation consists of unmetamorphosed sandstone and conglomerate that was deposited in the Thelon Basin, a continental basin in northern Canada. This study presents the first detailed sedimentological and sequence stratigraphic analysis of the expansive and under-studied western Thelon Basin, and its relation to uranium mineralizing processes in the vicinity of the Boomerang Lake unconformity-related U prospect. Four lithofacies comprise the Thelon Formation in the western Thelon Basin, consisting of breccia, conglomerate, sandstone, and minor mudstone. These are grouped into four facies associations that represent deposition in alluvial fans and braided fluvial streams. Three stratigraphic sequences were identified based on the recognition of widespread surfaces that mark significant periods of subaerial exposure or changes in depositional environment. These unconformity-bounded sequences are composite in nature and can be subdivided into six high-frequency sequences (HFS). Low-accommodation and high-accommodation systems tracts are delineated, instead of the traditional systems tracts that are linked to changes in relative sea level, as no marine deposits are recognized in the Thelon Formation. Two significant strataform paleohydrogeological units were distinguished in the Thelon Formation, based on the relative timing of key diagenetic events such as compaction, grain dissolution, and cementation. Diagenetic aquicludes became isolated from basinal fluid flow due to stylolite-catalyzed early diagenetic quartz cementation, and occlusion of secondary intergranular macroporosity by kaolinite and muscovite, and minor dickite during later peak diagenesis. Diagenetic aquifers conducted U-bearing basinal fluids throughout diagenesis and are marked by an abundance of peak diagenetic dickite. Sequence stratigraphic correlation indicates that diagenetic aquicludes, which were not effective at conducting U-bearing basinal fluids, overlie the unconformity at the Boomerang Lake prospect and extend along the entire strike length of a related exploration trend. In contrast, thick diagenetic aquifers lie approximately 10 km to the north of the Boomerang Lake prospect along a different exploration trend. The western and eastern portions of the Thelon Basin experienced depositional environments and diagenetic conditions distinct from one another, suggesting that they were separate sub-basins. The western Thelon Basin contains fewer of the well-cemented facies that compartmentalized basinal fluid flow than the eastern Thelon Basin, and contains a kaolin-dominated diagenetic mineral assemblage rather than a muscovite-dominated assemblage that is common in the eastern Thelon Basin. In these regards, the western Thelon Basin evolved more similarly to the eastern portion of the Athabasca Basin, where thick, regional, kaolin-rich diagenetic aquifers were fluid conduits for the movement of the U-rich fluids that formed high-grade unconformity-related U deposits.

Sequence stratigraphy of a Mesozoic carbonate platform-to-basin system in western Sicily

Abstract Sequence stratigraphic studies of the Triassic through Paleogene carbonate successions of platform, slope and basin in western Sicily (Palermo and Termini Imerese Mountains) have identified a sedimentary cyclicity mostly caused by relative oscillations of sea level. The stratigraphic successions of the Imerese and Panormide palaeogeographic domains of the southern Tethyan continental margin were studied with physical-stratigraphy and facies analysis to reconstruct the sedimentary evolution of this platform-to-basin system. The Imerese Basin is characterized by a carbonate and siliceous-calcareous succession, 1200–1400m thick, Late Triassic to Eocene in age. The strata display a typical example of a carbonate platform margin, characterized by resedimented facies with progradational stacking patterns. The Panormide Carbonate Platform is characterized by a carbonate succession, 1000–1200 m thick, Late Triassic to Late Eocene, mostly consisting of shallow-water facies with periodic subaerial exposure. The cyclic arrangement has been obtained by the study of the stratigraphic signatures (unconformities, facies sequences, erosional surfaces and stratal geometries) found in the slope successions. The recognized pattern has been compared with coeval facies of the shelf. This correlation provided evidence of sedimentary evolution, influenced by progradation and backstepping of the shelf deposits. The stratigraphic architecture of the platform-to-basin system is characterized by four major transgressive/regressive cycles during the late Triassic to late Eocene. These cycles, framed in a chronostratigraphic chart, allows the correlation of the investigated shelf-to-basin system with the geological evolution of the African continental margin during the Mesozoic, showing tectono-eustatic cycles. The first cycle, encompassing the late Triassic to early Jurassic, appears to be related to the late syn-rift stage of the continental margin evolution. The following three cycles, spanning from the Jurassic to Eocene, can be related to the post-rift evolution and to thermal subsidence changes.

From detached to attached carbonate buildup complexes — 3D seismic data from the upper Palaeozoic, Finnmark Platform, southwestern Barents Sea

Carbonate buildups were abundant during the Palaeozoic. Three-dimensional seismic data from the Finnmark Platform, Barents Sea, has been used to reconstruct the evolution of laterally extensive carbonate buildup complexes in space and time. The results suggest that the location of Upper Palaeozoic warm- and cool-water carbonate buildups from the Finnmark Platform is controlled by faults and the sea floor morphology at the time of their growth. A fluctuating sea level affected the growth of the carbonate buildups, but they also influenced their own environment by forming lagoons, atoll-like ridges and possibly areas with restricted circulation. Warm-water carbonate buildups, forming ridges and isolated mounds, occur in the Gipsdalen Group (latest Serpukhovian–mid-Sakmarian), where they initially grew in a detached platform setting. The carbonate buildups are several tens of kilometres long, up to 2.5 km wide and 300 m thick, and interpreted to consist of vertically stacked complexes of sub-seismic scale carbonate buildups. Evaporites were deposited and later subject to karstification, possibly during a period of sub-aerial exposure, before a transgression and the subsequent carbonate deposition and buildup growth bridged the detached platform with the attached platform. In the Bjarmeland Group (Lower Permian) 0.35–4.8 km wide, 1.5–27 km long and 60–420 m thick cool-water bryozoan-dominated straight, sinuous and continuous carbonate ridges or atoll-like ridges are located on top of the warm-water carbonate structures. Three-dimensional maps of Upper Palaeozoic carbonate buildups document their geomorphology, distribution and size through time. The lateral and vertical growth of carbonate buildups has been reconstructed, revealing how their distribution changed over time and with changing environmental settings.

Naktodemasis bowni: New ichnogenus and ichnospecies for adhesive meniscate burrows (AMB), and paleoenvironmental implications, Paleogene Willwood Formation, Bighorn Basin, Wyoming

Adhesive meniscate burrows (AMB) are common in alluvial paleosols of the Paleogene Willwood Formation, Bighorn Basin, Wyoming. AMB are sinuous, variably oriented burrows composed of a nested series of distinct, ellipsoidal packets containing thin, tightly spaced menisci subparallel to the bounding packet. Menisci are non-pelleted and texturally homogeneous with each other and the surrounding matrix. AMB were constructed most likely by burrower bugs (Hemiptera: Cydnidae), cicada nymphs (Hemiptera: Cicadae), and less likely by scarabaeid (Coleoptera: Scarabaeidae) or carabid beetles (Coleoptera: Carabidae), based on burrow morphology and comparison to similar structures produced by these organisms in modern soils. Extant burrowing insects excavate backfilled burrows in well-rooted A and upper B horizons of soils generally below field capacity depending on soil type. This study demonstrates that AMB are distinct morphologically from such previously described ichnofossils asBeaconites, Laminites, Scoyenia, Taenidium, andAncorichnus. Naktodemasis bowni, a new ichnogenus and ichnospecies, represents burrows composed of nested ellipsoidal packets backfilled with thin, tightly spaced, menisci subparallel to the bounding packet. The presence ofN. bowniindicate periods of subaerial exposure associated with pedogenic modification under moderately to well-drained soil conditions, or during periods of better drainage in imperfectly drained soils.N. bowni, therefore, can differentiate alluvial paleoenvironments from marine and lacustrine paleoenvironments, as well as periods of subaerial exposure of sediments deposited in aquatic settings.

Pegadas teromorfóides do Triássico Superior (Formação Santa Maria) do Sul do Brasil

Tracks of triassic vertebrates are abundant all over the world, but they were discovered only recently in Santa Maria Formation (Middle-Late Triassic, Southern Brazil), despite the rich osteological record in this unit. The studied material consists of rock samples containing theromorphoid footprints and tracks from Predebon outcrop (upper portion of Alemoa Member, Santa Maria Formation), located in the municipality of São João do Polêsine, Rio Grande do Sul State. Some of the footprints had been attributed to Dicynodontipus isp., while others had been described as Dicynodontipus protherioides isp. nov. Some footprints associated to these could not have been identified, being classified as Incertae sedis. The studied footprints do not correspond to undertracks and were produced in wet and very plastic substrate. After their production, a brief period of subaerial exposure leads to the hardening of the surface before its burial. The trackmaker of Dicynodontipus protherioides isp. nov. was quadrupedal, cursorial, with alternate pace and half-erect gait, but with little lateral movement of the vertebral column; the autopodia were plantigrade to semiplantigrade, without long claws and with falangeal and sole pads. The tracks were produced in walking gait with the tail touching the ground. The gleno-acetabular distance of the trackmaker was calculated as 2.68 cm. The studied footprints, including Dicynodontipus protherioides isp. nov. and Dicynodontipus isp., can be attributed to small-sized cynodonts, whose skeletons are found in the Santa Maria and Caturrita formations.

Palaeoenvironmental change in the Gulf of Carpentaria (Australia) since the last interglacial based on Ostracoda

Throughout the last glacial cycle, the region between Australia and New Guinea, which is today known as the Gulf of Carpentaria, has oscillated from open shallow marine conditions to a large, land-locked freshwater lake, including periods of subaerial exposure. Ostracod faunal assemblages and variations in the valve morphology, preserved in the sediments of the gulf, record these changes. A 14.8 m long core (MD972132) extracted from near the centre of the modern Gulf of Carpentaria, spanning the last 130 ka BP provides the basis of this study. Environmental facies were determined by R- and Q-mode cluster analysis of the ostracod assemblage data, including 72 species, and comparison with modern analogues from both the Gulf of Carpentaria and Southeast Asian region. Using these methods, six clearly distinct biofacies were identified: (i) open shallow marine facies, including bairdiids, pectocytherinids and cytherettids; (ii) shallow marine facies, dominated by Cytherella and Hemikrithe; (iii) marginal marine facies, including Xestoleberis and Praemunita; (iv) tidal channel facies, dominated by Loxoconcha; (v) estuarine assemblage, comprising Venericythere and Leptocythere; (vi) non-marine facies a. brackish lagoon/lake facies dominated by Cyprideis and Leptocythere; b. freshwater facies, including Ilyocypris, with Cyprinotus and Cypretta. In addition, morphological variation of valves of the more “plastic” species, in particular Cyprideis australiensis and Leptocythere hartmanni, was observed and related to environmental variables, including salinity and solute composition. These two species occur through more than half of the length of the core and show significant variability. C. australiensis with irregular-shaped sieve pores is associated with heavily reticulated valves of L. hartmanni, around 90–70 ka BP, indicating increased salinity and carbonate-limited environments. Conversely, C. australiensis with round sieve pores occurs with finely reticulated valves of L. hartmanni from around 50 ka BP, in fresher, carbonate-rich environments. The preservation of valves was also noted so as to infer post-depositional effects, revealing shoreline features, channel activity and subaerial exposure. Combined ostracod biofacies and morphological analysis reveals a fluctuating marine environment through Marine Isotope Stage (MIS) 5, terminating in a restricted lagoon around 90 ka BP, followed by an extensive period of subaerial exposure in this region of the gulf. MIS 3 marks a return of marine conditions and an increase in fluvial activity. Lake Carpentaria then appears as a permanent feature, at least to the current 60 m depth contour, from around 40 ka BP, with the freshest water occurring around 16 cal. ka BP, prior to the most recent marine transgression, at 10.8 cal. ka BP. Although the presence of Lake Carpentaria had previously been identified, this study extends the knowledge of the region through the last glacial cycle and provides greater detail of the sequential biofacies.

Laboratory simulation of terrestrial meteorite weathering using the Bensour (LL6) ordinary chondrite

Abstract— Laboratory dissolution experiments using the LL6 ordinary chondrite Bensour demonstrate that meteoritic minerals readily react with distilled water at low temperatures, liberating ions into solution and forming reaction products. Three experiments were performed, all for 68 days and at atmospheric fO2 but using a range of water/rock ratios and different temperatures. Experiments 1 and 2 were batch experiments and undertaken at room temperature, whereas in experiment 3, condensed boiling water was dripped onto meteorite subsamples within a Soxhlet extractor. Solutions from experiment 1 were chemically analyzed at the end of the experiment, whereas aliquots were extracted from experiments 2 and 3 for analysis at regular intervals. In all three experiments, a very significant proportion of the Na, Cl, and K within the Bensour subsamples entered solution, demonstrating that chlorapatite and feldspar were especially susceptible to dissolution. Concentrations of Mg, Al, Si, Ca, and Fe in solution were strongly affected by the precipitation of reaction products and Mg and Ca may also have been removed by sorption. Calculations predict saturation of experimental solutions with respect to Al hydroxides, Fe oxides, and Fe (oxy)hydroxides, which would have frequently been accompanied by hydrous aluminosilicates. Some reaction products were identified and include silica, a Mg-rich silicate, Fe oxides, and Fe (oxy)hydroxides. The implications of these results are that even very short periods of subaerial exposure of ordinary chondrites will lead to dissolution of primary minerals and crystallization of weathering products that are likely to include aluminosilicates and silicates, Mg-Ca carbonates, and sulfates in addition to the ubiquitous Fe oxides and (oxy)hydroxides.

Late Miocene to Early Pliocene depositional history of the intramontane Florina–Ptolemais–Servia Basin, NW Greece: Interplay between orbital forcing and tectonics

Upper Miocene and Lower Pliocene shallow lacustrine deposits from the Florina–Ptolemais–Servia Basin in northwest Greece show a distinct m-scale sedimentary cyclicity of alternating marls and lignites or clays, which were shown to be primarily related to precession-induced climate variations ([van Vugt, N., Steenbrink, J., Langereis, C.G., Hilgen, F.J., Meulenkamp, J.E., 1998. Magnetostratigraphy-based astronomical tuning of the early Pliocene lacustrine sediments of Ptolemais (NW Greece) and bed-to-bed correlation with the marine record. Earth Planet. Sci. Lett. 164 (3–4) 535–551; Steenbrink, J., Van Vugt, N., Hilgen, F.J., Wijbrans, J.R., Meulenkamp, J.E., 1999. Sedimentary cycles and volcanic ash beds in the lower Pliocene lacustrine succession of Ptolemais (NW Greece): Discrepancy between 40Ar/ 39Ar and astronomical ages. Palaeogeogr. Palaeoclimatol. Palaeoecol. 152, 283–303; Steenbrink, J., Van Vugt, N., Kloosterboer-van Hoeve, M.L., Hilgen, F.J., 2000. Refinement of the Messinian APTS from sedimentary cycle patterns in the lacustrine Lava section (Servia Basin, NW Greece). Earth Planet. Sci. Lett. 181 (3–4) 161–173]). Three additional sections and drill cores from the same basin are studied using a combination of cyclostratigraphy, magnetostratigraphy and 40Ar/ 39Ar dating. Together with the earlier studied sections, they provide a continuous succession that documents the depositional history of the intramontane Neogene basin fill. Sedimentation in the basin started at around 8 Ma ago with deposition of coarse-grained terrigenous-clastics in an alluvial environment. From ∼ 7.1 to ∼ 6.1 Ma ago, diatomaceous marls were deposited in a lacustrine environment, frequently interrupted by deposition of clays and local xylite-type lignite seams. Hereafter, deposition of alternating coarse- and fine-grained terrigenous-clastic alluvial sediments took place, followed by a gradual shift towards lacustrine sedimentation at around 5.5 Ma. Deposition of alternating fine-grained clastics (silts and clays) and biochemical sediments (carbonates, marls and thin lignites), with episodic periods of sub-aerial exposure continued until ∼ 5.23 Ma. From then onwards up to ∼ 3.9 Ma, regular alternating lignites and marls were formed in a shallow lacustrine environment. Precession-induced fluctuations of regional climate primarily defined the m-scale variations in lithology as observed in all members of succession, whereas the 100- and 400-kyr eccentricity cycle had a clear impact on larger-scale lithological alternations in parts of the succession as well. A marked similarity in the ages of fundamental changes in depositional environments between the Florina–Ptolemais–Servia Basin and the marine basins in the Mediterranean is observed, suggesting (but not necessarily implying) that they have a common, possibly tectonic origin.

The Nature of Zeolite Occurrences in Deposits of the Olduvai Basin, Northern Tanzania

AbstractThe Plio-Pleistocene deposits of the Olduvai Basin in northern Tanzania consist of a sequence of lacustrine and fluvial sediments. They contain various amounts of zeolite minerals, the formation of which is related to an interaction of volcanic material or detrital clays with saline alkaline lake water and groundwater. Petrographic characteristics of zeolite occurrences provide information about their conditions of formation. They were studied for all four main stratigraphical units that are recognized at Olduvai (Beds I to IV), sampled in the southeastern part of the basin. In the lake-margin deposits of Bed I and the lower part of Bed II, chabazite is the dominant zeolite mineral accompanied by phillipsite and minor amounts of erionite and clinoptilolite. Chabazite commonly occurs as part of altered volcanic rock fragments, characterized by partial or complete dissolution of volcanic glass and the formation of chabazite inside vesicles, following the development of thin smectite coatings. It also formed within the sediment matrix, requiring extended periods of impregnation of the deposits by saline alkaline solutions. Chabazite also occurs extensively as coatings and infillings of pores, developed during periods of subaerial exposure which were characterized by high groundwater levels. Phillipsite formed at a later stage, from more evolved solutions, with higher K/Na ratios than during chabazite formation. The fluvial deposits of Bed IV, Bed III and the upper part of Bed II have a high analcime content. They also contain various amounts of chabazite, phillipsite and natrolite. All zeolite minerals mainly occur in pores. The predominance of analcime indicates a higher salinity and alkalinity than during the preceding period with sedimentation and diagenesis in a lake margin environment. Early development of zeolite occurrences, shortly after the deposits became exposed during breaks in sedimentation, is recorded for some intervals, where zeolites are covered by illuvial clay coatings or by sparitic carbonate cement. In most intervals, however, zeolites mainly formed at a later stage.

Paleokarst and reservoir porosity in the Ordovician Beekmantown Dolomite of the central appalachian basin

A karst-unconformity play at the top of the Ordovician Beekmantown Dolomite is judged to have great petroleum potential in Ohio and adjacent states; wells have high ultimate reserves and large areas remain untested. To better understand the origin, development, and distribution of Beekmantown porosity, we conducted a petrologic-stratigraphic study of cores and thin sections from 15 oil and gas wells. The massive dolomite, characterized by a hypidiotopic-idiotopic texture, formed by the replacement of stacked peritidal carbonate cycles. Secondary porosity occurs at two scales: (1) mesoscopic—breccia porosity, solution-enlarged fractures, large vugs, and caverns, and (2) microscopic—intercrystalline, intracrystalline, molds, small vugs, and microfractures. Mesoscopic pores (providing the major storage capacity in this reservoir) were produced by intrastratal solution and collapse of carbonate layers, whereas microscopic pores (connecting the larger pores) generally formed by the leaching of individual carbonate grains and crystals. Most pore types developed during periods of subaerial exposure across the carbonate bank, tied to either the numerous, though brief falls of relative sea level during Beekmantown deposition or more importantly the prolonged Knox unconformity at the close of sedimentation. The distribution of reservoir-quality porosity is quite heterogeneous, being confined vertically to a zone immediately below the unconformity and best developed laterally beneath buried hills and noses of this erosion surface. The inferred, shallow flow of ground water in the Beckmantown karst, primarily below topographic highs and above a diagenetic base level close to the water table, led to this irregular distribution of porosity.