川西北地区中三叠统顶部溶蚀型喀斯特特征及地质意义^*     

吴冰  金鑫  时志强  Nereo Preto  杜怡星 《古地理学报》2018,20(1):133-146

中三叠统顶部溶蚀型喀斯特在什邡金河剖面及四川盆地大部分地区已有报道。在近期的野外工作中,在川西北地区绵竹汉旺观音崖、江油黄莲桥和江油马鞍塘剖面天井山组近顶部、顶部也识别出这一古特提斯面,其形成与强烈的构造活动或全球海平面下降造成的岩层长时间的暴露有关。其表现形式主要为: (1)暴露溶蚀间断沉积面;(2)溶沟、溶缝及其内的碳酸盐、陆源碎屑充填物;(3)暴露期黏土层;(4)喀斯特角砾。其中暴露溶蚀间断沉积面上的沉积构造以鸟眼、层状孔洞构造最为典型,在绵竹汉旺观音崖剖面最为发育,孔、洞充填物主要为亮晶方解石和白云石;溶沟、溶缝及其内的碳酸盐、陆源碎屑充填物在江油马鞍塘、汉旺观音崖、什邡金河等剖面均有发育,这些溶缝、溶沟一般近于竖直发育,宽度及深度不一,被碳酸盐角砾、亮晶方解石、白云石或陆源黏土等所充填;暴露期黏土层主要见于江油黄莲桥剖面,共发育2套,厚5~20 cm,与铁质矿物共生,其上下发育深灰色潟湖沉积;喀斯特角砾主要见于汉旺观音崖剖面,喀斯特角砾最大可达15×20 cm大小,呈棱角状充填于富含泥质和有机质的暗色碳酸盐基质中。天井山组近顶部溶蚀型喀斯特的发现可为区域地层等时对比提供依据,与其有关的古岩溶作用可在天井山组(或雷口坡组)顶部碳酸盐岩中形成以溶蚀孔、洞、缝为主要储集空间的古岩溶型储集层。  相似文献   

Primary dolomite in the Late Triassic Travenanzes Formation,Dolomites, Northern Italy: Facies control and possible bacterial influence          下载免费PDF全文

Nereo Preto  Anna Breda  Jacopo Dal Corso  Christoph Spötl  Federico Zorzi  Silvia Frisia 《Sedimentology》2015,62(3):697-716

In the late Carnian (Late Triassic), a carbonate‐clastic depositional system including a distal alluvial plain, flood basin and sabkha, tidal flat and shallow carbonate lagoon was established in the Dolomites (Northern Italy). The flood basin was a muddy supratidal environment where marine carbonates and continental siliciclastics interfingered. A dolomite phase made of sub‐micrometre euhedral crystals with a mosaic microstructure of nanometre‐scale domains was identified in stromatolitic laminae of the flood basin embedded in clay. This dolomite is interpreted here as primary and has a nearly stoichiometric composition, as opposed to younger early diagenetic (not primary) dolomite phases, which are commonly calcian. This primary dolomite was shielded from later diagenetic transformation by the clay. The stable isotopic composition of dolomite was analyzed along a depositional transect. The δ¹³C values range between ca ?6‰ and +4‰, with the most ¹³C‐depleted values in dolomites of the distal alluvial plain and flood basin, and the most ¹³C‐enriched in dolomites of the tidal flat and lagoon. Uniform δ¹⁸O values ranging between 0‰ and +3‰ were found in all sedimentary facies. It is hypothesized that the primary dolomite with mosaic microstructure nucleated on extracellular polymeric substances secreted by sulphate reducing bacteria. A multi‐step process involving sabkha and reflux dolomitization led to partial replacement and overgrowth of the primary dolomite, but replacement and overgrowth were facies‐dependent. Dolomites of the landward, clay‐rich portion of the sedimentary system were only moderately overgrown during late dolomitization steps, and partly retain an isotopic signature consistent with bacterial sulphate reduction with δ¹³C as low as ?6‰. In contrast, dolomites of the marine, clay‐free part of the system were probably transformed through sabkha and reflux diagenetic processes into calcian varieties, and exhibit δ¹³C values of ca +3‰. Major shifts of δ¹³C values strictly follow the lateral migration of facies and thus mark transgressions and regressions.  相似文献   

Origin of facies zonation in microbial carbonate platform slopes: Clues from trace element and stable isotope geochemistry (Middle Triassic,Dolomites, Italy)     

Nereo Preto  Andreas Klügel  Tobias Himmler  Marco Franceschi 《Sedimentology》2019,66(1):81-101

Limestones containing radiaxial fibrous cements were sampled along the southern slope of the late Anisian (Middle Triassic) Latemar carbonate platform in the Dolomites, northern Italy. The Latemar upper slopes comprise massive microbial boundstone, whereas lower slopes are made of clinostratified grainstone, rudstone and breccia. Samples are representative of a seawater column from near sea‐level to an aphotic zone at about 500 m water depth. Radiaxial fibrous cements were analyzed for carbon (δ¹³C) and oxygen (δ¹⁸O) stable isotopic composition, as well as major and trace element content, to shed light on the origin of the slope facies zonation. The δ¹³C vary between 1·7‰ and 2·3‰ (Vienna Pee‐Dee Belemnite), with lowest values at palaeo‐water depths between 70 m and 300 m. Radiaxial fibrous cements yielded seawater‐like rare earth element patterns with light rare earth element depletion (Nd_SN/Yb_SN ≈ 0·4), superchondritic yttrium/holmium ratios (≈55) and negative cerium anomalies. Cadmium reaches maximum values of ca 0·5 to 0·7 μg/g at palaeo‐water depths between 70 m and 300 m; barium contents (0·8 to 1·8 μg/g) increase linearly with depth. The downslope patterns of δ¹³C and cadmium suggest increased nutrient and organic matter contents at depths between ca 70 m and 300 m and point to an active biological pump. The peak in cadmium and the minimum of δ¹³C mark a zone of maximum organic matter respiration and high nutrient and organic matter availability. The base of this zone at ca 300 m depth corresponds with the transition from massive microbial boundstone to clinostratified grainstone, rudstone and breccia. The microbial boundstone facies apparently formed only in seawater enriched in organic matter, possibly because this organic matter sustained benthic microbial communities at Latemar. The base of slope microbialites on high‐relief microbial carbonate platforms may be a proxy for the depth to maximum respiration zones of Palaeozoic and Mesozoic periplatform basins.  相似文献   

Discrimination between marls and limestones using intensity data from terrestrial laser scanner     

Marco Franceschi  Giordano Teza  Nereo Preto  Arianna Pesci  Antonio Galgaro  Stefano Girardi 《ISPRS Journal of Photogrammetry and Remote Sensing》2009,64(6):522-528

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