Fossil footprints from the Dakota Group (Cretaceous) John Martin Reservoir, Bent County, Colorado: New insights into the paleoecology of the Dinosaur Freeway

The John Martin Reservoir tracksites from the Dakota Group of Bent County, in southeastern Colorado form part of the mid Cretaceous Dinosaur Freeway characterized by abundant ornithopod footprints (Caririchnium). Over 350 tracks (331 Caririchnium, 1 Magnoavipes, 22 crocodiles, and a few pterosaurs) were discovered at 10 new tracksites. All tracks were found as natural casts, including Caririchnium trackways that were still in-situ, parallel and regularly spaced, suggesting gregarious behavior. Most crocodile tracks are swimming tracks that consist of three or four scratch marks also with parallel orientations in many cases. A few pterosaur tracks consist of pes footprints and scratch marks suggesting swimming or floating activity in shallow water. This is the first pterosaur evidence from the Dakota Group. The Caririchnium size structure from John Martin Reservoir is consistent with the size structure tendency of the whole Dinosaur Freeway, which shows larger track size in the north. This tendency could be interpreted as evidence for more than one ornithopod species spread out across the Dinosaur Freeway. The alternative, that there was one ornithopod species that migrated north and south seasonally is less likely. The John Martin paleoecosystem is interpreted from the track evidence as a well-vegetated coastal plain environment with many ornithopods and a few theropods on land, crocodiles in the water, and pterosaurs in the skies and on the water.     

略谈藏东吉塘群

[摘 要]以往的研究工作将藏东吉塘群分两个组:上部以片岩、变质砂砾岩为主的地层名酉西组;下部以片岩、片麻岩、混合岩为主的地层名恩达组,时代均属前震旦纪。近几年,开展1:25 万编图项目的研究,恩达组可细分为两个岩性段:混合岩段和以片岩、片麻岩、变粒岩、斜长角闪岩及大理岩共同组成的深变质岩段。酉西组中获得新的同位素年龄:240±12Ma( 锆石U-Pb)、226±2Ma( 白云母Ar-Ar),时代可能为古生代,更名为酉西岩群。恩达组的混合岩中新获得锆石SHRIMP U-Pb 同位素年龄254±8Ma、232±9Ma、227±1.8Ma,原1:20 万类乌齐、拉多幅曾于相当恩达组的花岗片麻岩中获得锆石U-Pb年龄253±9Ma、195Ma,这一岩段的混合岩可能是华力西至燕山期变形变质的花岗岩侵入体,应从吉塘群中分离出去。原恩达组中除混合岩外的地层更名为吉塘岩群,时代属古中元古代。     

SHRIMP U-Pb zircon geochronology and Nd-Sr isotopic study of the Mamianshan Group: implications for the Neoproterozoic tectonic development of southeast China

Northwestern Fujian contains abundant well-studied Precambrian basement, and was a composite terrane in Cathaysia during the Neoproterozoic; however, its magmatic activity, petrogenesis, and tectonic evolution remain controversial. This article focuses on the geochronology and geochemistry of the Neoproterozoic Group in order to resolve the above problems. We provide new SHRIMP U-Pb zircon dating for the Mamianshan Group: 851.9 ± 9.2 to 825.5 ± 9.8 Ma for the Longbeixi Formation, 796.5 ± 9.3 Ma for the Dongyan Formation, and 756.2 ± 7.2 Ma for the Daling Formation. These ages document the existence of Neoproterozoic magmatism in the northwestern Cathaysia Block. Dongyan Nd-Sr isotopic data show that mafic amphibolite schists, mafic greenschists, and quartzofeldspathic schists were derived from a more depleted mantle (initial ε_Nd ? +5.5 and ⁸⁷Sr/⁸⁶Sr ratio 0.703409643), a mixture of depleted mantle and crustal components (initial ε_Nd ? ?1 and ⁸⁷Sr/⁸⁶Sr ratio 0.702045282–0.704147714), and late Palaeoproterozoic continental crustal materials (initial ε_Nd < ?1 and ⁸⁷Sr/⁸⁶Sr ratio 0.71083603), respectively. These new data, together with previous studies, suggest a bi-subduction-collision orogenic model for the Neoproterozoic evolution of the Yangtze and Cathaysia blocks. Our plate tectonic scenario involves earlier NW-dipping subduction during 1.0 Ga–860 Ma along the southeastern margin of the Yangtze Block and later NW-dipping subduction near the northwestern margin of the Cathaysia Block starting at ca. 850 Ma. The 796.5 ± 9.3 Ma age of the volcanic Dongyan Formation suggests that the final assembly of the Yangtze and Cathaysia blocks probably occurred after ca. 800 Ma. The 756.2 ± 7.2 Ma age of the Daling Formation indicates that post-orogenic extensional magmatism took place after 800 Ma along the northwestern margin of Cathaysia.     

西昆仑甜水海-麻扎地块上石炭统恰提尔群B组沉积环境分析

本文在详细描述甜水海-麻扎地块上石炭统恰提尔群B组地层剖面基础上,结合野外沉积构造、室内薄片鉴定、粒度分析成果,讨论了该组的岩石组合特征和沉积环境。其岩性组合为细砂岩、粉砂岩、泥岩、页岩等碎屑岩类,为一套湖陆过渡的三角洲沉积,可进一步划分为三角洲平原、三角洲前缘与前三角洲3个沉积亚相,其物源区为再循环造山带、混合物源区。     

欧龙布鲁克地块西段达肯大坂岩群物质组成及变形特征研究

达肯大坂岩群是欧龙布鲁克地块结晶基底的重要组成部分,在形成过程中保存了丰富的物质记录和演化信息,是研究欧龙布鲁克地块乃至我国西部古老地壳及构造演化的窗口。通过对达肯大坂岩群详细的物质组成及构造剖面研究,根据岩石组合及变质-变形特征,将其进一步细化为混合片麻岩岩段、(混合岩化)流变片麻岩岩段、条带状片麻岩岩段、片岩岩段和大理岩岩段,野外调查表明,达肯大坂岩群中发育多期构造变形作用,根据野外变形特征及各期变形的叠加改造关系,初步建立了达肯大坂岩群相对变形序列,为区域构造过程的探讨与对比提供了新的基础资料。     

Geology of coal bearing Palaeogene sediments,onshore Torquay Basin,Victoria

New mapping at Anglesea coal mine, and coal resource and deep groundwater drilling have provided new perspectives on the economically important Eastern View Group coal bearing sedimentary succession in the onshore Torquay Basin. In the Anglesea Syncline, the upper 35 m thick brown coal seam of the Eastern View Group is overlain by a low angle unconformity. Units overlying the coal seams include high energy, cross cutting sand channels of the Boonah Formation and lower energy channel and interchannel systems of the overlying Salt Creek and Anglesea Formations. The mine section can be correlated from borehole data with the Eastern View Group and Demons Bluff Group exposed in coastal cliff sections along the adjacent Anglesea to Torquay coast. Recently drilled coal and groundwater exploration bores provide new data on the extent of the coal measures in the Anglesea area, and details of the underlying Tertiary succession that include typical Otway Basin units such as the Pember Mudstone and Pebble Point Formations. The stratigraphy below the coal measures suggests that the Otway Ranges were not present during Palaeogene times. The rank of the brown coals on and around the Otway Ranges is higher than any other Tertiary coals in onshore Victoria, and they preserve similar patterns of rank distribution to the high rank black coals in the underlying Lower Cretaceous Otway Group. Evidence for large overburden thicknesses is lacking, and the high ranks may have been augmented by higher than normal geothermal gradients in the Early Tertiary. Comparisons between the observed depositional cycles, sequence stratigraphic cycles, and worldwide coastal onlap curves suggest that the observed disconformity boundaries are sequence boundaries that provide a chronostratigraphic framework. Sequences present may include TB4.1–4.5 in the overburden units, and TB 2.4–3.6 in the coal bearing interval.     

Soldiers Cap Group iron‐formations,Mt Isa Inlier,Australia, as windows into the hydrothermal evolution of a base‐metal‐bearing Proterozoic rift basin

The Proterozoic Soldiers Cap Group, a product of two major magmatic rift phases separated by clastic sediment deposition, hosts mineralised (e.g. Pegmont Broken Hill‐type deposit) and barren iron oxide‐rich units at three main stratigraphic levels. Evaluation of detailed geological and geochemical features was carried out for one lens of an apatite‐garnet‐rich, laterally extensive (1.9 km) example, the Weatherly Creek iron‐formation, and it was placed in the context of reconnaissance studies of other similar units in the area. Chemical similarities with iron‐formations associated with Broken Hill‐type Pb–Zn deposit iron‐formations are demonstrated here. Concordant contact relationships, mineralogy, geochemical patterns and pre‐deformational alteration all indicate that the Soldiers Cap Group iron‐formations are mainly hydrothermal chemical sediments. Chondrite normalised REE patterns display positive Eu and negative Ce anomalisms, are consistent with components of both high‐temperature, reduced, hydrothermal fluid (≥250°C) and cool oxidised seawater. Major element data suggest a largely mafic provenance for montmorillonitic clays and other detritus during chemical sedimentation, consistent with westward erosion of Cover Sequence 2 volcanic rocks, rather than local mafic sources. Ni enrichment is most consistent with hydrogenous uptake by Mn‐oxides or carbonates. Temperatures inferred from REE data indicate that although they are not strongly enriched, base metals such as Pb and Zn are likely to have been transported and deposited prior to or following iron‐formation deposition. Most chemical sedimentation pre‐dated emplacement of the major mafic igneous sill complexes present in the upper part of the basin. Heating of deep basinal brines in a regional‐scale aquifer by deep‐seated mafic magma chambers is inferred to have driven development of hydrothermal fluids. Three major episodes of extension exhausted this aquifer, but were succeeded by a final climactic extensional phase, which produced widespread voluminous mafic volcanism. The lateral extent of the iron‐formations requires a depositional setting such as a sea‐floor metalliferous sediment blanket or series of brine pools, with iron‐formation deposition likely confined to much smaller fault‐fed areas surrounded by Fe–Mn–P–anomalous sediments. These relationships indicate that in such settings, major sulfide deposits and their associated chemical sediment marker horizons need not overlie major igneous sequences. Rather, the timing of expulsion of hydrothermal fluid reflects the interplay between deep‐seated heating, extension and magmatism.     

Discussion and Reply Lithostratigraphic revision and correlation of the Oligo‐Miocene Murray Supergroup,western Murray Basin,South Australia

Sequence‐stratigraphic interpretations of the 4200 m‐thick Palaeoproterozoic (1700–1650 Ma) Mt Isa Group and underlying Surprise Creek Formation identify three unconformity‐bounded packages termed the Prize, Gun and Loretta Supersequences. Siliciclastic rocks of the Surprise Creek Formation and Warrina Park Quartzite comprise the Prize Supersequence. Rapid facies changes from proximal, conglomeratic fluvial packages to distal, fine‐grained and deep‐water, rhythmites characterise this supersequence. Conglomeratic intervals in the Mt Isa area reflect syndepositional movement along basin‐margin faults during the period of supersequence initiation. A major unconformity, which extends over a period of about 25 million years, separates the Gun and Prize Supersequences. In the Leichhardt River Fault Trough uplift and incision of Prize sedimentary rocks coincided with emplacement of the Sybella Granite (1671±8 Ma) and Carters Bore Rhyolite (1678±2 Ma) and the removal of an unknown thickness of Prize Supersequence section. Deep‐water, turbiditic rhythmites of the Mt Isa Group dominated the Gun and Loretta Supersequences. Tempestites are present over discrete intervals and represent times of relative shallowing. High accommodation and sedimentation rates at the base of the Gun Supersequence resulted in the deposition of transgressive nearshore facies (uppermost Warrina Park Quartzite) overlain by a thick interval of deep‐water, siltstone‐mudstone rhythmites of the Moondarra Siltstone and Breakaway Shale. With declining rates of siliciclastic sedimentation and shallowing of the succession, calcareous sediments of the Native Bee Siltstone prograded over the deeper water deposits. Two third‐order sequences, Gun 1 and 2, characterise these lower parts of the Gun Supersequence. An increase in accommodation rates near the top of the Native Bee Siltstone in Gun 3 time, resulted in a return to deep‐water sedimentation with deposition of dolomitic rhythmites of the Urquhart Shale and Spear Siltstone. The Pb–Zn–Ag ore‐hosting interval of the Urquhart Shale is interpreted to occur in progradational highstand deposits of the Gun 3 Sequence. In the Leichhardt River Fault Trough the Loretta Supersequence boundary forms a correlative conformity. Coarser grained and thicker bedded sediments of the Kennedy Siltstone comprise lowstand deposits at the base of this cycle. These sediments fine up into the transgressive, deep‐water, siliciclastic facies of the Magazine Shale, which in turn are truncated against the Mt Isa Fault.     

Pre‐Tabberabberan deformation in eastern Tasmania: A southern extension of the Benambran Orogeny

Recumbent folding in eastern Tasmania affected turbidites containing Lower to Middle Ordovician (Bendigonian Be1 to Darriwilian Da3) fossils, but not stratigraphically overlying turbidites containing Silurian (Ludlow) graptolites, and is of a timing consistent with Ordovician to Silurian Benambran orogenesis on the Australian mainland. Two subsequent phases of upright folding post‐date deposition of turbidites containing Devonian plant fossils but pre‐date intrusion of Middle Devonian granitoids, and are of Tabberabberan age. A closely spaced disjunctive cleavage (S₂), associated with the first phase of Tabberabberan folding, everywhere cuts a slaty cleavage (S₁) associated with the earlier formed recumbent folds. However, refolding associated with development of S₂ is not always clear in outcrop and it is proposed that coincident tectonic vergence between the two events has resulted in reactivation of recumbent D₁ structures during the D₂ event. The transition to rocks not affected by recumbent folding coincides with a marked change in sedimentology from shale‐ to sand‐dominated successions. This contact does not outcrop but, from seismic data, appears to dip moderately to the east, and can only be explained as an unconformity. The current grouping of all pre‐Middle Devonian turbidites in eastern Tasmania into the one Mathinna Group is misleading in that the turbidite sequence can be subdivided into two distinct sedimentary packages separated by an orogenic event. It is proposed that the Mathinna Group be given supergroup status and existing formations placed into two new groups: an older Early to Middle Ordovician Tippogoree Group and a younger Silurian to Devonian Panama Group.     

Amelia Creek: a Proterozoic impact structure in the Davenport Ranges,Northern Territory

The Amelia Creek impact structure is located in Australia's Northern Territory in folded Palaeoproterozoic strata of the Davenport Ranges (20°51'S, 134°53′E). An impact origin is confirmed by presence of unequivocal shatter cones with apices that point upwards, and by planar microstructures in quartz grains from target sandstones of the Hatches Creek Group. Aeromagnetic, advanced spaceborne thermal emission and reflection radiometer (ASTER), and X-band synthetic aperture radar (X-SAR) images show an area of anomalous deformation in which smooth regional trends are disrupted by arcuate features at a 10 km radius to the north and south of the shock-metamorphosed rocks. However, no arcuate forms are apparent to the east and west of these shocked rocks, and instead, large south-southwest-trending faults are present about 6 km away on both sides. Despite pervasive shatter coning, typical of the central region of complex impact structures, no structural uplift is apparent, but instead the shocked rocks lie at the southern toe of a north-northeast-trending syncline. These shatter cones overprint and post-date the Palaeoproterozoic regional deformation, and thus, the impact structure has not been refolded and its abnormal form is likely due to pre-existing structure in the target rocks and/or an oblique impact. Small pockets of undeformed Late Neoproterozoic and Middle Cambrian strata are exposed in palaeovalleys in the central region of the structure, constraining the time of the impact to the Proterozoic.