摘要:

ABSTRACTDistinctive peritidal tepee antiform structures, buckled margins of saucer‐like megapolygons are common in marine vadose fenestral and pisolitic limestones and/or dolomites of carbonate platform sequences and occur in intertidal and supratidal carbonates ranging in age from Silurian to Holocene. These megapolygons commonly form and are sometimes truncated before the deposition of the next sedimentary layer. The megapolygons result from the expansion of surface sediments by as much as 15%. The expansion is caused by the following continuously repeated sequence of processes: (1) Desiccation and thermal contraction causing small fractures; (2) phases of wetting causing enlargement of fractures; (3) phases of crystallization of calcium carbonate and other minerals causing the enlargement, fill and cementation of the fractures. Precipitation is from brines and meteoric waters; (4) hydration of minerals, thermal expansion, breaking waves and faulting may add to this disruption.The development of the tepee fabric can be traced from an initially cemented subaerial fenestral crust, exhibiting expansion and compressional structures, to a completely disrupted and brecciated sediment riddled by a labyrinth of fractures and solution cavities. These spaces are filled by numerous phases of internal marine and fresh‐water cement and sediment, the latter containing penecontemporaneous or younger marine faunas.Peritidal tepees are useful tools for geologic reconstruction and provide evidence of subaerial exposure; a tropical to subtropical climate; and back‐beach or back‐barrier deposition. Proper identification of tepees is of economic importance, because they provide good early porosity and permeability for petroleum entrapment and a site for mineralization. Aesthetically, tepee rocks are a fine kaleidoscopic decorativ

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1977.tb00254.x

出版商:Blackwell Publishing Ltd    

年代:1977

数据来源: WILEY

摘要:

ABSTRACTThe Messinian Vena del Gesso Basin in the Northern Apennines is filled by very thick (up to 35 m) beds of coarse crystalline gypsum (selenite) associated with thinner carbonate and shaly (euxinic) intercalations. The conventional Usiglio model of salt fractionation does not apply to this evaporitic sequence for the following reasons: carbonate which underlies gypsum is not evaporitic but algal in origin; most gypsum did not precipitate from surface brines but at and below a sediment‐water interface occupied by algal mats; a significant portion (10–80%) of gypsum beds is composed of redeposited selenite which was removed from the margins and transported toward the centre of the basin by slope‐controlled currents and gravity flows (debris flows).We call this process cannibalistic because of its intraformational character (connected with evaporative fall of water level) and volumetric importance.A recurrent vertical pattern of six main facies (euxinic to gypsum fanglo‐merates) is interpreted as a bathymetric, regressive cycle controlled by both sedi‐mentological and tectonic‐eustatic factors. The inferred environmental setting is a residual turbidite trough (Marnoso‐arenacea) evolving abruptly toward lagoonal conditions and filled up to sea level by evaporitic and mechanical (mostly fluvial) processes. Repeated inundations of restricted‐marine water started the depositional cycle thirteen or

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1977.tb00255.x

出版商:Blackwell Publishing Ltd    

年代:1977

数据来源: WILEY

摘要:

ABSTRACTFive coarsening upward shallow marine sandstone sequences (2–10 m thick), are described from the late Precambrian of North Norway, where they occur in a laterally continuous and tectonically undeformed outcrop.The sequences consist of five facies with distinct assemblages of sedimentary structures and palaeocurrent patterns. Each facies is the product of alternate phases of sedimentation during relatively high‐ and low‐energy periods. Facies 1 to 4 are interpreted as representing prograding, subtidal sand bars. Sand bar progradation occurred during the highest energy periods when unidirectional currents flowed to the northwest, depositing trough cross‐bedded sandstones (facies 3 and 4) on the bar crests and flanks, and sheet sandstone beds (facies 1 and 2) in the offshore environments. Weaker northwesterly flowing currents continued during moderate energy fair weather periods. Low energy fair weather periods were dominated by wave processes, which formed largescale, low‐angle, westerly inclined surfaces on the bar flanks (facies 4) and wave rippled sandstone beds (facies 2) and flat laminated siltstone layers (facies 1) in the offshore environments.One sand bar was dissected by channels and infilled by tabular cross‐bedded sandstones (facies 5). Bipolar palaeocurrent evidence, with two modes separated into two laterally equivalent channel systems, suggests deposition by tidal currents in mutually evasive ebb and flood channels.The inferred processes of these sand bars are compared with those associated with modern storm‐generated and tidal current generated linear sand ridges. Both are influenced by the interaction of relatively low and high energy conditions. The presence of the tidal channel facies, however, combined with the inferred strong bottom current regime, is more analogous to a tidal current hyd

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1977.tb00256.x

出版商:Blackwell Publishing Ltd    

年代:1977

数据来源: WILEY

摘要:

ABSTRACTChannels, with maximum depths of over 40 m, have been cut into the top of a coarsening upward sequence, in the Upper Carboniferous of northern England. The channels are interpreted as deep, distributaries cut into delta slope deposits.Giant cross‐bed sets, up to 40 m thick, are the major channel infilling facies. Internal erosion surfaces occur within the giant cross‐beds and some are draped by micaceous, silty sandstone. Previously interpreted as deltaic sedimentation units, the cross‐beds are now thought to have been produced mainly by side attached, alternate bars and the drapes are interpreted as low stage deposits.Cosets of medium scale cross‐beds overlie the giant cross‐beds. These are thought to be deposits of dunes and smaller bars on top of the transverse bars and in the shallower parts of the distributary. The giant cross‐beds are commonly underlain by unlaminated sandstone but may be underlain by undulatory bedding. The latter was apparently produced by ridges, parallel to the current, spaced between 9 and 23 m apart.The north of England appears to have had a major river by present day standards, for at least part of the upper Ca

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1977.tb00257.x

出版商:Blackwell Publishing Ltd    

年代:1977

数据来源: WILEY

摘要:

ABSTRACTPhosphatized limestones from the Pourtales Terrace, Straits of Florida, have undergone fresh‐water diagenesis, as shown by several selective petrologic features such as dissolution of aragonitic skeletal material and preservation and recrystallization of Mg‐calcite. This interpretation is supported by the fact that greater than 94% of the uranium in these samples is in the U(VI) oxidation state.On the deeper Pourtales Escarpment, phosphatized limestones show no alteration features indicative of fresh‐water diagenesis. The uranium in these samples is a mixture of U(IV) and U(VI) with oxidation‐state ratios similar to other sea‐floor phosphorites.Our interpretation of these data is that uranium in the terrace samples was oxidized when that area was emergent. The karsted surface of the terrace, therefore, is a result of subaerial exposure of this portion of the Floridan platform. The escarpment samples were never above sea level, and so more closely retain their original uranium oxidation‐s

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1977.tb00258.x

出版商:Blackwell Publishing Ltd    

年代:1977

数据来源: WILEY