Intraformational Conglomerates Research Articles

Inter‐tuff correlation and architecture within the Moor Cliffs Formation, Lower Old Red Sandstone, southwest Wales: a multi‐disciplinary approach for reservoir characterization

AbstractThe laterally continuous coastal exposures of the Moor Cliffs Formation of the Anglo‐Welsh Basin have allowed a highly detailed 2D reconstruction of the floodplain. Whole‐rock trace element geochemical and spectral gamma‐ray (SGR) correlation are combined with traditional sedimentological analysis allowing outcrop and subsurface techniques to be compared. Micaceous, quartz‐rich and well‐sorted fluvial channel sandstones form up to 10% of the thickness in this low net‐to‐gross succession. High Th/K ratios derived from SGR analysis distinguish sandstones from the surrounding fine‐grained sediments, but lateral facies variations prevent the correlation of fluvial channel intervals using SGR without outcrop control. Red‐brown micaceous siltstone and claystone constitute 65 to 95% of the outcrop thickness and contain abundant pedogenic features characteristic of modern Vertisols. The most mature Vertisol horizons and the non‐pedogenically altered siltstones/claystones do not show significant differences in their SGR response. Intraformational conglomerates comprising carbonate and siltstone clasts within a siltstone matrix are common above laterally extensive erosion surfaces, and can be clearly recognized from a sharp spike in Nb/Y and Zr/Y ratios. Major tuff horizons, which form chronostratigraphic marker horizons throughout the Moor Cliffs Formation, are also characterized by a distinctive geochemical signature, enabling calibration of the geochemical and sedimentological datasets. Geochemical and SGR analytical techniques are complementary and assist in enhancing the confidence of correlations created by traditional techniques in the Moor Cliffs Formation. Where multiple correlation scenarios have been interpreted due to a lack of diagnostic marker beds or biostratigraphic control, the SGR and geochemical data can increase confidence in a particular interpretation and hence decrease uncertainty in geological models of low net‐to‐gross systems in the subsurface. Copyright © 2004 John Wiley & Sons, Ltd.

Middle Paleozoic paleomagnetism of east Kazakhstan: post-Middle Devonian rotations in a large-scale orocline in the central Ural–Mongol belt

In order to test different hypotheses concerning the Paleozoic evolution of the Ural–Mongol belt (UMB) and the amalgamation of Eurasia, we studied Middle Devonian basalts from two localities (11 sites) and Lower Silurian volcanics, redbeds, and intra-formational conglomerates from three localities (20 sites) in the Chingiz Range of East Kazakhstan. The Devonian rocks prove to be heavily overprinted in the late Paleozoic, and a high-temperature, presumably primary, southerly, and down component is isolated at only four sites from a homoclinal section. Most Silurian redbeds are found to be remagnetized in the late Paleozoic; in contrast, a bipolar near-horizontal remanence, isolated from Silurian volcanics, is most probably primary as indicated by positive tilt and conglomerate tests. Analysis of paleomagnetic data from the Chingiz Range shows that southward-pointing directions in Ordovician, Silurian, and Devonian rocks are of normal polarity and hence indicate large-scale rotations after the Middle Devonian. The Chingiz paleomagnetic directions can be compared with Paleozoic data from the North Tien Shan and with the horseshoe-shaped distribution of subduction-related volcanic complexes in Kazakhstan. Both paleomagnetic and geological data support the idea that today's strongly curved volcanic belts of Kazakhstan are an orocline, deformed mostly before mid-Permian time. Despite the determination of nearly a dozen new Paleozoic paleopoles in this study and other recent publications by our team, significant temporal and spatial gaps remain in our knowledge of the paleomagnetic directions during the middle and late Paleozoic. However, the paleomagnetic results from the Chingiz Range and the North Tien Shan indicate that these areas show generally coherent motions with Siberia and Baltica, respectively.

Channel iron deposits of the Hamersley Province, Western Australia

The fluvial goethite‐hematite channel iron deposits (CID) of fhe Pilbara region of Western Australia, formerly known as the ‘Robe Pisolite’, represent a major source of iron ore mined in the Hamersley Province. The CID occupy Early Tertiary palaeochannels that are typically less than 1 km but range to several kilometres in width and from 1 to ∼⃒100 m thick. The Robe palaeochannel is the longest, with CID partly preserved over a distance of 150 km, whereas the Marillana palaeochannel contains CID along 80 km of its length. The CID range from goethitic mudstone to fine hematite‐goethite gravel and intraformational conglomerate varying in distribution along the channels in a range of massive, bedded and altered types. The granular type that forms the bulk of the ore is typically ooidal, comprising pelletoids with goethite cortices around hematite nuclei (which often consist of fossilised wood), with typically abundant coarse goethitised wood fragments, a varied proportion of peloids, minor pisoids, and a diverse porous goethitic matrix. The Robe deposits tend to have more spherical pelletoids, with more regularly developed layering, and smaller cores than CID from the Yandi deposits of the Marillana valley. Generally, the presence of ferruginised fossil wood is the principal diagnostic field criterion used to distinguish CID from other comparable goethite‐hematite detrital deposits. Electron microprobe X‐ray maps reveal that lattice Al and Si increase from the hematitic nuclei to the matrix, and then the cortex. In the Robe CID, alumina in the nuclei varies between 0.5 and 2.8%, whereas at Yandi, alumina in the nuclei ranges from 0.2 to 0.7%. The goethite cortex of a granule is usually richer in both alumina and silica than its nucleus, in a cortex : nucleus ratio of ∼⃒2:1 at Robe and 3:1 at Yandi. The nucleus and cortex of individual pelletoids generally exhibit a similar trace‐element distribution, which can often be related to the surrounding country rock and, with the exception of Al, is different to that shown by the matrix. A visual classification (labelled co‐manual) based on the relative amount of cortex (co), matrix (ma) and nucleus (nu) enables an estimate of the deleterious minor elements, mainly aluminium (al), for separating samples into ore, near‐ore and non‐ore.

Events of sediment deformation and mass failure in Upper Cretaceous estuarine deposits (Cametá Basin, northern Brazil) as evidence for seismic activity

Event beds recording Late Cretaceous synsedimentary seismicity occur in the Cametá Subbasin, northern Brazil. The deposits consist of nearly 40 m of kaolinized, fluvio-estuarine deposits. The lower fluvial (tidal-influenced) complex, which carries a high-quality commercial kaolin, includes genetically related channel fill, floodplain, crevasse splay, and mud plug deposits. The upper estuarine deposits contain trace fossils indicating marine-influenced settings ( Thalassinoides, Ophiomorpha, Teichichnus, Diplocraterion, Skolithos, Cylindrichnus, Rhizocorallium, and Planolites), and include heterolithic, sandy, and bioturbated units, interpreted as tidal-flat, storm-overwash, and low-energy estuarine bay deposits, respectively. In the upper portion of the estuarine lithosomes, four successive events (events 1–4) of synsedimentary brittle and ductile deformation and large-scale sediment failure, anomalous in estuarine settings, were recognized and attributed to earthquakes. Event 1 is recorded by heterolithic and bioturbated deposits with vertical to subvertical fractures that disappear into underlying undeformed deposits. Event 2 is recorded by thin, sharply bounded sections composed of heterolithic mudstone, and very fine-grained sandstone that shows convolute folds and pillow-like structures. Event 3 is documented by intraformational conglomerate and breccia, and minor mudstones and sandstones, with abundant trace fossils ( Skolithos and Thalassinoides). Event 4 is characterized by vertical to subvertical fractures and faults with homogenized sandstone intrusions along their planes, mostly developed in the upper parts of the deposits corresponding to Event 3. A detailed analysis of facies relationships, combined with consistent orientations of brittle deformation structures and the regional tectonics of the Cametá subbasin, suggests earthquakes as the triggering mechanism for sediment deformation and large-scale mass failure.

Reworked calcretes: their significance in the reconstruction of alluvial sequences (Permian and Triassic, Minorca, Balearic Islands, Spain)

The Permian and Triassic of Minorca (Balearic Islands) consists of a 670-m-thick, red, alluvial succession that includes in situ calcrete profiles and reworked calcrete material. In the Permian succession, the calcretes vary from laminar forms developed on the Carboniferous basement to weakly developed nodular calcretes in fluvial sediments. The palaeosols in the Triassic are mostly dolomitic, and the profiles reach up to Stage III of soil development (Spec. Pap.-Geol. Surv. Am. 203, (1995) 1). The clasts, formed through reworking of the palaeosol profiles, are about 0.5–10 cm across and include mosaics of calcite/dolomite crystals, brecciated clasts, rhizolith fragments, and aggregates of clay and/or silt. These clasts appear in three different types of deposits. Type 1 corresponds to lenticular bodies that fill small scour surfaces, and consists only of intraformational conglomerates. These deposits are interpreted as ephemeral channels and sheet-floods that represent the interfluvial drainage systems that captured only the precipitation falling on the alluvial plain. Type 2 includes sand dune 3-D bodies with flat bottoms and convex tops. These bodies are about 20 cm high and 2 m wide, and were formed by floodwaters that flowed down the levees of the major streams. Type 3 channel deposits contain reworked calcretes and extrabasinal clasts, which overlie erosive surfaces and are found in layers within cross-bedded sandstones and conglomerates. These are interpreted as channel-floor lag deposits of major channels that entered from distant uplands and drained the alluvial plain. Variations in the aggradation rates of the floodplain resulted in five different infill stages. In the lowstand to early transgressive interval, as in stages I (P1) and IV (B1), the fluvial deposits filled palaeovalleys; calcretes and reworked calcrete deposits were of difficult formation (apart from terraces) and preservation. Accommodation space was at its greatest in the transgressive, stages II (P2) and V (B2). This caused the greatest aggradation of the floodplains, which are formed of thick sequences of fine-grained sediments, isolated meandering channels, weakly developed calcretes (compound) and reworked calcrete deposits, mostly of types 1 and 2. The density of channels notably increased in stage III (P3), highstand interval, because of the reduction of accommodation space, this could favour the formation of composite or even cumulative palaeosols, but of difficult preservation. Reworked calcrete deposits are mostly of type 3, but types 1 and 2 are also recognised. The reworked calcrete deposits are an important part of the Permian and Triassic fluvial sediments and their occurrence and characteristics are important in order to interpret the infill of terrestrial basins and the construction of floodplains.

Sedimentology of liassic carbonates (Pirencik Tepe measured section) in the Aydincik (Içel) area, southern Turkey

Liassic carbonates are widespread in the Aydincik (Içel) area where they occur within the Tauride Orogenic Belt. They are represented by the Dibekli Member of the Jurassic to Early Cretaceous Cehennemdere Formation. Seven lithofacies have been delineated within the 489m thick Pirencik Tepe measured stratigraphic section.The facies are: (1) Dolomite (supratidal environment), (2) Microbial laminated limestone (intertidal environment), (3) Intraformational conglomerate (dominantly subtidal environment, (4) Megalodont-Orbitopsella-bearing micrite (shallow subtidal environment), (5) Gastropod-bearing micrite (restricted subtidal environment), (6) Mudstone/micrite (restricted shallow subtidal environment), and (7) Oolite-intraclast-bearing micrite/sparite (high energy intertidal environment).From base to top, the Pirencik Tepe measured stratigraphic section consists of predominantly dolomite, microbial laminated limestone, and cyclic carbonates which are composed of intraformational conglomerate, Megalodont-Orbitopsella-bearing micrite, and microbial laminated limestone. Among the cyclic carbonates, gastropod-bearing micrite and mudstone are irregularly present. The uppermost portion of the section is an oolite-intraclast-bearing micrite/sparite. The overall Liassic carbonate sequence shows a slightly deepening upward sequence in contrast to each cycle in the cyclicity. The average cycle duration is approximately 27–34kyr, which is due to the fifth order cyclicity. The cyclicity is mainly caused by sea level changes. In some areas, a tectonic effect was added to sea level changes, indicated by the lateral changes in the thickness of the intraformational conglomerate.

Palaeoenvironmental analysis of a Miocene basin in the high Taurus Mountains (southern Turkey) and its palaeogeographical and structural significance

Determination of the relationships between the southern, marine-dominated Miocene basins of south central Turkey and their continental hinterland in southern Turkey has traditionally been frustrated by the apparent absence of basin remnants within the Taurus Mountains. The Dikme basin, which seems to be an enclave of basin remnants within the Aladağ Mountains (Eastern Taurides), consists mainly of coarse-grained continental sediments of various facies. These mostly early–middle Miocene sediments were studied to determine the depositional environments and the factors controlling the basin formation and basin fill architecture, to attempt to close the information gap between the Adana Basin to the south and central Anatolian Miocene further to the north. A generally southwest-flowing axial fluvial system and interfingering coarse-grained marginal alluvial clastics derived from northwest and southeast were identified. The marginal facies to the northwest is bounded by a N 55° E-running structural lineament, that starts from the Ecemiş Fault Zone and in digital elevation models extends toward the north of the study area. Along this lineament, Miocene sediments onlap steep fault-line escarpments. Certain Miocene levels are tectonically disrupted, and an intraformational unconformity and boulder conglomerates are also well-developed in the Miocene sequence. The southeast boundary is similarly defined by a NE-trending fault that periodically elevated the adjacent Tufanbeyli autochthon, producing coarse clastics from this area. This boundary fault also induced fining-upwards vertical patterns and synsedimentary deformation in the marginal facies. Additionally, the central part of the basin exhibits a distinct fault-defined morphology characterized by small-scale (tens of metres to 150 m high) valley-and-sill topography. A thin marine interval was also encountered in the southernmost part of the basin, indicating that the clastic system originating around this area debouched into a Miocene sea situated further to the south. The proposed palaeogeography and basin fill model suggests that the Dikme basin and similar Miocene remnants, all controlled mainly by a northeast-running extensional or transtensional fault system, may have been parts of the terrestrial hinterland that supplied sediment to rapidly subsiding marine areas further south, such as the Adana Basin.

Tectonic control on the evolution of the fluvial systems of the Vinchina Formation (Miocene), northwestern Argentina

The Miocene Vinchina Formation accumulated in a large foreland basin is related to the uplift of the Andes Mountains. This 5100m thick unit was mostly deposited in fluvial environments, but short episodes of eolian and lacustrine sedimentation also occurred. Low-angle intraformational unconformities and dramatic facies changes define three depositional sequences. Sequence S1 is composed of sandstones and mudstones deposited in anastomosing river systems during a period of tectonic quiescence. Sequence S2, formed by conglomerates and sandstones with scarce intercalations of mudstones, rests on a low-angle erosional surface and represents deposition in fluvial braided systems. S2 probably resulted from progradation of clastic wedges after an episode of uplift of the fold-thrust belt. Sequence S3, dominated by sandstones, mudstones, and sparse intraformational conglomerates, was deposited in ephemeral braided and meandering rivers governed by a complex balance between subsidence and supply. Fluvial styles in the Vinchina Formation allowed the recognition of three major stages in the foreland basin evolution. During the underfilled stage (S1), high subsidence rates favored the development of axial anastomosing rivers with well-preserved floodplain sediments. Early post-tectonic sedimentation marks initial overfilled conditions (S2), characterized by high sediment supply and progradation of clastic wedges as braided rivers deposits. Finally, more advanced overfilled conditions (S3) were reached when sediment supply matched subsidence, giving birth to ephemeral braided and meandering systems.

Earliest crinoids: New evidence for the origin of the dominant Paleozoic echinoderms

The oldest crinoids have been discovered in Early Ordovician strata of the western United States. A set of emergent crinoid traits based on these and other early crinoids enables reinterpretation of crinoid origins and early history. The new fossils retain primitive echinoderm characteristics, including ambulacral floor plates and largely unorganized cup plating, a first for crinoids. They lack shared derived characteristics linking them to other stalked echinoderms, including blastozoans. Contrary to current widespread opinion, crinoids originated as an independent group during the Cambrian, apparently from an edrioasteroid ancestor. All four major Paleozoic crinoid clades had evolved by the early Ibexian (Tremadocian), and this initial diversification slightly preceded those of most other Paleozoic evolutionary fauna components. These earliest crinoids attached to carbonate hardgrounds developed on sponge-algal mounds, intraformational conglomerates, and grainstones.

Sedimentology of the Triassic-Jurassic boundary beds in Pinhay Bay (Devon, SW England)

New exposures in Pinhay Bay (SE Devon) of the White Lias (Langport Member of the Lilstock Formation) and basal Blue Lias reveal rapidly changing palaeoenvironments during the Triassic-Jurassic (T- J) boundary interval. During deposition of the topmost White Lias a soft seafloor of micritic mudstone was lithified and bored. The resultant hardground was locally eroded, probably in a shallow marine setting, to form a spectacular intraformational conglomerate that was itself lithified. Brief subaerial emergence then followed and produced a fissured and pitted top surface to the White Lias. The regression was short lived and rapid transgression at the base of the Blue Lias established organic-rich shale deposition with a small framboidal pyrite population and low Th/U ratios indicative of a stable, sulphidic lower water column (euxinic conditions). The White Lias/Blue Lias contact thus records a short duration, high amplitude relative sea-level change. This sea-level oscillation has also been postulated for other T -J boundary sections in Europe, although the failure to identify it in regional-scale sequence stratigraphic studies is probably due to its brief duration. Deposition of the basal beds of the Blue Lias was marked by a discrete phase of syn-sedimentary folding and small growth fault activity that may record a regional pulse of extensional tectonic activity.

Paleomagnetism and geochronology of the Late Cretaceous‐Paleogene island arc complex of the Kronotsky Peninsula, Kamchatka, Russia: Kinematic implications

The remnants of ancient island arcs are exposed in the Achayvayam‐Valagina and East Peninsulas tectonic zones of the Olutor‐Kamchatka region, northeast Russia. Geochemically, Late Cretaceous to Paleogene island arc complexes of the East Peninsulas zone are similar to intraoceanic island arcs, and the East Peninsulas zone is regarded as the extinct Kronotsky island arc. Paleomagnetic and biostratigraphic studies of Late Cretaceous and Paleogene island arc rocks were carried out at 21 sites in the eastern part of the Kronotsky Peninsula, which belongs to the East Peninsulas zone. Characteristic remanent magnetizations of both polarities isolated from most sites pass reversal and fold tests; the Cretaceous result is also supported by a positive conglomerate test on lava boulders from an intraformational conglomerate. Mean inclinations of Late Cretaceous, Ypresian and Bartonian rocks correspond to paleolatitudes of 44.8° ± 8.0°N, 38.6° ± 3.5°N, and 45.1° ± 7.0°N, respectively. These values are ∼20° lower than the corresponding reference values for the North American plate for the Cretaceous and Ypresian and ∼14° lower for the Bartonian. Therefore northward transport of the Kronotsky island arc is indicated. Kinematic evolution of the Kronotsky island arc in the Late Cretaceous‐Paleogene was reconstructed using published kinematic parameters. In addition, we incorporated into analysis the geological and published paleomagnetic data for the Late Cretaceous and Tertiary island arc complexes of the Cape Kamchatka Peninsula from the East Peninsulas zone and the Late Cretaceous island‐arc complexes of the Achayvayam‐Valagina zone. As a result, two alternative scenarios of kinematic evolution for the Kronotsky island arc are proposed. According to both of them, the Kronotsky island arc was moving with the continental plate in Late Cretaceous time and with the Pacific plate from the beginning of the Paleocene until docking with the Eurasian margin in Miocene‐Pliocene time.

Taenidium and Associated Ichnofossils in Fluvial Deposits, Cretaceous Tuscaloosa Formation, Eastern Alabama, Southeastern U.S.A

Exposures of the Tuscaloosa Formation in eastern Alabama (U.S.A.) provided an opportunity to examine ichnofossils and ichnofabrics within Cretaceous fluvial facies. Floodplain mudstones are completely bioturbated and dominated by backfilled burrows most allied with Taenidium serpentinum. These burrows likely reflect the work of insects or oligochaetes that occupied soft, moist proximal floodplain substrates. Although the affinities and ethology of the tracemakers and the precise environmental conditions that prevailed during burrowing cannot be confidently inferred, Tnenidium‐dominated ichnofabrics of the mudstones can be assigned to the Scoyenia ichnofacies in the narrower sense of several recent authors (e.g., Buatois and Mángano, 1985). Ichnofossils are rare to absent in Tuscaloosa sandstones. The tops of some thin crevasse‐splay sands within mudstone‐dominated intervals were intensely disrupted by Taenidium producers. Large, vertically extensive, chambered(?) burrow systems, presumably dug by some unknown vertebrate organisms, locally were excavated in abandoned channel sands. In situ ichnofossils apparently are absent altogether in conglomeratic fades. However, intraformational conglomerates in channel‐fill sequences locally are composed almost entirely of Taenidium backfill segments that survived erosion from semi‐consolidated floodplain muds and subsequent fluvial transport and, thus, represent a form of concentration‐type ichnofossil‐lagerstätte.

A new Silurian palaeolatitude for eastern Avalonia and evidence for crustal rotations in the Avalonian margin of southwestern Ireland

SUMMARY Palaeomagnetic data are presented from Mid-Silurian (Homerian, Upper Wenlock,~425 Ma) sediments from the Dingle Peninsula, SW Ireland, which forms part of thenorthern margin of the Palaeozoic microcontinent of Avalonia. Three remanencecomponents were recognized. After removal of a low-temperature component (‘L’),oriented parallel to the present Earth field at the sampling area, two higher-stabilitycomponents were isolated: an intermediate-unblocking-temperature component (‘I’)with mean in situ D=196.9°, I=11.0°, a95=10.8, with a corresponding palaeopole at330.0°E, 30.6°S(dp=5.6, dm=11.0), and a high-unblocking-temperature component(‘H’) with mean tilt-corrected D=218.6°, I=22.1°, a95=7.9, with a correspondingpalaeopole at 309.5°E, 18.3°S(dp=4.4, dm=8.4). A primary (Wenlock) age is indicatedfor the ‘H’-component by a positive intraformational conglomerate test, whereas the‘I’-component is thought to be a secondary mid-Carboniferous partial remagnetization.These data confirm that the sector of the Iapetus Ocean between Avalonia andLaurentia was essentially closed, within the limits of palaeomagnetic resolution, by theWenlock. There is still, however, a discrepancy between the declinations recorded bysimilar-aged sequences to the north and south of the Iapetus Suture. These point toeither an approximately 30° clockwise rotation of the entire Avalonian microcontinentrelative to Laurentia during closure, or local vertical axis rotations of the samplingsites in southern Britain.

The Salt That Wasn't There: Mudflat Facies Equivalents to Halite of the Permian Rustler Formation, Southeastern New Mexico

Four halite beds of the Permian Rustler Formation in southeastern New Mexico thin dramatically over short lateral distances to correlative clastic (mudstone) beds. The mudstones have long been considered residues after post-burial dissolution (subrosion) of halite, assumed to have been deposited continuously across the area. Hydraulic properties of the Culebra Dolomite Member have often been related to Rustler subrosion. In cores and three shafts at the Waste Isolation Pilot Plant (WIPP), however, these mudstones display flat bedding, graded bedding, cross-bedding, erosional contacts, and channels filled with intraformational conglomerates. Cutans indicate early stages of soil development during subaerial exposure. Smeared intraclasts developed locally as halite was removed syndepositionally during subaerial exposure. We interpret these beds as facies formed in salt-pan or hypersaline-lagoon, transitional, and mudflat environments. Halite is distributed approximately as it was deposited. Breccia in limited areas along one halite margin indicates post-burial dissolution, and these breccias are key to identifying areas of subrosion. A depositional model accounts for observed sedimentary features of Rustler mudstones. Marked facies and thickness changes are consistent with influence by subsidence boundaries, as found in some modern continental evaporites. A subrosion model accounts for limited brecciated zones along (depositional) halite margins, but bedding observed in the mudstones would not survive 90% reduction in rock volume. Depositional margins for these halite beds will be useful in reconstructing detailed subsidence history of the Late Permian in the northern Delaware Basin. It also no longer is tenable to attribute large variations in Culebra transmissivity to Rustler subrosion.

The Salt that Wasn't there: Mudflat Facies Equivalents to Halite of the Permian Rustler Formation, Southeastern New Mexico: CURRENT RIPPLES

ABSTRACT Four halite beds of the Permian Rustler Formation in southeastern New Mexico thin dramatically over short lateral distances to correlative clastic (mudstone) beds. The mudstones have long been considered residues after post-burial dissolution (subrosion) of halite, assumed to have been deposited continuously across the area. Hydraulic properties of the Culebra Dolomite Member have often been related to Rustler subrosion. In cores and three shafts at the Waste Isolation Pilot Plant (WIPP), however, these mudstones display flat bedding, graded bedding, cross-bedding, erosional contacts, and channels filled with intraformational conglomerates. Cutans indicate early stages of soil development during subaerial exposure. Smeared intraclasts developed locally as halite was removed syndepositionally during subaerial exposure. We interpret these beds as facies formed in salt-pan or hypersaline-lagoon, transitional, and mudflat environments. Halite is distributed approximately as it was deposited. Breccia in limited areas along one halite margin indicates post-burial dissolution, and these breccias are key to identifying areas of subrosion. A depositional model accounts for observed sedimentary features of Rustler mudstones. Marked facies and thickness changes are consistent with influence by subsidence boundaries, as found in some modern continental evaporites. A subrosion model accounts for limited brecciated zones along (depositional) halite margins, but bedding observed in the mudstones would not survive 90% reduction in rock volume. Depositional margins for these halite beds will be useful in reconstructing detailed subsidence history of the Late Permian in the northern Delaware Basin. It also no longer is tenable to attribute large variations in Culebra transmissivity to Rustler subrosion.

Depositional setting of the Lower Old Red Sandstone at Pantymaes Quarry, central South Wales: new perspectives on the significance and occurrence of ‘Senni Beds’ facies

Abstract The 200 m long face of Pantymaes Quarry, central South Wales, exposes part of the upper (Dittonian) Red Marl Group (Lower Old Red Sandstone). A lower Sandstone Facies Association (up to 15 m thick) comprises interbedded grey-green micaceous sandstones, pebbly intraformational conglomerates, and grey siltstones. Major bounding surfaces define stacked channel complexes, representing the deposits of braided river systems. An erosively overlying Mudstone Facies Association (up to 15 m thick) comprises tabular-bedded, bioturbated and calcretized red siltstones and fine sandstones representing overbank deposits, possibly from a meandering system. The change in fluvial style is interpreted as a local response to pulses of oblique-slip movement on nearby fault systems. The period of fault activity resulted in the braided Sandstone Facies Association, the Mudstone Facies Association recording a gradual return to a meandering system. Individual pulses of activity produced the stacked channels within the Sandstone Facies Association. The Sandstone Facies Association is similar in character to the overlying Senni Beds, suggesting that ‘Senni Beds’ may describe a recurring, possibly tectonically controlled, braided channel facies in the Lower Old Red Sandstone.

Paleomagnetism of the 550 Ma Skinner Cove volcanics of western Newfoundland and the opening of the Iapetus Ocean

The Skinner Cove Formation of western Newfoundland is an alkali volcanic suite transported in the Ordovician onto Laurentia's Iapetus Ocean margin. It has previously yielded a U–Pb zircon age of 550.5 +3 −2 Ma, has no penetrative deformation and has only been affected by zeolite facies metamorphism. At 10 sites, flows and dykes of the Skinner Cove Formation exhibit a stable characteristic `A' remanence carried by magnetite. This `A' remanence is shown to be primary by an intraformational conglomerate test using 20 trachybasalt clasts (whereas 11 alkali basalt clasts were overprinted, probably because they originally contained titanomagnetite from which rutile exsolved soon after burial by overlying flows). After tilt-correction, the mean `A' remanence direction is southeast and down ( D=144°, I=32°; α 95=11°, k=21, N=10 sites). The paleolatitude calculated from the ten `A' site virtual geomagnetic poles is 19°S ± 9°, which likely represents the paleolatitude of Laurentia's Iapetan margin at ∼550 Ma. Evidence that the Skinner Cove Formation originated at this margin includes its structural position within the Humber Arm Allochthon, its within-plate trace element geochemistry, and its similarity in age to other alkali magmatism that more certainly represents Laurentia's Iapetan margin. Comparison with other Laurentian paleomagnetic data implies that Laurentia drifted very rapidly northward from the south polar region to the equator between ∼570 and 550 Ma. We suggest that the start of this rapid northward drift at ∼570 Ma marked the onset of Iapetus sea-floor spreading between Laurentia and West Gondwana. Further well-constrained paleomagnetic data of ∼570 to ∼550 Ma age are needed from Laurentia to test its high rate of northward drift, and from West Gondwanan cratons to test that they remained relatively stationary.

A Late Triassic lake system in East Greenland: facies, depositional cycles and palaeoclimate

The Upper Triassic Fleming Fjord Formation of the Jameson Land Basin in East Greenland contains a well-exposed succession, 200–300 m thick, of lake deposits. The Malmros Klint Member, 100–130 m thick, is composed of cyclically bedded intraformational conglomerates, red siltstones and fine-grained sandstones and disrupted dolomitic sediments (paleosols). The cyclicity is composite with cycles having mean thicknesses of (25), 5.9 and 1.6 m. The overlying Carlsberg Fjord beds of the Ørsted Dal Member, 80–115 m thick, are composed of structureless red mudstones rhythmically broken by thin greyish siltstones. This unit also has a composite cyclicity with cycles having mean thicknesses of 5.0 and 1.0 m. The uppermost Tait Bjerg Beds of the Ørsted Dal Member, 50–65 m thick, can be divided into two units. A lower unit is composed of cyclically bedded intraformational conglomerates or thin sandstones, red mudstones, greenish mudstones and yellowish marlstones. An upper unit is composed of relatively simple cycles of grey mudstones and yellowish marlstones. Recognized cycles have mean thicknesses of 5.6 and 1.6 m. The lake deposits contain evidence of seasonal, orbital and long-term climatic change. Seasonal change is documented by numerous desiccation surfaces especially in the Malmros Klint Member and Carlsberg Fjord beds, orbital change is suggested by the composite cyclicity, and long-term climatic change is indicated by the systematic upwards change in sedimentary characteristics of the lake deposits. The sedimentary features of the Malmros Klint Member suggest lacustrine deposition in a dry climate that fluctuated between desert and steppe conditions, the Carlsberg Fjord beds probably record lacustrine lake deposition in a rather constant dry (steppe) climate, while the Tait Bjerg Beds record lake sedimentation in a climate that fluctuated between dry (steppe) and warm moist temperate. In the Tait Bjerg Beds the upward change in cycle characteristics indicates a shift towards more humid conditions. Climatic deductions from sedimentary facies are in good agreement with climate maps of Laurasia, as simulated by numerical climate models. Palaeomagnetic data indicate a northward drift of East Greenland of about 10° from ca. 25°N to ca. 35°N in the Middle to Late Triassic. The Fleming Fjord Formation which represents ca. 5 m.y. of the Late Triassic interval was deposited during latitudinal drift of 1–2°. It is possible that the observed long-term upward shift in climatic indicators within the formation can be ascribed to plate drift, but southward shift of climatic belts could also have been of importance.

The Mupe Bay oil seep: a detailed organic geochemical study of a controversial outcrop

Abstract The oil seep in an intraformational conglomerate at Mupe Bay has attracted much interest as providing possible evidence of oil generation in the Wessex Basin by the early Cretaceous (Wealden). Previous organic geochemical studies investigating whether oils of different maturity exist in the clasts and matrix of the conglomerate have been complicated by the effects of heavy biodegradation of the oil. In this work, we present detailed molecular organic geochemical data from a significantly larger suite of samples than has been previously studied. These have been screened using a multivariate statistical approach to identify those samples which have been least influenced by biodegradation. Conventional molecular maturity parameters applied to the subset of samples which have suffered no detectable modification to their biological marker (hopane and sterane) distributions indicate that the oil in the clasts is of exactly the same maturity as that in the matrix. However, this observation does not preclude two phases of staining, although it remains to be proven that the clasts contained oil at the time of their deposition.

Heterogeneity and lithotype distribution in ancient deep-sea canyons: Point Lobos deep-sea canyon as a reservoir analogue

An evolution and history of filling is proposed for an exceptionally exposed ancient deep-sea canyon on a Paleocene oblique-slip tectonic margin which, on a number of scales, reveals, successive phases of canyon activity. The quantitative methods adopted for this study make it of direct use to modellers as an example of reservoir heterogeneity in an ancient canyon fill, where facies distribution from boreholes can be scaled up to reconstruct the reservoir, using the methods outlined in this paper. The Point Lobos submarine canyon, near Carmel in California, provides a complete cross-section of an ancient canyon, with a fill which displays a whole range of channel morphologies, and laterally extensive coverage of the internal architecture of associated conglomerate packages and related debris flows. This paper presents quantitative documentation of the canyon-fill sediments and canyon-wide fill packages, on scales which vary from bed-to-bed analysis, reflecting processes in operation during individual events, to canyon-wide analysis, reflecting the overall evolution of the canyon. The northern and southern canyon margins are both exposed, and the Paleocene fill onlaps the subvertical canyon wall. The canyon was incised into Cretaceous granodiorite. The fill comprises five thick sequences which correspond to five successive phases of sediment deposition within the canyon. Each sequence typically consists of resedimented conglomerates that are stacked and channelised, with a vertical architecture which resembles that of subaerial braided stream deposits. These are overlain by channelised turbidite sandstones, interbedded with intraformational conglomerates and mud-chip breccias. These in turn are overlain by mudstones and shales, which are commonly slumped and disturbed. Published classification schemes that show the range of deep-water facies were found insufficient to describe the Point Lobos canyon fill. Methods were developed for recording the lithologic proportions of the fill. Twenty-three lithotypes were defined, which show the variation in the sedimentary units, and a range of processes are proposed to explain each lithotype. These methods produced a quantitative database of lithologic proportions for the canyon fill. Over 80% of the fill comprises sandstone and conglomerate. Conglomerate is more common near the canyon margins. Mud and silt comprise the other 20% of the canyon fill; predominantly as laterally extensive, slumped packages which cap the coarser sequences. The distribution of these lithotypes within the fill was found to have a cyclicity, with five cycles of sedimentation recorded, and lateral transport of material from the canyon walls a primary feature.