Cryptic sequence boundaries in braided fluvial successions

In braided fluvial deposits, consisting of monotonous successions of sandstone or conglomerate, it may be difficult to distinguish regionally significant bounding surfaces (sequence boundaries) from autogenic channel-scour surfaces. Major surfaces may be characterized by erosional relief and draped by lag deposits, but not all sequence boundaries show these characteristics. Other clues to the presence of a major surface are sharp changes in detrital composition, shifts in regional palaeocurrent trends and evidence of early diagenesis of the sandstones immediately below the sequence boundary. Examples of these attributes of cryptic sequence boundaries are illustrated from three Mesozoic units in the Colorado Plateau area of the United States. In the Chinle Formation (Triassic), near Moab, Utah, angular intraformational unconformities overlie sandstones showing evidence of early diagenesis. In the Castlegate Sandstone (Upper Cretaceous) of east-central Utah, a cryptic sequence boundary can be discriminated from other erosion surfaces by the evidence of detrital petrography and early diagenesis. Palaeocurrent data indicate changes in regional palaeoslope at two sequence boundaries within this unit. Evidence of early diagenesis is also present at a sequence boundary in the Kayenta Formation (Jurassic) of westernmost Colorado.     

Source-area controls on the composition of beach and fluvial sands on the southern side of the Gibraltar Strait and Western Alboran Sea (Flysch Basin, Internal and External, Domains, Northern Rif Chain)

The southern side of Gibraltar and the Western Alboran Sea of the northern Rif coasts and rivers provide a natural field laboratory for sampling modern sand at different scales: small catchment basins (first order) and rivers draining mountain belts (second order). The Rifian chain represents a deformed and uplifted thrust-belt and related forelands composed of Palaeozoic nappes, metamorphic and plutonic basement, and their sedimentary Mesozoic and Cenozoic siliciclastic and carbonate cover, respectively. The present physiography of the Rif Chain is shaped by a rugged mountainous relief drained by different scale catchment basins that supply the nearby coastal and marine deep-sea environments. The analysis of the composition of modern fluvial and beach sands is useful for the interpretation of transported sediments by surface processes from the continent toward coasts and later to deep-water environments.Modern beach and fluvial sands of the southern side of Gibraltar and the Western Alboran Sea display three distinct petrologic littoral provinces, from the east to the west and from the north to the south, respectively, designated as: (i) the Tangier–Bel Younech Littoral Province with 90% of sand derived from erosion of Flysch Nappes (Flysch Basin Domain); (ii) the Bel Younech–Sebta Littoral Province with 64% of sand fed mainly by the metamorphic Units of Upper Sebtides and (iii) the Sebta–Ras Mazari Littoral Province with 74% of sand supplied from the epimetamorphic Palaeozoic Ghomaride Nappes and Alpine cover rather than Mesozoic and Cenozoic sedimentary successions of the “Dorsale Calcaire” Units. Comparison of detrital modes of fluvial and coastal marine environments highlights their dispersal pathways and drainage patterns of actualistic sand petrofacies.     

燕山承德地区晚侏罗世盆地充填记录及对盆缘构造作用的指示

承德地区滦平—承德和大杖子—新城子晚侏罗世沉积盆地带分别与丰宁—隆化逆冲带和尚义—平泉逆冲带相邻。盆地充填的土城子组在垂向上可划分出三个主要由辫状河冲积平原及辫状河三角洲沉积体系组成的盆地相。沉积物岩屑和砾石成分统计表明 ,滦平—承德盆地北缘岩屑岩性相在垂向上表现出三个基底变质岩岩屑向上增多的旋回 ,而滦平—承德盆地南缘及大杖子—新城子盆地则由碳酸盐岩和碎屑岩岩屑表现出三个向上增多的旋回。盆地沉积学特征表明 ,滦平—承德盆地南北边缘分别受控于尚义—平泉北缘和丰宁—隆化逆冲带 ,而大杖子—新城子盆地受控于尚义—平泉南缘逆冲带。如果承德断层发生了长距离逆冲推覆活动 ,其活动时间绝非土城子组同沉积期和之后 ,应该发生于髫髻山组沉积之后和土城子组沉积之前。盆地中三个岩屑岩性相旋回和盆地相旋回共同地反映了盆缘逆冲带三次抬升剥露过程     

新疆东部地区大地构造问题探讨

根据最新的航磁资料,主要探讨了新疆东部基底性质及岩相分区,指出在天山地区除普通存在古生界浅变质基底外,于吐鲁番—哈密盆地中南部深层还存在强磁性基底.库鲁克塔格地区由于元古界多为弱磁性或无磁性岩石,因而显示负磁场面貌,该区与南天山、塔里木盆地北部为区域负背景异常,推测它们为同一深部因素控制的构造区.磁场资料可清楚地圈定出塔里木地台东部边界.文中还对原中天山隆起带出现的升高变化异常提出了解释依据.     

Petrography and temporal changes in petrofacies of deep-marine Ainsa–Jaca basin sandstone systems, Early and Middle Eocene, Spanish Pyrenees

A petrographic study of 157 samples from the Early to Middle Eocene deep-marine sandy systems, Ainsa–Jaca basin, Spanish Pyrenees, shows that each system has a characteristic petrofacies. Three main petrofacies are recognized. Petrofacies 1 sandstones comprise mainly siliciclastic grains (≥80%), subordinate terrigenous carbonate grains and negligible intrabasinal grains. Petrofacies 2 hybrid arenites are characterized by significant amounts (≥10%) of intrabasinal carbonate grains. Petrofacies 3 calcilithites contain relatively abundant (≥10%) extrabasinal carbonate grains. On the basis of these petrofacies, a revised correlation of the sandy systems is proposed between the more proximal Ainsa basin, and the more distal Jaca basin sediments, now separated by the Boltaña anticline, across which it is impossible to actually trace out individual beds or sandstone packages between both basins. The arenite composition in the Ainsa and Jaca basins is interpreted as being controlled mainly by synsedimentary tectonic processes that led to changes in sediment sources during basin evolution.     

An 800 million-year detrital zircon record of continental amalgamation: Tarim basin,NW China

Sedimentary provenance studies based on detrital zircon ages have proved to be a powerful tool for elucidating detailed patterns of erosion and sediment dispersal on and adjacent to the continents. This approach is commonly used to examine processes that occur over discrete intervals of geologic time, but it can also be used to address fundamental questions concerning the longer-term evolution of the continental crust. Central Asia, for example, consists of a Palaeozoic accretionary collage that contrasts with the cratonal cores of continents such as North America, which were fully consolidated during the Proterozoic. Detrital zircon provenance may thus provide important insights on the comparative evolution of these distinctly different styles of continental growth. The Tarim sedimentary basin is interpreted to be underlain by a micro-continental block that accreted to central Asia during the late Palaeozoic. Detrital zircon in pre-Permian strata of the northern Tarim basin includes Precambrian grains derived from Tarim basement, and Palaeozoic grains related to active tectonism along its margins. During this phase of tectonic amalgamation, overall sediment maturity increased up-section, consistent with similar observations from other cratons. However, in Tarim these trends reverse dramatically in Mesozoic and Cenozoic strata deposited after amalgamation of Tarim with central Asia. Palaeozoic zircon grains derived from the Tian Shan become dominant, Precambrian grains drop to trace abundances, and sandstone composition shift abruptly to immature litharenite. Moreover, a progressively increasing temporal gap occurs between zircon age and depositional age, reflecting the near absence of post-amalgamation magmatic or high-grade metamorphic rocks within the Tian Shan. Collectively, these observations support a model of Tarim and other central Asian basins as islands of long-lived tectonic stability, within an orogenic collage that has remained mobile for >250 million years.     

致密砂岩气储层有效性识别和定量评价 ——以鄂尔多斯盆地东南部上古生界山西组一段为例

储层有效性识别和定量评价是制约致密砂岩油气勘探的关键难题。以鄂尔多斯盆地东南部上古生界山西组一段为研究对象,从储层非均质性研究入手,开展砂岩微观—岩芯—测井尺度的细致观测,将含气储层砂岩划分为三种主要的岩石相,其中贫塑性颗粒岩屑石英砂岩构成了物性和含气性相对较好的有效储集岩石。运用薄片和岩芯数据标定不同岩石相的常规测井响应,建立了基于主成分统计分析的有效储集岩石测井识别模型,认识了有效储层的空间分布规律。将储层有效性研究从微观尺度的岩石相描述拓展到宏观尺度的量化表征,实现了致密砂岩气有效储集空间分布的定量评价。     

典型矿集区成矿地质事件研究和填图方法初探

针对中等尺度范围(或中等比例尺)区域成矿潜力和已知矿山的深部及外围资源评价等问题，试图用成矿地质事件方法开展相关研究。成矿地质事件评价方法，就是在恢复矿床形成过程并建立成矿模式的基础上，通过对矿源岩系的构造变形岩相形迹追索来恢复与成矿有重要意义的地质事件及其演化过程，研究在这一系列地质事件中成矿物质的分布规律和逐步富集成矿的标志及程度，并用以进一步探索、指导区域资源潜力评价和填图找矿的方法。应用这一思路对南岭中段锡多金属矿和胶东矿集区金矿进行了区域成矿地质事件研究和地质填(编)图方法的初步探讨。     

Petrotectonic setting of the provenance of Lower Siwalik sandstones of the Himalayan foreland basin,southeastern Kumaun Himalaya,India

Detailed petrography and modal analysis of 35 sandstone thin sections was carried out to determine petrotectonic setting of the provenance of the Lower Siwalik molasse of southeastern Kumaun Himalaya. The sandstones are fine‐ to coarse‐grained (0.14–0.63 mm), poorly‐ to moderately‐sorted and comprise lithic arenites, sublithic arenites and lithic greywackes. The sandstones invariably belong to the quartzolithic QtFL (Qt, total quartz; F, feldspar; L, lithic grains) and QmFLt (Qm, monocrystalline quartz; Lt, lithic grains plus polycrystalline quartz) petrofacies, and indicate their derivation from a quartzose‐ and transitional‐recycled orogen provenance under sub‐humid climatic conditions. The framework composition of the sandstones comprises abundant monocrystalline and polycrystalline quartz and low‐ to high‐grade metamorphic rock fragments, along with subordinate feldspar, characterized by low ratios of plagioclase to total feldspar, and accessory minerals. The framework composition and petrofacies characters of these texturally submature sandstones suggest their derivation mainly from the nearby located Great Himalaya terrane and subordinately from the Tethys and Lesser Himalayan terranes. A comparison of the data presented here with the previous similar data from Lower Siwalik of northwestern Pakistan, northwestern India, south‐central Kumaun, western Nepal and southeastern Nepal reveals that like the Lower Siwalik rivers in other sections, the Lower Siwalik rivers of the southeastern Kumaun too drained large parts of the Great Himalayan terrane and some parts of the Tethys and Lesser Himalayan terranes.     

乌尔逊凹陷沉积成岩体系与油气分布

乌尔逊凹陷地处海拉尔盆地中央，是一个面积较大、勘探程度较高的箕式凹陷。其主要储集层为基岩风化壳、下白垩统的铜钵庙组、南屯组及大磨拐河组。已在其中的两个弧形构造带20余口井获商业油气流。受西缘主断裂控制，该凹陷内形成了4类构造体系，同时也控制形成了西陡东缓的古地形。南屯组、大磨拐河组的各类沉积相以不完整的环带状展布，由此划分出7类构造岩相带，即乌西断阶扇三角洲构造岩相带、乌中深洼湖相构造岩相带，吞－巴中央隆起三角洲构造岩相带、乌东斜坡辫状三角洲构造岩相带、苏仁诺尔弧形断隆近岩水下扇－浊积扇构造岩相带、黄旗庙鼻状隆起扇三角洲构造岩相带、巴彦塔拉弧形断垒冲积扇构造岩相带。分析了南屯组、大磨拐河组的储层特征，并确定5种成岩相，即压实成岩相、碳酸岩交代及胶结成岩相、溶蚀溶解成岩相、自生矿物充填成岩相、硅质沉淀成岩相等。成岩相的差异导致各构造岩相带内的油气藏类型和含油规模的不同。溶蚀溶解成岩相孔渗性最好，苏仁格尔、巴彦塔拉构造岩相带以这种成岩相为主，并已发现一定规模的油气储量。而吞－巴、黄旗庙构造岩相带则是下一步突破的方向。