Esk Trough—Yarraman Block contact,an unconformity: Its nature and implications for regional tectonics

Detailed field study in southeast Queensland has resulted in the interpretation of an unconformity at the base of the Esk Trough sequence at its contact with the Yarraman Block (Maronghi Creek beds and associated intrusions). Previously this contact had been considered to be faulted. The nature of the unconformity is very variable with the Esk Formation resting on freshly eroded surfaces, on mature palaeosols and on an immature palaeosol. Immediately above the unconformity, the Esk Formation variably comprises scree breccia, fluvial conglomerate and arenite, and alluvial fan conglomerate and arenite. North‐northwest‐south‐southeast‐striking faults are associated with the unconformity. Where the unconformity parallels these faults, it retains a relatively constant character, but where it is cut by these faults, it shows greater variability, a relationship interpreted to result from contemporaneous tectonism. The Glen Howden Fault extends into structurally disturbed areas previously described as ‘fractured anticlines’ and ‘complex anticlines’, which are here interpreted as flower structures and associated features. The south‐southeast extension of the Glen Howden Fault strikes obliquely across the Esk Trough to finally pass into the South Moreton Anticline previously interpreted as a positive flower structure, and resolves structural and stratigraphic observations that previously appeared anomalous. Inferred strike‐slip movement in the Esk Trough resulted from Early to Middle Triassic north‐northwest‐south‐southeast oblique transtension followed by Late Triassic transpression, and similar tectonism probably affected adjacent portions of the Yarraman Block.     

Carboniferous palaeogeography of the Yarrol and New England Orogens,eastern Australia

During the Carboniferous Period the Yarrol and New England Orogens comprised an active depositional margin east of cratonised parts of Australia. Patterns of deposition within the orogens were probably controlled by dextral shear systems believed responsible for tectonism and the positions of the various depositional elements (volcanic chain, shelf, slope and basin, pull‐apart troughs and graben), and global changes in sea level. These patterns are illustrated by a series of non‐palin‐spastic palaeogeographic reconstructions.

In the Early Carboniferous, similar patterns of deposition existed within the western volcanic chain, marine shelf, and eastern slope and basin provinces of both orogens. Sediments were deposited in two cycles. They range from volcanic fluvial and marine sandstone to siltstone, mudstone and turbidites. Complex depositional patterns within shelfal regions are shown in detailed palaeogeographic reconstructions.

This uniform pattern changed during the latest Visean and Namurian, with the uplift of the New England Arch, subsidence of a non‐marine graben (Werrie Trough) to the west, and development of a new shelf in the east. The Werrie Trough received volcanics as well as fluvial and glacigene sediments, and the shelf marine sandstone and siltstone. The Yarrol Orogen was unaffected by tectonism but there was a change in provenance.

Late in the Carboniferous the Yarrol Orogen was restructured by the intrusion of granitoids into the former volcanic chain, and development of the Yarrol and North D'Aguilar Troughs as probable pull‐apart basins. In the New England Arch, deformation and metamorphism were followed by intrusion of S‐type granitoids. A comparable episode of deformation and metamorphism affected the southeastern part of the Yarrol Orogen at the end of the Carboniferous Period. This partial cratonisation of the mobile zone was a prelude to widespread basin formation during the Permian Period.     

Editor's note

Postcratonization intrusions in the Yilgarn Block of Western Australia are predominantly dykes with high length/width ratios and sharp contacts with minimal thermal metamorphism of country rock. Dyke frequency and number of relative age relationships increase towards the exposed margins of the Yilgarn Block. Dykes in the northern, southern and western margins of the Yilgarn Block are mafic, ranging from gabbro to magnetite‐rich leucodolerite, and have apparently been intruded over a long time interval in response to periodic reactivation of the tectonically active craton margins. Dykes in the central Yilgarn Block range from porphyritic olivine picrite to magnetite‐rich quartz dolerite and display a spectrum of chemical compositions with an overall trend of tholeiitic iron‐enrichment. The concentration of both Archaean and Proterozoic rocks of high‐Mg nature in the central Yilgarn Block is suggestive of a fundamental control, perhaps in the mantle source area, and also indicates that ultramafic magmas were generated in the area over an extensive time interval. Dykes in the central Yilgarn Block were emplaced in tensional fractures from the Late Archaean until the culmination of a major marginal crust‐forming event at about 2000 Ma. On the basis of the limited data available, the dykes are similar to poststabilization swarms in other cratonic nucleii.     

Provenance comparisons between the Nambucca Block,Eastern Australia and the Torlesse Composite Terrane,New Zealand: connections and implications from detrital zircon age patterns

Detrital zircon U–Pb LAM-ICPMS age patterns for sandstones from the mid-Permian –Triassic part (Rakaia Terrane) of the accretionary wedge forming the Torlesse Composite Terrane in Otago, New Zealand, and from the early Permian Nambucca Block of the New England Orogen, eastern Australia, constrain the development of the early Gondwana margin. In Otago, the Triassic Torlesse samples have a major (64%), younger group of Permian–Early Triassic age components at ca 280, 255 and 240 Ma, and a minor (30%) older age group with a Precambrian–early Paleozoic range (ca 1000, 600 and 500 Ma). In Permian sandstones nearby, the younger, Late Permian age components are diminished (30%) with respect to the older Precambrian–early Paleozoic age group, which now also contains major (50%) and unusual Carboniferous age components at ca 350–330 Ma. Sandstones from the Nambucca Block, an early Permian extensional basin in the southern New England Orogen, follow the Torlesse pattern: the youngest. Early Permian age components are minor (<20%) and the overall age patterns are dominated (40%) by Carboniferous age components (ca 350–320 Ma). These latter zircons are inherited from either the adjacent Devonian–Carboniferous accretionary wedge (e.g. Texas-Woolomin and Coffs Harbour Blocks) or the forearc basin (Tamworth Belt) farther to the west, in which volcaniclastic-dominated sandstone units have very similar pre-Permian (principally Carboniferous) age components. This gradual variation in age patterns from Devonian–late Carboniferous time in Australia to Late Permian–mid-Cretaceous time in New Zealand suggests an evolutionary model for the Eastern Gondwanaland plate margin and the repositioning of its subduction zone. (1) A Devonian to Carboniferous accretionary wedge in the New England Orogen developing at a (present-day) Queensland position until late in the Carboniferous. (2) Early Permian outboard repositioning of the primary, magmatic arc allowing formation of extensional basins throughout the New England Orogen. (3) Early to mid-Permian translocation of the accretionary wedge and more inboard active-margin elements, southwards to their present position. This was accompanied by oroclinal bending which allowed the initiation of a new, late Permian to Early Triassic accretionary wedge (eventually the Torlesse Composite Terrane of New Zealand) in an offshore Queensland position. (4) Jurassic–Cretaceous development of this accretionary wedge offshore, in northern Zealandia, with southwards translation of the various constituent terranes of the Torlesse Composite Terrane to their present New Zealand position.     

扬子西缘元古宙峨边群烂包坪组地层归属及其锆石SHRIMP U-Pb年代学证据

扬子西缘峨边群地层序列一直以来存在争议,其时代归属缺乏地质年代学约束。通过对扬子西缘元古宙峨边群各组、段岩石组合特征、沉积充填序列及其沉积接触关系的综合分析,初步确定了峨边群烂包坪组地层归属。该地层应位于原峨边群上部,代表一次新的构造旋回开始,已不属于峨边群地层范畴。在烂包坪组下部凝灰岩中首次获得的(779.3±15.7) Ma锆石SHRIMP U Pb年龄,进一步证实其形成于新元古代,而非中元古代。(2 737±30) Ma和(2 480±29) Ma的两个单颗粒锆石SHRIMP U Pb年龄,表明扬子西缘峨边-金口河地区可能存在古老的新太古代结晶基底。这些研究成果和新的认识将有助于推动扬子西缘峨边群研究工作的深入开展。     

胜利青东区块沙河街组井壁稳定钻井液技术

青东区块位于济阳坳陷青东凹陷北部断阶带.沙河街组的地质构造复杂,局部断层发育,断层以下地应力作用明显,沙三段泥岩硬而脆,层理性强,从而使钻探中井壁失稳严重,曾经多次发生复杂事故,轻者反复划眼,严重者则卡钻,严重地制约该区块的勘探开发进程.通过优选铝胺抑制、封堵、防塌钻井液体系,并且在施工中采用合理的钻井液液柱压力支撑和联合多元协同抑制-强封堵-合理地控制流变性等技术措施,配合相应的现场维护处理工艺,保证了青东古1井沙河街组的井壁稳定,使该井提前完钻,钻井周期缩短了20％.     

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巴颜喀拉构造带二叠—三叠纪岩相特征及构造演化

特提斯洋的形成与演化问题是青藏高原重大基础地质问题之一, 通过多年的野外观察、分析测试和综合研究, 结合覆盖全区及相邻地区的1∶25万区域地质调查资料及其他前人研究成果, 尤其是对巴颜喀拉构造带二叠—三叠纪地层、岩相特征及构造古地理环境进行了系统研究, 并探讨了其构造演化, 以期对提高青藏高原特提斯洋演化历史和潘吉亚大陆形成特征等方面的研究工作有所禆益.巴颜喀拉构造带未出露前二叠纪地层, 二叠—新近纪地层均有出露, 尤以三叠纪地层广泛出露为其主要特征.其中, 二叠—三叠系主要为海相沉积, 比较连续, 尤以海相三叠系最具特色, 著名的巴颜喀拉山群横贯全区, 分布广泛, 厚度巨大, 侏罗—第四系主要为陆相河湖沉积.二叠系黄羊岭群岩性为碎屑岩、碳酸盐岩夹火山岩, 自下而上表现为浅海相-深海、半深海相-浅海相沉积演化特征; 三叠系主要为巴颜喀拉山群, 岩性单调, 主要为砂泥质类复理石沉积, 局部地区夹钙质及火山物质, 沉积环境总体表现为浅海相—深海、半深海相—滨浅海相—陆相沉积演化序列.二叠—三叠纪构造古地理环境表现为拉张裂陷形成洋(海)盆-汇聚、部分碰撞形成残留洋(海)盆、前陆盆地—拉张裂陷形成洋(海)盆—汇聚、部分碰撞形成残留洋(海)盆、前陆盆地—完全碰撞造山, 海水退出, 进入陆相沉积演化的历史.巴颜喀拉地区是塔里木—中朝陆块与南方大陆(冈瓦纳陆块)之间古特提斯洋域的主洋盆所在地区之一, 与其南部龙木错—双湖洋盆共同构成古特提斯洋域的双洋域.      

华北地块南缘白大山群的层序、时代及地质意义

笔者等近期对华北地块南缘豫皖交界处出露的一套层序连续完整的白大山群进行了剖面测制,在该套地层的碳酸盐岩中发现了中、晚奥陶世牙形刺分子Belodina compressa,Panderodus gracilis,Pseudobelodina dispansa(?)以及苔藓虫、海绵骨针、小壳类、古介形类等化石,填补了该地区一直无可靠化石记录的空白。同时依据地层层序特征、岩性组合、生物群面貌、古地理沉积环境,并结合大量灰岩薄片鉴定结果,将白大山群时代划归早古生代。同时,将华北地块南缘界限向北推移至蒋集-龙潭一线,并认为本区牙形刺生物群应属北方型牙形刺地理分区。     

阿拉善地块北缘朱拉扎嘎毛道晚二叠世火山岩的发现

研究区位于阿拉善地块北部的朱拉扎嘎毛道地区,根据前人研究,该区还未发现火山岩出露。通过对朱拉扎嘎毛道地区的岩石样品野外观察和室内薄片鉴定,确定该区存在球粒流纹岩。该套球粒流纹岩玻璃质含量较高,约75%,玻璃质呈球粒状,有脱玻化现象,其中还包含大量自形程度较高的长石颗粒,可能表明球粒流纹岩在喷发之前捕获有围岩成分。锆石U Pb年龄表明,该套球粒流纹岩的主要喷发时间为晚二叠世。根据LuHf同位素组成,U Pb年龄为240~325 Ma的锆石,εHf(t)全为负值(-39.2~-13.0),且模式年龄远远大于U Pb年龄,表明形成这套球粒流纹岩的岩浆来源于古老地壳再循环。而研究区位于华北板块西部块体的北缘,所以该套球粒流纹岩是由华北板块西部块体北缘的古老地壳物质再循环产生。