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doi: 10.1111/j.1365-3091.1969.tb00873.xpmid: N/A
SUMMARY Study of 176 ft. (53.7 m) of core from the Arab/Darb Formation of the Umm Shaif Field, Abu Dhabi Marine Areas, has revealed a sequence of sediments which can be related to nine distinct cycles of sabkha formation. The sabkha cycle consists of a basal algal grainstone/boundstone (which is interpreted as a shoal) passing upwards through lagoonal dolomite, intertidal algal mat and into a final supratidal development of nodular anhydrite and associated dolomite. Dolomiti‐sation and the formation of nodular anhydrite were early‐stage products of diagenesis. Poikilotopic anhydrite is common in the lagoonal dolomites but it is thought that this was not formed until the sediment was completely lithified. Stylolites, although of small amplitude, are more common in dolomite than in limestone; a generalisation which is at variance with earlier observations.
doi: 10.1111/j.1365-3091.1969.tb00874.xpmid: N/A
SUMMARY Along the eastern coast of the Gez Gölö, a salt lake formerly belonging to the larger “Paleo‐Tuz Gölö” in central Anatolia, about 6.5 m of lake sediments of probably Pleistocene age were studied. The sediment series consists of fine‐grained unconsolidated dolomite muds representing the basin sedimentation of the old lake. Nine lithified beds (oomicrites, intramicrites, intraoomicrites, intrabiomi‐crites and dololutites with smaller amounts of allochems) are intercalated. These beds were deposited in very shallow water in the littoral zone of the paleo‐lake. There is direct evidence that the beds were occasionally exposed to the air after deposition. This led to desiccation (with mud cracks, breaking up of intraclasts) and subaerial cementation due to the influx of fresh water (rain water, springs). These now lithified carbonate sedimentary rocks were submerged and subsequently covered with mud. During this sub‐aquatic phase the rock beds as well as the unconsolidated muds were dolomitized by an extremely high Mg/Caratio of the lake water which in the present Tuz Gölü amounts to about 150! It cannot be excluded, however, that dolomitization already took place or was initiated during the subaerial phase (“supratidal dolomite”).
doi: 10.1111/j.1365-3091.1969.tb00875.xpmid: N/A
SUMMARY Bored surfaces in Middle Jurassic limestones in northeastern France indicate syn‐sedimentary lithification. The sedimentary structures and textures, and age relationships between the bored carbonates and the argillaceous sediments above them suggest that the lithification has occurred in both submarine and intertidal environments. The diagenetic fabrics which have resulted from this early marine lithification include three types of calcite druse, echinoderm overgrowths, and microcrystalline cements. Most of these cements are comparable with those forming today in inter‐ and subtidal environments of the Persian Gulf. The localization of bored surfaces (“hard grounds”) at the tops of regressive carbonate sequences is interpreted as being the result of slow carbonate sedimentation and lithification of the Jurassic sea‐floor prior to the onset of argillaceous colder, or deeper‐water sedimentation.
doi: 10.1111/j.1365-3091.1969.tb00876.xpmid: N/A
SUMMARY Detailed documentation and interpretation is given of a thin band of limestone nodules in the Lower Lias of Dorset, which has been subjected to early lithification, exhumation on the sea bed and incrustation and boring by organisms in a more turbulent and shallow‐water regime than that of deposition. Final burial has been followed by extensive pyritization. The stratigraphical gap during which these events took place is less than two ammonite zones.
doi: 10.1111/j.1365-3091.1969.tb00877.xpmid: N/A
SUMMARY Absence of compaction, intraformational breccias, resedimention, internal sediments and synsedimentary hardgrounds indicate early lithification of finegrained carbonate rocks. One of the factors controlling early lithification is the purity of lime mud. Less than 2% of insoluble residue (especially clay minerals) favours cementation and recrystallisation before further sediment accumulation causes compaction. Thus, early lithification is terminated in or near the environment of sedimentation. “Electrodiagenesis” is considered to be a possible mechanism for cementation.
doi: 10.1111/j.1365-3091.1969.tb00878.xpmid: N/A
SUMMARY Dolomitic intraclastic sediments intercalated in a cyclic limestone‐dolostone sequence of the Wettersteinkalk represent a rock type with reworking fabrics demonstrating early lithification and dolomitization. The extreme flatness and the angularity of the pebble‐sized doloclasts, the lack in plastically deformed dolomitic fragments, and clasts of previously cementated arenites and oncolites testify to a pre‐reworking consolidation which, besides biogenic stabilization and dehydration, requires an early cementation. In the different types of doloclasts cementation both by dolomite and by calcite has been confirmed by stained peels and thin‐sections, electron‐microphotographs, and electron‐microprobe analysis. The prevailing rock type, a very poorly sorted mixture of doloclasts and minor amounts of rounded limeclasts with a pelletal calcite mud matrix is interpreted as storm sediment formed by the introduction of subtidal lime muds to dolomite incrusted supratidal flats.
doi: 10.1111/j.1365-3091.1969.tb00879.xpmid: N/A
SUMMARY Fibrous calcite is the main cement in the voids and cavities of Devonian fore‐reef limestones in Germany. Cementation occurs in some cases simultaneously with encrustation by blue‐green algae as well as internal deposition of calcarenites and calcilutites. These observations lead to the conclusion that the fore‐reef voids were exposed to light and were still in free communication with the sea, when the cementation began. Furthermore, redeposited fragments of former cavity fillings indicate that the formation of cavity‐filling fibrous calcium carbonate cement was already completed before the sea‐ward erosion took place. It is believed that the precipitation of fibrous calcium carbonate in most of the Devonian fore‐reef limestones, which show no signs of dissolution, occurred in a shallow subtidal environment. Only the voids showing larger solution phenomena were probably exposed to fresh water in the inter‐ to supratidal zone.
MILLER, DONALD G.; RICHARDS, ADRIAN F.
doi: 10.1111/j.1365-3091.1969.tb00880.xpmid: N/A
SUMMARY A comparison is made between the void ratio and pressure relationships resulting from a laboratory consolidation test and a sedimentation‐compression computation on a short core of calcareous mud or ooze of low plasticity. Geo‐technical measurements of grain size, bulk density, Atterberg limits, water content, vane shear strength, pore‐water salinity, and carbonate content are graphically related to depth in the core. Results of the laboratory consolidation test on this material differ markedly from the in‐place relationship between void ratio, or water content, and the effective overburden pressure, or burial depth, shown by the sedimentation‐compression curve. The previous maximum consolidation pressure, based on laboratory consolidation test data, is about 60 times greater than the computed in‐place effective overburden pressure. An explanation for this difference would include the different magnitudes of time available for consolidation, cementation occurring in‐place, and orientation of the constituents. It is suggested that results of the consolidation test on carbonate muds or oozes should be interpreted with caution for geological and engineering purposes.
doi: 10.1111/j.1365-3091.1969.tb00881.xpmid: N/A
SUMMARY The present paper describes the first stages of echinoderm overgrowth by calcite, observed in partially cemented limestones from boreholes in the floor of the Persian Gulf. The overgrowths which develop as spires in the direction of the c‐axis of calcite show all essential features that were inferred in a previous paper for the early stages in development of overgrowths in fully cemented Jurassic limestones.
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