摘要:

ABSTRACTMiddle Cambrian rocks of the Georgina Basin contain both phosphatic and organic‐rich sediments, the former often occurring as commercially viable phosphate deposits, and the latter as minor oil shale occurrences. Both kinds of rocks occur in the Hay River Formation penetrated by Bureau of Mineral Resources (BMR) stratigraphic corehole Tobermory 14. Detailed sampling and analysis of this core has been undertaken to assess the chemostratigraphy of the Hay River Formation; to examine the possible use, as a palaeoceanographic tool, of changes in δ13C values of carbonates and kerogens; and to gain an understanding of phosphate deposition in this part of the basin.The Upper Hay River Formation was deposited in a normal marine environment characterized by oxic bottom conditions. It contains several episodes of significant trace metal enrichment which appear to indicate periodic replacement of the water body. The Lower Hay River Formation, however, was deposited in an abnormal marine environment low in sulphate and Mo and enriched in P. Its carbonate phase has positive δ13C values 1–1.5%0enriched over the accepted Cambrian marine carbonate value of ‐0.57 ± 0.17%0. The very low sulphate concentration, Mo‐depletion and spiking of carbonate δ18O values suggests there was an influx of deeper ocean water of sufficient volume to maintain prolonged stable anoxic bottom waters. These data support a previous suggestion that phosphogenesis in the Georgina Basin was related to ‘Oceanic A

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1988.tb01235.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTEstimates for the rate of concretionary growth in shales are based on models which assume that growth is diffusion‐controlled. However, laboratory and field studies of CaCO3precipitation in organic carbon‐rich sediments indicate that surface reactions control growth, due to inhibition by various dissolved species. The spatial distribution of carbonate concretions in the Jet Rock (Lower Jurassic, England) is also inconsistent with diffusion‐controlled precipitation of CaCO3into concretions, and growth must have been at least partly surface reaction‐con

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1988.tb01236.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTScanning electron micrographs of crypto‐crystalline (<4 μm) dolomites found in cores taken from the lower Eocene Facha Member reservoir of the Zella and Aswad fields in Libya show that they consist of tiny globular bodies mixed with anhedral crystals, producing a chalky texture. The sedimentary sequence indicates deposition in upper intertidal/supratidal arid environments and in saline lagoons. The crypto‐crystalline dolomite is facies‐controlled and probably formed as a primary precipitate, similar to those reported from present‐day subaqueous environments in the Coorong of South Australia and Holocene supratidal crusts off Florida.Such chalky‐textured dolomites could form local fluid‐migration barriers within hydrocarbo

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1988.tb01237.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTTwo oolites in the Dinantian (Mississippian/Lower Carboniferous) of Glamorgan, SW Britain, were deposited in similar depositional environments but have contrasting diagenetic histories. The Brofiscin and Gully Oolites occur in the upper parts of shallowing‐upward sequences, formed through strandplain progradation and sand shoal and barrier growth upon a southward‐dipping carbonate ramp. TheBrofiscin Ooliteis characterized by a first‐generation cement of equant calcite spar, preferentially located at grain‐contacts and forming non‐isopachous fringes around grains, interpreted as meteoric vadose and phreatic in origin. Isopachous fibrous calcite fringes of marine origin are rather rare and occur only at a few horizons. Burial compaction was not important and porosity was occluded by poikilotopic calcite spar. Fitted grain‐grain contacts locally occur and could be the result of near‐surface vadose dissolution‐compaction. Syntaxial overgrowths on echinoderm debris are common. Pre‐compaction overgrowths are cloudy (inclusion‐rich) and probably of meteoric origin, and post‐compaction overgrowths are inclusion‐free. By contrast, theGully Oolitehas little first‐generation cement. However, marine fibrous calcite is common in oolitic intraclasts, as isopachous fringes of acicular calcite crystals closely associated with peloidal internal sediment; and early equant, drusy calcite spar occurs in the uppermost part of the Gully, beneath a prominent palaeokarst where pedogenic cements also occur. The major feature of Gully diagenesis is burial compaction, resulting in extensive grain‐grain dissolution and microstylolitic grain contacts, and post‐compaction poikilotopic spar occluded remaining porosity.The Brofiscin Oolite is pervasively dolomitized up‐dip but the Gully Oolite for the most part only contains scattered pre‐compaction dolomite rhombs and late veins of baroque dolomite, with less pervasive dolomitization.The difference in diagenetic style of the two Dinantian oolites is attributed to prevailing climate. The paucity of early meteoric cements in the Gully is a result of an arid climate, and this is supported by the nature of the capping palaeokarst. The abundant meteoric cements in the Brofiscin reflect a more humid climate, and effective meteoric recharge also resulted in up‐dip pervasive mixing‐zone dolomitization. The style of early diagenesis in these two oolites exerted a major control on the later burial diagenesis: in the Brofiscin, the early cements inhibited grain‐grain dissolution and pressure solution, while these processes operated extensively in the Gully Oolite. Thus, prevailing climate can influence a limestone's diagenetic history fro

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1988.tb01238.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTLaminar calcretes are described from the Lower Carboniferous of South Wales, the Upper Jurassic of southern England and the Upper Jurassic‐Lower Cretaceous of northern Spain. They are interpreted as calcified root‐mats (horizontal root systems) and are compared with other examples in the geological record and with possible modern analogues. All three occurrences consist of virtually identical, centimetre to decimetre‐thick, locally organic carbon‐rich, laminar micrites containing up to 50% by volume of millimetre‐sized typically calcite‐filled, tubular fenestrae set in an irregular but very finely laminated matrix. It is suggested that root‐mat calcretes are probably very common in the geological record in peritidal, lacustrine margin and floodplain deposits, but owing to their crudely biogenic microstructure, they more closely resemble cryptalgal laminites than do other laminar calcretes. The recognition of such root‐mat calcretes in sedimentary sequences not only provides evidence of subaerial exposure and vegetation cover but can also indicate positions of palaeo‐water‐tables in ce

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1988.tb01239.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTAggradation of intertidal sediment in upper Turnagain Arm, a macrotidal estuary located in southern Alaska near Anchorage, began because of subsidence caused by sediment consolidation and tectonic lowering of the land during the earthquake of March 27, 1964. At the head of the Arm, decrease in percent sand of surface sediments with time and return of rodents, pulmonate gastropods, trees, bushes, and grasses to the intertidal region, indicate that the rate of sediment deposition has slowed and the Upper Turnagain Arm area has returned to a state of preearthquake equilibrium only 17 years after the event. Comparison of channel locations before and after the earthquake indicates the possibility of regional postseismic tilting of the estuary.

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1988.tb01240.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTThe Sur debris slide and associated debris flow cover more than 1000 km2of the eastern part of Monterey submarine fan and extend from the base of the continental slope near the apex of the fan to the Monterey fan valley. The flow is generally confined between the continental slope and remnants of an older channel (Monterey East fan valley). The hummocky surface of the debris slide and diffuse echo returns from the surface of the nearly acoustically transparent debris flow, as seen in 3.5 kHz profiles, are common to large submarine slides described from passive‐margin continental‐slope and rise deposits. Piston cores from the Sur slide recovered contorted and sheared fan turbidite units as well as clast‐ and matrix‐supported mudlump debris flows. The cores show that the slide debris is overlain by 0.23–0.87 m of mud and turbidite sand that limit the age of the slide to latest Holocene, although more than one stage of emplacement is

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1988.tb01241.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTA transverse crescentic draa in the Algodones dune field, California, was monitored for a year using surface process mapping, aerial photography and supplemental wind measurement. The draa is oriented by the long‐term resultant wind, whereas its superimposed features are in equilibrium with the bedform‐modified secondary airflow. Surface airflow and the movement of superimposed bedforms is typically oblique or parallel to the draa brinkline, particularly on the lee slope. Comparison of measurements of draa movement and sand deposition on the lee slope, with expected rates calculated from wind data and draa size, confirm that there is a significant component of sand flow parallel to the draa brinkline. The internal structure being generated at the base of the draa lee slope is inferred from the surface processes active there. Within the space of a kilometer two types of compound cross‐strata, separated by an area of simple cross‐strata, are being produced. This has significant implications for interpretations of ancient aeolian strata. Variations in internal structure types found in lateral sequence may be generated by one complex bedform, and these cross‐strata may be simple or compound. Second‐order bounding surface orientations indicate resultant primary palaeowind directions; compound cross‐strata dip directions indicate secondary flow conditions. The existence of cross‐strata dip directions oblique or perpendicular to the second‐order surface indicates longitudinal secondary flow on the lee face, but not necessarily a longitudinal or oblique draa. Without further detailed knowledge about various draa configurations and behaviour, stratification attributed to draas can be used only to interpret activity on the lo

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1988.tb01242.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTSand transport rates were measured using a vertical sand trap along the intertidal zone of a beach in North Devon, England, together with simultaneous monitoring of the wind speed on the beach and moisture levels in the surface layers of sand. The results of 88 sand trap samples in a wide range of wind speeds showed that moisture levels up to 14%, in the top 1–2 mm of the beach sand, have no discernible effect on the transport rates. Transport rates measured from areas of the beach where the moisture was below this level are compared with the rates predicted by seven expressions based on theoretical and wind tunnel research together with the empirical results of other published research. Measured transport rates range from 0.0001 to 0.22 kg m‐1s‐1. The results indicate that expressions based on a power relation between the wind speed and the transport rate, and which include a threshold velocity term, provide the best estimates of the observed transport

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1988.tb01243.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY

摘要:

ABSTRACTModels of sediment threshold by grain pivoting or sliding over underlying particles are examined in order to explore their application to evaluations of selective entrainment of gravel by flowing water. Of special interest is whether such process‐based models provide satisfactory evaluations of flow competence and the movement of large clasts by floods. A detailed derivation is undertaken, focusing first on the fluid flow and forces at the particle level. The resulting threshold equation for the particle‐level velocity is then modified to yield the mean entrainment stress for the flow as a whole. This approach is appropriate for considerations of selective entrainment of grains of varying sizes within a deposit, the sorting being due to their relative projection distances above the bed and the dependence of their pivoting angles on grain size and shape. The resulting threshold equations contain a number of coefficients (e.g. drag and lift) whose values are poorly known, but can be constrained by requiring agreement with the Shields curve for the threshold of grains in uniform deposits. If pivoting coefficients based on laboratory measurements with tetrahedral arrangements of particles are used in the models, smaller degrees of selective sorting are predicted than found in the field measurements of gravel entrainment. However, if reasonable modifications of those coefficients are made for expected field conditions, then the models yield good agreement with the data. Sliding models, where sorting is due entirely to projection distances of the grains above the bed, yield somewhat poorer agreement with the field data; however, the sliding models may have support from laboratory experiments on gravel entrainment in that the data and theoretical curves have similar concave trends. The existing measurements lack documentation of the mechanisms of grain movement, so it is not possible to conclusively determine the relative importance of grain pivoting versus sliding. In spite of such uncertainties, the results are encouraging and it is concluded that pivoting and sliding models for grain entrainment do have potential for field computations of selective entrainment and flow compete

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1988.tb01244.x

出版商:Blackwell Publishing Ltd    

年代:1988

数据来源: WILEY