摘要:

ABSTRACTContinental carbonates of the basal part of the Guarga Formation, Upper Eocene, have been studied in the field and in the laboratory. Both geographic pattern and vertical sequences served as a basis for sedimentary models. The pattern reveals that carbonate deposition was greater in the western part of the Tremp‐Graus Basin than in the eastern part. Studies of slabs and thin sections suggest high energy deposition in the eastern part of the basin and low energy deposition in the western part. Gross lithology, petrography, and faunal/floral associations indicate sedimentary environments which include calcimorphic palaeosoils, caliche nodule zones, solid caliche nodule zones, caliche crusts, fluvial carbonates, paludal carbonates, and pond/lake carbonates. Distribution pattern of the carbonate bodies and lithologies seem to be controlled by alluvial‐fan processes. They are typical fining‐upward sequences in the sense of Collinson (1978) which furthermore can be assigned to a channel‐bound group and a succession independent of fluvial channels. Referring to fan‐physiography the sequences can be subdivided into fluvial proximal interchannel, fluvial distal interchannel, lacustrine distal interchannel, nearshore lacustrine‐paludine, and lacustrine offshore carbonate facies. The two last categories are distal alluvial plain and/or fan‐edge associations. The presence of especially charophyte oogonia adjacent to pseudomorphs of lenticular gypsum in the lacustrine carbonates of the western Tremp‐Graus Basin indicates that their depositional environment was that of shallow lakes with alternating salinities.Diagenetic alterations consist of three major phenomena: (a) general cementation of the carbonates by low‐Mg calcite, (b) pseudomorphs of lenticular gypsum, and (c) silification. The relationship of gypsum and silica diagenesis to the regional distribution of lacustrine carbonates suggests early diagenetic processes for their formation. Gypsum pseudomorphs are small and are found to be replaced by Fe‐calcite throughout the whole area, or are large and replaced preferentially by silica in the west only. There is experimental evidence (Cody, 1979) that distribution of gypsum morphology and crystal sizes can be related to increased salinities and increased organic production in the west.Chert‐gypsum fabrics from that area resemble the ‘silex nectique’ of Cayeux (1929). The occurrence of this noteworthy type of chert together with length‐slow chalcedony is an indication of relatively early silification of the gypsum. It is suggested that the sediments containing the silicified lenticular gypsum hosted brines of partly marine origin which alternately became brackish or fresh.An attempt was made to check the influence of evaporation during carbonate deposition by determining the stable isotope composition of the carbonates and of the chert. The data show no distinct variations with respect to the presumed palaeoenvironments. The ranges in the carbonates (δ13C − 5,0 to −7,9; δ18O −4,5 to −7,7) fit among the data given in the literature. The values derived from chert (δ18O 26,9 to 31 SMOW) support a concept of alternating salinities.Distribution of carbonate lithofacies indicate a systematic change in sedimentation from fluvial/ephemeral to permanent lacustrine from east to west. Diagenetic alterations suggest an increase of salinities of pore waters in the same direction. In addition the lacustrine limestones in the west were due to frequent changes in pore‐water salinity. Similar sequences lacking subtidal and/or lagoonal fossil associations but including pseudomorphs of carbonate and chert after lenticular gypsum were described from the Lower Purbeckian of Dorset (West, 1964, 1975, 1979), and from the Tertiary of the Paris Basin (Fontes, 1968). Guarga limestone environments of especially the western Tremp‐Graus Basin probably resembled the modern semi‐arid ephemeral saline lakes in Australia (Burne, Bauld&De Dekker, 1980). Gypsum precipitation in these environments can be compared to the one in modern sabkhas from the Mediterranean coast of Egypt (

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1982.tb00084.x

出版商:Blackwell Publishing Ltd    

年代:1982

数据来源: WILEY

摘要:

ABSTRACTThe environments in which carbonate diagenesis proceeds have been documented in previous studies of Holocene and late Pleistocene sediments and limestones on Barbados, West Indies. Variations in the carbon and oxygen isotopic composition of limestones, produced during early freshwater diagenesis, have been observed in this study to occur in specific patterns. Six potentially useful patterns emerge when one views stable isotope data within a stratigraphic framework: (1) the subaerial exposure surface is characterized by strongly12C‐enriched limestones. δ13C compositions of underlying limestones grow progressively heavier with increasing depth; (2) the subaerial exposure surface may also be marked by slight18O‐enrichment; (3) an abrupt shift in δ18O values may differentiate sediments above the exposure surface from those below; (4) sediments altered in the marine‐meteoric mixing zone may be characterized by positive covariance between their δ18O and δ13C compositions; (5) the vadose‐phreatic boundary may be marked by a sharp increase in δ13C values in the seaward portions of a fresh groundwater system; and (6) samples contemporaneously altered in a single fresh groundwater system within an areally restricted region should display a narrow range of δ18O and a wide range of δ13C compositions.Analysis of samples from five Palaeozoic and Mesozoic formations, which contained petrographic evidence of early freshwater diagenesis, showed that isotope patterns similar to those observed in Barbados limestones have been preserved in rocks as old as Mississippian.These isotope patterns could prove to be useful for identifying diagenetically induced porosity trends in carbonate rocks. They might be used to identify limestones diagenetically altered in meteoric environments, to identify mixing zone cements and dolomites, and to trace the regional and vertical distributions of early meteoric groundwater systems in ancient carbon

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1982.tb00085.x

出版商:Blackwell Publishing Ltd    

年代:1982

数据来源: WILEY

摘要:

ABSTRACTIrregular, scalloped erosion surfaces in the shelf carbonate sequences of the lower Eke Formation (Ludlow Series, upper Silurian) in the east of Gotland, Sweden, comprise series of shallow hollows separated by sharp‐crested ridges, and cavities with sculptured, undercut walls, cut into lithified sediment. These represent analogues of the solution basins formed in modern coastal and subaerial karst terrains. Discrete erosional cavities merge on enlargement by breaching of the intervening walls to leave remnant, tapering ridges. Sets of the ridges and basins seen in surface view show an average width of basins of 1–2 m, with relief of 40–50 cm and pronounced N‐S axes for the ridges; this alignment may reflect the local drainage direction. The lowermost erosion surface passes laterally into a planar, mineralized horizon at the top of the underlying Hemse Group that was resistant to and forms the base level of erosion. Because of restricted exposure of higher Eke Formation sediments the upper limit of erosion remains unknown. There is no evidence of caliche or subaerial diagenetic textures, but solution vugs are common in the eroded limestones. Marine hard‐bottom biota attached to some surfaces, and transition from scalloped to planar surfaces indicate erosion in tidal zones, but subaerial karstic erosion is also inferred. The resubmerged karst‐eroded topography is overlain by shallow marine carbonates, including small organic buildups. Finely‐layered stromatolitic mats developed over the initial infill, in subtidal environments, and grew to form domed mounds within the erosional cavities. They abut sharply against bounding side walls and overhangs. Some emergence is evident from desiccation features in the upper parts of mounds. Biostratigraphical evidence dating the events from initial uplift and karstic erosion to covering of the drowned relief topography places the whole sequence within upper

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1982.tb00086.x

出版商:Blackwell Publishing Ltd    

年代:1982

数据来源: WILEY

摘要:

ABSTRACTWashover sand bodies commonly develop along microtidal coastlines in beach/barrier island or spit settings. Wave runup, usually in conjunction with an abnormally high water level, may overtop the most landward berm of the beach and the foredune crest, if one exists, to produce overwash and subsequent runoff across the more landward subaerial surface. Two main elements of the resulting deposit are the washover fan and runoff channel. Newly formed, small‐scale washover deposits were examined along the Outer Banks, North Carolina, near Pt Mugu, California, and at Presque Isle (Lake Erie), Pennsylvania. The fans were formed in response to unidirectional landward transport, and the runoff channels in response to unidirectional flow usually in a landward direction, but sometimes in shore‐parallel then seaward direction. Where overwash carried across the fan surface and entered a pond or lagoon, a small‐scale delta (microdelta) developed. In this case, the washover fan consisted of two subfacies, the wetted, but ‘subaerial’ part of the fan and the subaqueous washover delta. Flow associated with the development of the fan and runoff channel produced distinctive sets of bedforms and internal stratification.High velocity discontinuous surges moving across the fan surface resulted in the development of a plane bed and subhorizontal to low‐angle (landward dipping) planar stratification which comprised the major part of the fan. Similarly, rhomboid forms were produced by high velocity sheet flow across the fan surface. Where flow carried into a standing body of water, delta‐type foreset strata developed. For this case, the lateral structural sequence was subhorizontal, planar stratification merging landward into landward dipping, delta (tabular) foreset strata. In the runoff setting, where flow became channelized and continuous, both upper‐flow and lower‐flow regime currents were typical. Upper‐flow regime bedforms included antidunes, standing waves, and plane beds. The most commonly observed lower‐flow regime bedforms included microdelta‐like bars, low‐amplitude bars, linguoid ripples, and sinuous‐crested current ripple trains.The sets of sedimentary structures comprising modern washover sand bodies provide criteria for the identification of similar deposits in ancient sediments and for more specific interp

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1982.tb00087.x

出版商:Blackwell Publishing Ltd    

年代:1982

数据来源: WILEY

摘要:

ABSTRACTThe English Channel and its Western Approaches constitute a 700 km long epicontinental sea located in a temperate environment and in a tectonic setting where subsidence is minimal.A sedimentological survey of this region reveals quite distinct provinces: a central sector (Western Channel) where predominantly bioclastic sediments are widely represented, bounded by two mainly terrigenous‐rich zones, the outer terrace of the Celtic Sea and the Eastern Channel. Lateral sedimentary variations of decreasing grain size are interpreted in terms of current velocity patterns. The various types of such sequences may relate to the degree of mixing of older lithoclastic sediments with the Holocene bioclastic supply.A scenario for the evolution of the recent sedimentation of this epicontinental sea is presented. Starting from a permanent marine zone—at least since the Würm period—that bounds the Bay of Biscay, the Holocene transgression progressed over the English Channel. In the Western Channel earlier terrigenous deposits were gradually overlapped by bioclastic sediments that originated on the pre‐Mesozoic rocky substratum and which were particularly extensive off the Armorican Massif. In the subsequently submerged Eastern Channel the pre‐Holocene clastic source has undergone comparatively less modification and still crops out in most of the area. With the opening of the Straits of Dover, at about 9000 yr BP, submersion was complete and new hydrodynamic conditions developed as the eustatic level stabilized. The present sediment distribution is in harmony with the hydrodynamic setting except on the southern rise of the Celtic Sea where both morphology and sedimentary patterns still largely reflect pre‐Holocen

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1982.tb00088.x

出版商:Blackwell Publishing Ltd    

年代:1982

数据来源: WILEY

摘要:

ABSTRACTThe San Andreas fault system in northern California forms an 80–90 km wide zone of right‐lateral shear. Extensional tectonism within this broad shear zone is indicated by both Neogene silicic volcanic rocks that gradually young in the direction of shear propagation to the north‐west and by numerous Neogene faultbounded structural basins filled with thick non‐marine sequences.The Little Sulphur Creek basins, three well‐exposed 1·5–2 km wide pull apart basins within this shear system, have sedimentation patterns analogous to those of much larger pull‐apart basins. They were formed and subsequently deformed by east‐west extension and by north‐west to south‐east‐orientated right‐slip concurrently with basin filling. Palaeocurrent and maximum‐clast size data indicate both lateral sediment transport from fault‐bounded basin margins and longitudinal transport down the basin axes.The basins are filled primarily with coarse alluvial‐fan and streamflow deposits derived from a surrounding igneous, sedimentary, and metamorphic provenance. Two of the basins contain basin‐plain‐type lacustrine turbidites that grade laterally into distal alluvial fan, fan‐delta, and sublacustrine delta deposits. Talus deposits along the south‐west margin of the basins contain megabreccia indicative of active uplift. Structures indicative of dewatering, liquefaction, and slumping

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1982.tb00089.x

出版商:Blackwell Publishing Ltd    

年代:1982

数据来源: WILEY

摘要:

ABSTRACTIt is well known that sediment sorting according to size, shape and density occurs, but the exact mechanisms involved are poorly understood. To assess the effects of size and density, sand‐size spheres of two densities were transported and deposited under controlled flume conditions. Observations on the motion of discrete particles show that grains smaller than bed‐roughness grains move continuously and have the same transport velocities regardless of density. For grains near and slightly larger than the roughness, movement is intermittent and, for a given size, heavy particles move more slowly than lights. For grains much larger than bed roughness grains, movement is continuous over the rough surface and light and heavy grains have nearly the same transport velocities.Analyses of bulk sediment deposited from plane‐bed transport, show that the size and proportion of heavies decreases and that of lights increases with distance transported. For ripple bed transport, however, the size relations between associated light and heavy grains remains essentially unchanged with transport distance and the proportion of light and heavy grains is extremely variable.These results suggest that size‐density sorting in plane‐bed transport is a function of the transportabilities identified in the discrete grain studies but that sorting in ripple‐bed transport is related to deposition on, and recycling through, the bed forms. Application of these findings to the concept of hydraulic equivalence implies that some indication of bed configuration may be necessary for the concept t

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1982.tb00090.x

出版商:Blackwell Publishing Ltd    

年代:1982

数据来源: WILEY

摘要:

ABSTRACTLongitudinal helical vortices are generally considered to be the cause of longitudinal sedimentary bedforms. In tidal oscillatory flows, however, it is not clear how a regular system of vortices will become fixed for long enough to establish the bedforms.It is proposed that the presence of standing internal waves, with axes parallel to the tidal current, have provided the basic flow pattern on which longitudinal helical vortices develop in Southampton Water. The observed furrows then developed in a pattern determined by the standing internal waves.

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1982.tb00091.x

出版商:Blackwell Publishing Ltd    

年代:1982

数据来源: WILEY

摘要:

ABSTRACTA miniature, 9 m‐wide floodplain, developed along a gravel‐washing effluent stream, shows features such as levées, crevasse splays and floodbasins which compare with their larger‐scale counterparts. For sediments deposited overbank, median size decreases exponentially with distance from the channel whilst sorting increases, with coarser sediment on the outside of a meander bend. Overbank flows are only a few grain diameters in depth near the channel. This study shows potentially useful systematic relationships in floodplain sediment textures, but it involves only one of a possible variety of floodplain types dominated by overbank sedimentation. This suggests that further exploration of overbank depositional processes is desirable as an aid to field interpre

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1982.tb00092.x

出版商:Blackwell Publishing Ltd    

年代:1982

数据来源: WILEY

摘要:

ABSTRACTThe benthic foraminiferQuinqueloculina impressaReuss, was buried in various types of sediment in order to assess its capability for producing sediment disturbances and thus, ichnofossils. Silts and silty‐clays showed distinct burrows; fine sands did not. Two types of burrows were produced: fairly straight, vertical burrows from 4 cm below the water‐sediment interface to within 1 cm of the sediment surface, and a horizontal and vertical maze‐like burrow system in the top centimetre of the sediment. Individuals moving on the sediment surface also produced visible trails. When the sediments were dried the burrows were always destroyed; in some cases the surface trails were preserved. We propose that the vertical burrows are escape structures and that the horizontal and vertical maze‐like structures are living burrows.Ichnofossils similar to the escape structures and surface structures have been described. Presence of these ichnofossils indicates a low energy sedimentary environment and a lack of macrofaunal bioturbation. The presence of escape structures may indicate a pulsatory pattern of sedime

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1982.tb00093.x

出版商:Blackwell Publishing Ltd    

年代:1982

数据来源: WILEY