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STEVE KERSHAW 《Geology Today》1988,4(6):202-206
Stromatoporoids are an important group of fossil sponges that form some of the largest reef complexes in the fossil record and are valuable in palaeoecological and facies analysis. They are frequently large and easily studies in the field and in hand specimen. Using their growth forms it is possible to determine some features of sediment deposition rates and changes in their growth direction during life show episodes of turbulence that affected the sea bed. This article intriduces them, and gives practical help to interpret thier growth forms in quaries and cliffs where you may see them on fieldwork. You will realized just how absorbing these fossil sponges can be. 相似文献
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STEVE KERSHAW SYLVIE CRASQUIN MARIE‐BÉATRICE FOREL CARINE RANDON PIERRE‐YVES COLLIN ERDAL KOSUN SYLVAIN RICHOZ AYMON BAUD 《Sedimentology》2011,58(3):739-755
The Permian–Triassic Boundary sequence at Çürük Dag, near Antalya, Turkey, begins with a major erosion surface interpreted as being the Late Permian lowstand, on which lies ca 0·4 m of grainstone/packstone composed of ooids, peloids and bioclasts. Most ooids are superficial coats on fragments of calcite crystals presumed to be eroded from crystal fans which are no longer present. The erosion surface is smooth and shows no evidence of dissolution; the grainstone/packstone contains intraclasts of the underlying wackestone, proving erosion. Next are 15 m of microbialite comprised of interbedded stromatolites, thrombolites, plus beds of planar limestones with small‐scale erosion. The latter comprise a complex interlayering of stromatolitic, thrombolitic and peloidal fabrics and precipitated crystal fans, which form a hybrid of microbialite and inorganic carbonate, together with bioclastic debris and micrite. The Çürük Dag microbialite sequence is repetitious; the lower part is more complex, with abundant stromatolites and hybrid microbialites. Some of the stromatolites are themselves hybrids composed of peloids and crystal fans. In the upper part of the sequence stromatolites are missing and the rock is composed mostly of recrystallized thrombolites that develop upwards from tabular to domal form. The domes form directly below small breaks in microbialite growth where very thin shelly micrites and grainstones/packstones are deposited. Repetition of facies may be controlled by sea‐level change; a deepening‐up model is consistent with the evidence. Stromatolites (with abundant crystal fans) dominate in shallower water, deepening through hybrid microbialite and interlayered sediments to thrombolite, probably no more than a few tens of metres deep, followed by breaks and renewal of microbialite growth. An interpretation of open marine fully oxygenated waters for microbialite growth is consistent with ongoing parallel work that has identified Bairdioid ostracods in the microbialite, a group known to be open marine. However, other researchers have proposed low oxygen conditions for Permian–Triassic boundary facies globally, so work continues to confirm whether the Çürük Dag microbialite grew in dysoxic or normally oxygenated conditions. The principal stimulus for post‐extinction microbialites is likely to be carbonate supersaturation of the oceans. The microbialite sequence is overlain by a further 25 m of grainstone/packstone (without microbialite), followed by Early Triassic shales. Overall, microbialites form a thin aggradational sequence during an overall relative sea‐level rise, consistent with global eustatic rise following the Late Permian lowstand. 相似文献
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ZHAO Wenzhi YANG Xiaoping Steve KERSHAW ZHANG Baomin 《《地质学报》英文版》2006,80(5):684-692
Lower Silurian mud mounds of the Shinuilan Formation, located in the southern Sichuan Basin, China, have developed in open shelf settings in deeper water than shallow-water reef-bearing limestones that occur in the region. An integration of the outcrop, drill data and seismic profiles show that contemporaneous faults have controlled the boundary and distribution of the sedimentary facies of Lower Silurian rocks in the southern Sichuan Basin. Mounds appear to have developed in the topographic lows formed by synsedimentary faulting, on the sheff of the Yangtze Platform. Average mound thickness is 20 m, maximum 35 m. Mounds are composed mainly of micrite, possibly microbially bound, and are overlain by shales. Mound tops are preferentially dolomitized, with the Mg^2＋ source probably from the clay content of the mound-top carbonate. Microfacies analysis and reconstruction of the diagenetic history reveal that the mound tops have higher porosity, and are gas targets; in contrast, mound cores and limbs show pores filled by three generations of calcite cement, and therefore have a low gas potential. 相似文献
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Geology is primarily afield study, and no matter what information is collected from detailed study of hand specimens and thin sections in the laboratory, this must be related to the outcrops in order to develop a geological interpretation of a sequence of rocks. Here we focus on the type of information that should be collected when studying sedimentary rocks in the field, and highlight the main techniques employed. 相似文献
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STEVE KERSHAW 《Geology Today》1993,9(5):187-190
Gotland is an island in the central Baltic, long recognized as the most outstanding outcrop of Silurian shallow-water marine sediments in the world. These represent deposition in tropical environments in an epeiric sea on the small continent of Baltica. The sediments are interbedded limestones and shales with subordinate sandstones and are developed as a carbonate platform on the underlying Precambrian, Cambrian and Ordovician. Particularly spectacular are reef deposits, rich in stromatoporoid sponges, which occur throughout the Gotland sequence. The sequence is almost undisturbed and provides an excellent field laboratoy to study a variety of Silurian facies. 相似文献
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STEVE KERSHAW 《Geology Today》1990,6(2):55-60
Current problems with the ozone layer and rising carbon dioxide levels have highlighted the importance of the atmosphere, with its delicate balance of gases. In fact, the atmosphere has been changing since the Earth formed. From anoxic beginnings to its current oxygenated state, it has influenced depositional processes on the Earth's surface and helped the development of active, oxygen-breathing life forms that culminated in ourselves. Evidence for changes in the atmosphere is found in the sediments accumulated on the Earth's surface, where fossils are also preserved. In this article the major geological aspects of the atmosphere's evolution are considered; in particular, the role of oxygen is aired. 相似文献
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