Separate Sub-basins Research Articles

Concurrent 1. Presentation for: The role of faulting in Northern Carnarvon’s sub-basins in transferring very high overpressure

Presented on Tuesday 17 May: Session 1 Australia’s Northern Carnarvon Basin has experienced a varied geological history with periods of extension and thermal subsidence giving rise to separate sub-basins. Between the sub-basins the sequences of strata, their age, depth and rate of burial, and rock type vary greatly. Contrasts in rock type and burial rates are key variables that control generation of overpressure and, as such, create a wide range of pore pressure conditions from hydrostatic pressure to overpressure levels approaching values of vertical stress. Often these contrasting pressures can be situated relatively proximal to one another in both a horizontal and lateral sense. To improve pre-drill prediction capabilities for well planning in Northern Carnarvon, a study was completed in 2013 that included over 600 exploration wells and included reporting on areas at increased risk of high overpressure. One of the study findings was that reservoirs at sub-basin boundaries were often where the pressure was most variable and difficult to predict. The reason pressure was difficult to predict in these areas was due to high overpressure being transferred via faults at the margin of early Jurassic to early Cretaceous depocentres. In 2015, Hoskin et al. published the paper ‘Influence of faulting on reservoir overpressure distribution in the Northern Carnarvon Basin’ and this extended abstract looks at the Winchester-1 well, drilled after data were compiled for the original study, that validates the model presented in 2015, and also poses additional questions about the transfer of pressure. To access the presentation click the link on the right. To read the full paper click here

Coastal palaeoenvironmental record of Late Bronze to Iron Age harbour development at Liman Tepe-Clazomenae, western Anatolia, Turkey

Liman Tepe-Clazomenae, located in the southern Bay of Izmir, Turkey, was an important Early Bronze Age to Classical Period trading port and cultural centre in the eastern Aegean. The mainland harbour, now submerged ~1.5–2 m below present sea level, is one of the best-preserved examples of an Iron Age (Archaic Period; ca. 7th–6th c. BCE) semi-enclosed harbour (>5 ha) with engineered breakwater structures. A multi-proxy study (micropalaeontology, micro-XRF core scanning) was conducted on seven harbour sediment cores and integrated with geophysical data to map the harbour structures and document coastal palaeoenvironmental changes. Bathymetry and side-scan mapping revealed two broad (>35 m) rubble-constructed breakwater structures and a submerged headland that divided the harbour into two separate sub-basins. Linear magnetic anomalies within the eastern breakwater indicate a buried pier structure, recording possible augmentation of a Late Bronze Age (LBA) or Early Iron Age (EIA) proto-harbour embayment. The harbour basin stratigraphy comprises foreshore and upper shoreface deposits overlying terrigenous clays across a marine transgressive surface. A distinctive silt-rich chemofacies with increased Ti/Ca and decreased Si marks a transition from a sandy marine shoreface to a low energy, sheltered LBA proto-harbour embayment. The Iron Age harbour construction (ca. 7th–6th c. BCE) is recorded by a rise in Rosalina, decreased Ti/Ca and the appearance of Archaic pottery. The harbour was in use from the Archaic to early Classical periods and served as Clazomenae's mainland commercial port.

The role of faulting in Northern Carnarvon’s sub-basins in transferring very high overpressure

Australia’s Northern Carnarvon Basin has experienced a varied geological history with periods of extension and thermal subsidence giving rise to separate sub-basins. Between the sub-basins the sequences of strata, their age, depth and rate of burial, and rock type vary greatly. Contrasts in rock type and burial rates are key variables that control generation of overpressure and, as such, create a wide range of pore pressure conditions from hydrostatic pressure to overpressure levels approaching values of vertical stress. Often these contrasting pressures can be situated relatively proximal to one another in both a horizontal and lateral sense. To improve pre-drill prediction capabilities for well planning in Northern Carnarvon, a study was completed in 2013 that included over 600 exploration wells and included reporting on areas at increased risk of high overpressure. One of the study findings was that reservoirs at sub-basin boundaries were often where the pressure was most variable and difficult to predict. The reason pressure was difficult to predict in these areas was due to high overpressure being transferred via faults at the margin of early Jurassic to early Cretaceous depocentres. In 2015, Hoskin et al. published the paper ‘Influence of faulting on reservoir overpressure distribution in the Northern Carnarvon Basin’ and this extended abstract looks at the Winchester-1 well, drilled after data were compiled for the original study, that validates the model presented in 2015, and also poses additional questions about the transfer of pressure.

A new lithostratigraphic framework for portions of the Pongola Supergroup within the Nkandla sub-basin, southern Kaapvaal Craton, South Africa; insights into Mozaan Group stratigraphy

AbstractA revised lithostratigraphic framework for Mozaan Group-equivalent strata within the Nkandla sub-basin is presented based on new field data, remote sensing and genetic sequence stratigraphic interpretations. Although previous literature has suggested that no Mozaan Group lithologies were deposited within the sub-basin, reinterpretations presented here indicate that 90% of the lithostratigraphy developed within the main basin occurs within the Nkandla and Mhlatuze inliers. Mozaan Group units previously defined as the Vutshini and Ekombe formations are correlated with stratigraphy from the lowermost Sinqeni Formation to the Gabela Formation. Although thinner than units within the type area in the main basin, thicknesses of the Sinqeni Formation are comparable to those observed within the White Mfolozi Inlier. A ~1 000 m composite reference profile is measured within the Mdlelanga Syncline of the Nkandla Inlier. Further profiles were measured for sequences in the Gem-Vuleka Syncline of the Nkandla Inlier, as well as within the Mhlatuze Inlier. These latter profiles, however, host only lower Mozaan Group strata. In all sections the basal portion of the sequence comprises two quartz arenite units, separated by a ferruginous shale, which hosts minor iron formation interbeds. This predominantly coarse-grained lower sequence is overlain by a shale-dominated succession with multiple sandstone interbeds. A prominent coarse-grained quartz arenite unit forms a distinct marker in the middle portion of the sequence. This is overlain by a sequence of shales and sandstones with two prominent igneous units present. Genetic sequence stratigraphic interpretations indicate cyclical deposition of dominantly shallow marine sediments with condensed sections, marked by iron formations or ferruginous shales, denoting periods of marine highstand along the southeastern margin of the Kaapvaal Craton. The evidence of Mozaan Group stratigraphy within the Nkandla sub-basin supports a passive margin tectonic model whereby deposition occurred in an arcuate shallow continental margin which opened to the southeast. The extension of Mozaan Group strata into the Nkandla sub-basin suggests that the Mozaan Basin likely formed a single depository rather than separate sub-basins as previously proposed.

Sources of Perennial Water Supporting Critical Ecosystems, San Pedro Valley, Arizona

ABSTRACT Stable O and H isotope data distinguish three sources for base flow in five reaches of the San Pedro River: (A) base flow and sub-flow from upstream reaches of the river; (B) bank storage derived from summer monsoon floodwater; and (C) water from the mountainous flanks of the river catchment. A and C support base flow in the sub-basin upstream of Sierra Vista. A, B, and C combine to support base flow near St. David. Source C in this area is ancient deep-basin groundwater. Source C dominates in Cascabel near Benson Narrows, with downstream additions from A. In Cascabel near Gamez Road, sources A and C combined to support base flow that had disappeared by 2019. Near Redington, source C appears to have operated through a limestone aquifer vulnerable to short-term drought. Groundwater sub-basins separated by impermeable sills in the riverbed are evolving into hydrologically separate sub-basins as base flow across the sills decreases. The decrease in base flow partly reflects regional long-term drought, which has been exacerbated by pumping. Additional groundwater demand from urban growth upstream of Benson is likely to cause further decline of base flow near St. David and Sierra Vista.

The Talostolida pellucens Complex in the Tropical Eastern Pacific: In Perspective (Mollusca: Gastropoda: Cypraeidae)

The Tropical Eastern Pacific (TEP) stretches south from the Gulf of California along the west American mainland and the adjacent coastal areas, and then continues to its terminus in northern Peru. There are also five deep water oceanic islands within the TEP. The TEP coastal area encompasses the coast to the edge of the continental shelf and includes the coastal offshore islands in the Gulf of Panama. The Gulf of California and the Gulf of Panama constitute separate sub-basins within the TEP and contain the greatest diversity of molluscan fauna within the coastal area. The five TEP oceanic islands are defined as: (1) the near atoll of Clipperton, and the islands of Cocos and Malpelo which are centered in the region and (2) the two archipelagos, Revillagigedo and Galapagos which are on the northern and southern peripheries of the TEP. Clipperton is the furthest of the five main oceanic islands from the American mainland at 1,100 km, while Malpelo is the closest at 435 km. All five are volcanic and are separated by abyssal depths from the mainland and each other, and have never been connected. The Cypraeidae of the TEP, including the Talostolida pellucens (Melville, 1888) complex, have only received the attention of scientists in relatively recent times, and only on a sporadic basis. This is due to the remote location of the offshore oceanic islands they inhabit, the difficulty of access, and the challenging collecting conditions which limit that activity. The literature is sparse and, with respect to the T. pellucens complex, confusing. While the description of the populations at Clipperton and Cocos Islands provided much current information, some additional perspective will hopefully shed more light on the status of the populations.

A high-resolution carbon-isotope record of the Turonian stage correlated to a siliciclastic basin fill: Implications for mid-Cretaceous sea-level change

Turonian strata of the Bohemian Cretaceous Basin, Central Europe, preserve a basin-scale record of shoreline transgressions and regressions, previously interpreted to have been strongly influenced by short-term eustatic cycles. Here, nearshore siliciclastic strata in two separate sub-basins are correlated to a multi-stratigraphic dataset generated from a new research core (Bch-1) drilled in offshore marine sediments of the central basin. A high-resolution δ13Corg record from Bch-1 is presented along with major- and minor-element proxies, TOC, carbonate content, terrestrial to marine palynomorph ratios, and detailed macro- and microfossil biostratigraphy. The 400m thick Turonian through Lower Coniacian interval permits correlation to the highest-resolution C-isotope curves available: all carbon-isotope events demonstrated by δ13Ccarb studies in the British Chalk, NW Germany and other reference sections in Europe are recognized in the δ13Corg curve from Bch-1.A number of short-term, basin-wide regressions in the Bohemian Cretaceous Basin, most likely reflecting eustatic falls, show a recurrence interval of 100kyr or less. This is an order of magnitude shorter than the timing of sea-level falls inferred from the New Jersey margin and the Apulian platform, interpreted to be driven by glacioeustacy. The estimated magnitude of the Bohemian Basin sea-level falls, typically 10–20m and generally <40m, indicates that the 2.4Myr period suggested by others to generate 3rd-order cycles, is too long to be the principal cycle generating unconformities in rapidly-subsiding basins, where the rate of eustatic fall must exceed the subsidence rate. Unconformities in low-accommodation settings such as passive margins most likely represent amalgamated records of multiple cycles of sea-level fluctuations of 100kyr scale, recognizable only in high-resolution datasets from expanded successions.Comparison of the δ13C excursions to the interpreted sea-level record has not yielded a clear causal link. A long-term ‘background’ δ13C cycle shows a duration close to the 2.4Myr long-eccentricity cycle, and shorter-term (1Myr scale) highs and lows in δ13C appear to broadly correspond to intervals characterised by more pronounced short-term sea-level highs and lows, respectively. However, on the scale of intermediate to short-term δ13C fluctuations, no systematic relationship between δ13C and sea-level change can be demonstrated.

Geologic Setting, Sedimentary Architecture, and Paragenesis of the Mesoproterozoic Sediment-Hosted Sheep Creek Cu-Co-Ag Deposit, Helena Embayment, Montana

The northern margin of the Helena Embayment contains extensive syngenetic to diagenetic massive pyrite horizons that extend over 25 km along the Volcano Valley-Buttress fault zone and extend up to 8 km basinward (south) within the Mesoproterozoic Newland Formation. The Sheep Creek Cu-Co deposit occurs within a structural block along a bend in the fault system, where replacement-style chalcopyrite mineralization is spatially associated mostly with the two stratigraphically lowest massive pyrite zones. These mineralized pyritic horizons are intercalated with debris flows derived from synsedimentary movement along the Volcano Valley-Buttress fault zone. Cominco American Inc. delineated a geologic resource of 4.5 Mt at 2.5% Cu and 0.1% Co in the upper sulfide zone and 4 Mt at 4% Cu within the lower sulfide zone. More recently, Tintina Resources Inc. has delineated an inferred resource of 8.48 Mt at 2.96% Cu, 0.12% Co, and 16.4 g/t Ag in the upper sulfide zone. The more intact upper sulfide zone displays significant thickness variations along strike thought to represent formation in at least three separate subbasins. The largest accumulation of mineralized sulfide in the upper zone occurs as an N-S–trending body that thickens southward from the generally E trending Volcano Valley Fault and probably occupies a paleograben controlled by normal faults in the hanging wall of the Volcano Valley Fault. Early microcrystalline to framboidal pyrite was accompanied by abundant and local barite deposition in the upper and lower sulfide zones, respectively. The sulfide bodies underwent intense (lower sulfide zone) to localized (upper sulfide zone) recrystallization and overprinting by coarser-grained pyrite and minor marcasite that is intergrown with and replaces dolomite. Silicification and paragenetically late chalcopyrite, along with minor tennantite in the upper sulfide zone, replaces fine-grained pyrite, barite, and carbonate. The restriction of chalcopyrite to inferred synsedimentary E- and northerly trending faults and absence of definitive zonation with respect to the Laramide Volcano Valley Fault in the lower sulfide zone suggest a diagenetic age related to basin development for the Sheep Creek Cu-Co-Ag deposit.

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.

A revision of the stratigraphy and geology of the south-western part of the Crotone Basin (South Italy)

The Crotone Basin, located on a stack of nappes piled up during the late Paleogene-Neogene, formed in the late Neogene to Quaternary as a forearc basin of the Ionian arc-trench system. The process of slab rollback caused rapid trench migration, resulting in an extensionaltranstensional regime persisting most of the time in the forearc area.The late Neogene tectonic evolution was strongly influenced by a NW-directed fault system, interpreted as basement wrench faults leading to partitioning of the basin into separate sub-basins subject to differential subsidence and mutual displacements. Major sequences identified in the area are regarded as tectono-stratigraphic sequences (TSS). The first of them was laid down in the late Serravallian (?) - Tortonian - early Messinian, during the basin opening stage and is bounded at the top by an erosional unconformity, which may be correlated with the well-known intra-Messinian event of the Mediterranean Salinity Crisis. The second TSS, of middle to late Messinian age, is characterized by strongly syntectonic deposits mostly derived from cannibalization of the lower Messiniansuccession, first infilling extensional troughs, then involved in an episode of sinistral transpression along the NW-trending fault system, which generated local overthrusts, sealed by a late Messinian erosional unconformity. The Messinian tectonics probably reflects the interplay between the processes linked to the kinematics of the Calabria block and those triggered by the Salinity Crisis. The erosional unconformity is overlain by widespread coarse fluvial conglomerates, which are the first onlapping term of the third TSS, represented by uppermost Messinian to lower Zanclean deposits, laid down in an extensional-transtensional regime. This TSS was closed by an important late Zanclean episode of dextral transpression along the NW-trending fault system, leading to inversion of the formerbasins, and limited SW-verging thrusts on fault-restraining bends.The unconformity sealing the structures has a clear expression in the northern, marginal part of the Crotone Basin and correlates downbasin with a conformable surface. The fourth TSS is characterized by a long-lasting phase dominated by extension-transtension, leading to high subsidence rate during the latest Zanclean to Early Pleistocene, and accommodating a thick succession of slope mudstones including clusters of diatomaceous bands mostly in the D. tamalis and D. brouweri Zones. In the northern part of the Crotone Basin two phases of drowning separated by an uplift pulse at ca. 2.55 Ma can be recognized, the second of which was a dramatic collapse, between 2.3 and 2.1 Ma. The fifth TSS is bounded at the base by an unconformity at the transition between «large» and «small» GephyrocapsaZones, i.e. at around 1.1-1.2 Ma, correlating basinwards with a conformable surface. The unconformity is erosional and locally angular in the marginal part of the basin, where it seals structures generated by a contractional event documented also elsewhere in the Calabria block. This event, which is accompanied by a strikeslip component, is inferred to be coeval to the Lower Pleistocene important transpressional episode along the Pollino shear system,which led to release of the Calabria block from the southern Apennines.The fifth TSS is characterized by resumed dextral transtension in the Middle Pleistocene along right-stepping NW-trending faults. This episode generated minor pull-apart sub-basins, showing spectacular growth structures in their infilling successions, which developed with shoaling trend up to inferred Marine Isotope Stages 9-8. The onset of shoaling trend was diachronous, being remarkably younger in the southern sub-basin. In the late Middle Pleistocene to Recent times extensional tectonics was dominating, accompanied by local gravity gliding towards the Ionian Sea, arguably triggered by increase in topographic gradient following hinterland uplift, and implying the activation of a linked, thin-skinned extensional and contractional NE- to NNE-directed fault system, with detachment surface possibly soling into Messinian evaporitic-mudstone deposits. It is concluded that the geologic evolution of the investigated forearc area was characterized by an alternation of long-lived stages of extension-transtension expressed by prolonged subsidence preceded by uplift pulses, and short-lived episodes of contraction-transpression.Major drowning episodes in the forearc area are thought to be coeval to the main phases of spreading in the Tyrrhenian basin.

Topographic Control of Basin and Channel Flows: The Role of Bottom Pressure Torques and Friction

Abstract The topographical control of western boundary currents within a basin and zonal jets in a channel is investigated in terms of the potential vorticity (PV) and barotropic vorticity (BV: the curl of the depth-integrated velocity) budgets using isopycnic, adiabatic wind–driven experiments. Along the western boundary, the wind-driven transport is returned across latitude lines by the bottom pressure torque, while friction is only important in altering the PV within an isopycnic layer and in allowing a closed circulation. These contrasting balances constrain the geometry of the flow through integral relationships for the BV and PV. For both homogenous and stratified basins with sloping sidewalls, the northward subtropical jet separates from the western wall and has opposing frictional torques on either side of the jet, which cancel in a zonal integral for BV but alter the PV within a layer streamline. In a channel with partial topographic barriers, the bottom pressure torque is again important in returning wind-driven flows along western boundaries and in transferring BV from neighboring wind-driven gyres into a zonal jet. The depth-integrated flow steered by topography controls where the bottom friction alters the PV, which can lead to different PV states being attained for separate subbasins along a channel.

A combined suspended particle and phosphorus water quality model: Application to Lake Vänern

A dynamic model describing the flow of phosphorus and suspended particulate matter for large fresh water lakes is proposed and validated against data from Lake Vänern in Sweden. The model was modified to handle two separate sub-basins, but increasing the horizontal resolution further by splitting the basins into coastal area and pelagial failed, as the model fit to experimental data deteriorated. Besides, the scant reference data available for the coastal areas makes this a dubious exercise. Parameters for the nutrient dynamics in the water column required less tuning (up to 60%) than the sedimentation and sediments (up to a factor 70). The fit to experimental data is good for the periods between 1900 and 1940 and that after 1980, but is less satisfactory for the more polluted conditions in the middle of the century. The model is applied to two scenarios: increased emissions from a pulp and paper mill by the lake, and decreased phosphorus emissions achieved by a combination of effects on farmland, woodland, and rural households. These two scenarios demonstrate the usefulness of a dynamic quantitative lake water quality model.

Buried oblique‐slip faults in the Irish Caledonides

AbstractDespite over a century of geological investigation, the Ordovician evolution of South Mayo, western Ireland, is still imperfectly understood. An example of this is the supposed lateral equivalence of two formations within the succession, the Rosroe and Derrylea Formations of Arenig age, exposed on opposite limbs of a major east–west syncline. These formations exhibit characteristics which suggest that they were not deposited in the same basin. Both formations contain tuff horizons. Geochemical analysis of these tuffs shows that each formation contains chemically distinct volcanic signatures suggesting deposition in separate sub‐basins. Previously the Rosroe Formation on the south limb of the syncline was considered the coarse‐grained proximal equivalent of the finer‐grained Derrylea Formation, both being deposited in a deep‐water fan environment. Previously published palaeocurrent data together with new data show the Rosroe Formation to have been derived from the northeast and therefore it cannot be the proximal equivalent of the Derrylea Formation. Additionally, the two formations show different and distinct associations of heavy mineral assemblages.It is suggested that one explanation for these data is that both formations were deposited in separate sub‐basins controlled by oblique slip sinistral faults, similar in some respects to the Cenozoic basins of the Gulf of California. In the Irish case these faults would have been largely buried by later Ordovician sedimentation. Some models for the Ordovician evolution of this area postulate the presence of an initial oceanic arc situated above a southward directed subduction zone. The presence of thick proximal submarine tuffs derived from an arc environment in the Rosroe Formation suggest that at least by this time the subduction zone was in fact northward directed and outboard of the arc. Copyright © 2002 John Wiley &amp; Sons, Ltd.

Modeling combined effects of forestry, agriculture and deposition on nitrogen export in a northern river basin in Finland.

The development of the spatial N export and retention model N_EXRET for large river basins is presented, utilizing remote-sensing-based land use and forest classification. Export coefficients describing the contribution from agriculture, forestry and peat harvesting were estimated based on empirical studies. Representativeness of forest treatment coefficients have been evaluated by use of data from a small, well-documented test catchment. Simulation results from the application of the model to the Oulujoki river basin (22,840 km2) are discussed. Model estimated N fluxes were compared with measured N fluxes in separate points of the river basin. Based on source apportionment, agriculture contributes 17% of the total export, varying between 8% in the uppermost subbasin and 38% in the lowermost subbasin close to the sea. Forestry contributes almost as much, 16%, with less pronounced variation (11-24%) between the different subbasins. In the separate subbasins, 7-37% of the incoming gross N export was retained. Based on mass balances and sensitivity analyses, retention was estimated to be in the range of 5-10 kg ha-1 yr-1 in lakes and 0-1 kg ha-1 yr-1 in peatlands. The model results were validated by testing to data from the 3 closely situated river basins. However, further testing is needed in river basins where land use and deposition patterns differ clearly from these northern basins.

Magnetic stratigraphy and sedimentology of Holocene glacial marine deposits in the Palmer Deep, Bellingshausen Sea, Antarctica: implications for climate change?

The Palmer Deep is a closed bathymetric depression on the Antarctic Peninsula continental shelf. It contains three separate sub-basins. These basins lie along a northeast–southwest axis with water depths ranging from >1400 m to the southwest (Basins II and III) to just over 1000 m to the northeast (Basin I). Six sediment piston cores were collected from the study region; these cores clearly demonstrate the varied sediment character for each basin. Sediments in Basin I are laminated and thinly bedded consisting of diatomaceous, pelagic/hemipelagic sediments, siliciclastic, terrigenous sediments, and ice rafted, hemipelagic sediments. In concurrence with other investigators, we propose that these laminations and thin beds represent climatically forced productivity cycles. Basin II and Basin III sediments alternate between pelagic/hemipelagic units and bio-siliceous mud turbidites. Correlations between cores are based on their remarkable magnetic susceptibility (MS) records which indicate alternating biogenic (low MS) and siliciclastic (high MS) dominated sedimentation; the bio-siliceous mud turbidites are characterized by intermediate to low MS values. Cores taken from within the main axis of the basins are expanded ultra-high resolution sections. A core collected on the sill between Basins II and III represents a condensed sediment section and may contain a complete Holocene record of changing paleoenvironments, one that records the transition from a glacial, ice shelf environment to an open marine, Holocene environment. A sharp drop in magnetic susceptibility at mid-core is a common sedimentological feature of each basin. Presently, we favor a climate change hypothesis for this magnetic lithostratigraphic transition which may reflect the termination of the Holocene Hypsithermal and a marked change in productivity dated ca. 2500 years BP.

A magnetostratigraphic study of the onset of the Mediterranean Messinian salility crisis; Caltanissetta Basin, Sicily

Abstract The Messinian ‘salinity crisis’ marked a dramatic climatic change in the Mediterranean region. The timing and extent of this event remain controversial, with conflicting models ranging from predictions of rapid and catastrophic dessication of all of the Mediterranean basin at once, to progressive draw-down of sea-level causing gradual onset of evaporite formation starting at the margins of the Mediterranean Sea. In this paper the preliminary magnetostratigraphy is described of two sections in the Neogene Caltanissetta basin, Sicily, containing the early Messinian Tripoli Formation, which pre-dates evaporite formation, and the overlying Calcare di Base shallow water carbonates that mark the first evaporite phase. The magnetisation of the sediments has been extensively overprinted by recent weathering, but primary or early diagenetic remanences which pass reversal and fold tests have been isolated. This has been possible because the sections have been tilted by Pliocene tectonic movements, thus rotating the pre-tilt remanence direction away from the later overprint direction, allowing resolution of the components by great circle analysis. The sections have been correlated with Krijsman et al .’s revision of the geomagnetic polarity time scale (GPTS) of Cande &amp; Kent. This was achieved by comparing variations in the sediment rate derived from the alternative matches with the GPTS with the observed proportions of diatomite (relatively fast deposition) and clay (slow deposition). The proposed correlation puts the base of both sections at or close to the start of the Messinian, but predicts diachroneity in the onset of Calcare di Base deposition between the two sections, as the base of the Calcare di Base falls in opposite polarity chrons in the two sections. The sections lie in two separate sub-basins controlled by thrust anticlines of the Maghrebian thrust belt. Structural and sedimentological evidence suggests that the onset of evaporite formation in these perched basins was strongly controlled by the extent and development of the emerging anticlines. The magnetostratigraphy provides a first positive test of this prediction of tectonically controlled diachroneity of facies. Further palaeomagnetic work is in progress to assess the relative timing of evaporite formation on Sicily in more detail.

A correlation between optical properties from satellite data and some indicators of eutrophication in Lake Garda (Italy)

Here we present the first results of a research project in the field of optical remote sensing from a satellite platform to monitor the lacustrine environment. Landsat Thematic Mapper imagery was considered, on the basis of good performances in the visible bands, which is particularly useful for studies of water quality. The study involves Lake Garda (Italy) and concerns the correlation between remote sensing observations of the surface characteristics of water bodies and the eutrophication trend, as derived from bottom sediment cores. Image processing provided radiance-derived quantities, by means of transects, from the shoreline to the pelagic environment, in two separate sub-basins marked by different eutrophication. A correlation between the recent and present eutrophication trends and the optical properties of the water, as seen by remote sensing, was observed. This approach can be seen as a new remote sensing application to the study of lacustrine environments, since it permitted a comparison of instantaneous information, taken from a space platform, with a long-term data set obtained from sediments deposited at the bottom of the lake.

The origin of Mediterranean interstitial waters—relics of ancient Miocene brines: A re-evaluation

Re-examination of interstitial waters from sixteen Mediterranean DSDP sites reveals that the high salinity values of these waters (up to 350 g/l) do not reflect dissolution of underlying evaporites, but are rather related to Miocene evaporated sea water trapped in the sediments and modified by diagenetic reactions and advection-diffusion. Only in two shallow sites was the chemical composition of the interstitial waters controlled by evaporite dissolution. The large variations in the chemistry of the interstitial waters among different sites suggests that the Mediterranean was divided into separate sub-basins with different chemical compositions. The composition of interstitial waters records the history of the Mediterranean since Early Miocene as follows: (1) salinization of the western Mediterranean during the Early and Middle Miocene; (2) intrusion of low salinity water in the eastern basin during the Middle Miocene; (3) desiccation by evaporation of the Mediterranean during the Messinian (Late Miocene). Other processes, such as sulphate reduction and dolomitization, took place along the sedimentary section. The diffusion-advection processes in the interstitial water, in addition to smoothing the contrasts within vertical profiles of chemical concentrations, caused an upward shift in the liquid phase profile in respect to the solid phase, changed the vertical trend in the case of an end-member which became exhausted, and brought about vertical changes associated with the porosity structure.

Basement structure of the Cuddapah Basin from gravity anomalies

An attempt has been made to understand the complex nature of gravity anomalies over the Cuddapah Basin and their relationship with geology, structure and tectonics. The gravity anomalies could delineate the approximate boundaries of the basin, which is characterized by a steep gravity gradient on its eastern margin and gentle gradients on all its other sides. Prominent gravity lineaments, which probably represent faults, separate sub-basins within the main basin. Previous studies have indicated that the southwestern gravity high is related to a basic body and that the Macherla high is thought to be due to a greenstone belt. The gravity high near Kurnool also appears to be due to a greenstone belt, which might extend into the basin beneath the sedimentary rocks. With the exception of these highs, the gravity picture of the Cuddapah basin is a broad low of about 35 mGal in amplitude. Regression analysis between residual gravity and basement depths inferred from aeromagnetic interpretation and deep seismic sounding (DSS) data suggest a probable density contrast of about − 0.07 g/cm 3 between the basement and the Cuddapah rocks. P wave velocities from DSS data support this view, with a negative density contrast of about 0.05–0.10 g/cm 3 between the basement and the sedimentary rocks. From the residual gravity, the basement structure map of the Cuddapah Basin has been prepared by using an effective density contrast of − 0.07 g/cm 3. The results indicate that the thickest pile of sedimentary rocks (about 10 km) occurs in the Nallamalai sub-basin. At many locations, the basement depths obtained in the present analysis corraborate those obtained from the analysis of other data.

Correlation of NW European Purbeck-Wealden (nonmarine Lower Cretaceous) as seen from the English type-areas

Correlation within and between the onshore basins is reviewed and revised. The English Purbeck and Wealden stratotypes (zoned by ostracods and palynomorphs) lie in separate sub-basins, overlap in time and are not fully correlated (especially the Wealden). The overall age range is Berriasian-earliest Aptian (∼ 141(146)-119(125) m.y. B.P.), with the top of the Purbeck Formation at the Berriasian-Valanginian boundary and the Wealden Group base about mid-Berriasian. North of the mid-European source-massifs the Purbeck-Wealden facies are almost wholly ‘Purbeck’ (Berriasian) in age, with only a little lower ‘Wealden’ (earliest Valanginian) above. Ostracodpalynomorph subdivision is good, and correlation with the English stratotypes also good as far east as north Germany except for the west and north Netherlands basins. The Purbeck records must be fairly complete except in Poland (Lower and lower Middle divisions only). Correlation of the Bornholm sequence is largely enigmatic. On the massifs, Wealden lithofacies in palaeokarstic pockets doubtfully equate with upper Wealden (Hauterivian-Barremian). Those in the Hautrage basin could include younger strata. South of the massifs, the Paris basin succession broadly resembles that in England and must also be substantially Berriasian to Barremian. Published evidence on the Purbeck is scant, but ostracods and palynomorphs allow correlation with parts at least of the three divisions of the English stratotype. Some direct correlation of the overlying Wealden has been achieved in the same way, aided indirectly by intercalations of marine Hauterivian and Barremian sediments. Purbeck-Wealden facies in the Boulonnais are curiously different from those in the Paris and English basins. Near the Channel coast the succession must be largely Portlandian, and exposures to the east reveal possible upper Wealden (Barremian). “Purbeckian” carbonates in the French and Swiss Jura, clearly Berriasian from their marine contexts, yield palynomorphs and ostracods showing that the Lower, Middle and Upper divisions of the Purbeck stratotype are represented in some degree. Event stratigraphy is used with caution owing to the risk of circular arguments. Thus the widespread occurrence of gypsum near the base of the sequences might not, as often assumed, have been generated by a regional climatic event. Likewise transgression-regression phases, marine or nonmarine, do not necessarily reflect eustatic changes of sea-level. Both dilemmas are encountered in the English basins. Only independent tests of synchroneity can establish the reality of such events. Future progress in correlation may be aided by the absolute dating of volcanogenic materials and washed-in contemporary glauconites.