Middle Cambrian Research Articles

Salt tectonics along the High Zagros Fault in Iran, faulting through welded salt walls

In salt-bearing foreland fold-thrust belts, precursor salt structures affect deformation and accommodate fault zones during orogenic shortening. The complex structure and the halokinetic history of these belts, including the spatiotemporal relationship between diapirism and thrust faulting, have yet to be fully understood. The well-exposed Ediacaran–Early Cambrian Hormuz salt diapirs and the Cambrian–Pliocene overburden stratigraphic sequences along the High Zagros Fault (HZF) in Iran are interesting examples in this regard. The detailed field investigation of the diapirs, the halokinetic deformations, and the thickness variations of the sedimentary units toward the flanks of eight salt structures along the fault show that the first movement of the Hormuz salt could be as old as the Middle Cambrian, occurring long before the initiation of regional folding in the early Miocene. This marks the first field-based and the oldest proposed time for the onset of Hormuz salt diapirism in the Zagros fold-thrust belt. Our field and remote-sensing mapping show the presence of precursor linear salt structures like salt walls. The Hormuz salt diapirs likely began to rise as elongated salt walls above extensional basement faults coeval with the Neo-Tethyan rifting in Early Mesozoic. The squeezing of the salt walls during Zagros shortening from Miocene resulted in the partial accommodation of the HZF as thrust welds. This spatiotemporal evolution, showing the interplay between salt diapirism and late thrusting, can be considered as an analog for other fold-and-thrust belts with pre-orogenic diapiric systems.

Herpetogaster collinsi from the Cambrian of China elucidates the dispersal and palaeogeographic distribution of early deuterostomes and the origin of the ambulacrarian larva.

The Cambrian Radiation represents one of the largest diversification events in Earth history. While the resulting taxonomic diversity is exceptional, relatively few of these novel species can be traced outside the boundaries of a single palaeocontinent. Many of those species with cosmopolitan distributions were likely active swimmers, presenting opportunity and means to conquer new areas, but this would not have been the case for sessile organisms. Herpetogaster is a lower to middle Cambrian (Series 2-Miaolingian, Stage 3-Wuliuan) genus of sessile, stalked, filter-feeding deuterostomes with two species, H. collinsi and H. haiyanensis, known respectively from Laurentia and Gondwana. Here, we expand the distribution of H. collinsi to Gondwana with newly discovered specimens from the Balang Formation of Hunan, China. This discovery raises questions on the origin of the genus and how sessile organisms were able to disperse over such a broad distance in the lower Cambrian. As Herpetogaster has been recovered at the base of the Ambulacrarian tree in recent phylogenies, a planktonic larval stage is suggested, which implies, that the last common ancestor of the Ambulacraria might have already had planktonic larvae or that such larvae developed multiple times within the Ambulacraria.

LATE CAMBRIAN (~490 MA) ARC VOLCANIC ROCKS OF THE NORTH TIANSHAN OF KYRGYZSTAN: AGE CONSTRAINTS FROM U–Pb ZIRCON DATING

In the present contribution, we aim to determine the age of the oldest arc-related volcanic units in the North Tianshan of Kyrgyzstan (NTS) to better understand the early evolution of subduction systems in the western part of the Central Asian Orogenic Belt. We conducted zircon U–Pb dating (LA–ICP–MS) on andesitic tuffs from the Sultansary and Tashtambektor formations, which were previously believed to be of early Cambrian and middle Cambrian to early Ordovician age, respectively. Our results reveal age estimates of 491 ± 3 and 487 ± 3 Ma for these formations, indicating that both units are late Cambrian in age. The volcanic rocks belong to a major magmatic arc with a heterogeneous basement that evolved in the NTS during the late Cambrian and early Ordovician.

U–Pb zircon ages from tuffaceous beds in the Terreneuvian to Cambrian Series 2 sections of Avalonian southern New Brunswick, Canada: new constraints on chronostratigraphic correlations and the Cambrian time scale

Abstract Avalonian sections in the Saint John area, southern New Brunswick, have long contributed to global understanding of Cambrian chronostratigraphy. A tuffaceous bed in the Ratcliffe Brook Formation (RBF) in the Somerset Street section dated at c. 531 Ma has traditionally been considered to post-date small shelly fossils attributed to the Watsonella crosbyi Zone in the Hanford Brook section. A fine-grained tuffaceous bed approximately 8 m stratigraphically lower in the Somerset Street section yields a chemical abrasion isotope dilution–thermal ionization mass spectrometry zircon age of 532.3 ± 0.3 Ma; a tuffaceous carbonate unit in the lower RBF in Hanford Brook gives an age of 531.5 ± 0.3 Ma. Crystal and crystal-lithic tuff beds near the top of the RBF yield ages of 520.3 ± 0.3 Ma (in Hanford Brook) and 519.1 ± 0.3 Ma (in Ratcliffe Brook). The new ages confirm the correlation between the Somerset Street and Hanford Brook sections based on acritarchs and make the association of small shelly fossils in the Hanford Brook section younger than 531 Ma. This result is relevant to ongoing discussions on the age of the base of undefined Cambrian Stage 2. The radiometric ages also support a young age for the upper part of the RBF, perhaps extending into Epoch 2.

Large euarthropod carapaces from a high latitude Cambrian (Drumian) deposit in Spain.

Deposits preserving non-biomineralized tissues and animals provide an unrivalled opportunity to study the evolution and radiation of early animal life. Numerous sites of Cambrian age are known from North America (Laurentia) and South China (East Gondwana), which provide a high resolution picture of the fauna at low latitudes. By contrast, our knowledge of Cambrian animals from higher latitudes is relatively poor. This patchiness in our knowledge of animal life during the radiation of animals in the Cambrian period limits our ability to understand and detect palaeogeographic trends and does not provide a full appreciation of animal diversity at this time. Here we report a new middle Cambrian (Drumian) site preserving lightly sclerotized euarthropod carapaces, sponges and palaeoscolecids near the village of Mesones de Isuela in the Iberian Chains (Spain). We describe three bivalved euarthropod carapace morphs, two comparable to those described from the only other high latitude Drumian deposit, the Jince Formation (Czechia), and one distinct from previous discoveries. These new findings highlight the importance of high latitude Gondwana Konservat Lagerstatten for understanding the palaeogeographical aspect of the radiation of early animals and suggest that bivalved euarthropods at high latitudes were larger than those at lower latitudes during the Cambrian.

A new arthropod resting trace from the middle Cambrian of Texas

A new arthropod resting trace from the middle Cambrian of Texas

The boundary between the Middle and Upper Cambrian strata in the Tarim Basin, NW China: Regional to global SPICE event correlation

The subdivision and correlation of the Cambrian System have long been controversial globally with multiple schemes. This is especially the case for the Tarim Basin, western China where regional and global correlations of the Middle-Upper Cambrian boundary are hampered by incomplete biostratigraphy. The Middle-Upper Cambrian boundary (T81) in the Tarim Basin is characterized by an abrupt switch of lithological and sedimentary facies, a sudden change of wireline log responses, and the contrasting seismic features, structural styles and carbonate platform architectures. The T81 boundary manifests in various forms and styles including syn-sedimentary unconformity, overlapping unconformity, tectonic unconformity, multi-stage superimposed unconformity, parallel unconformity and conformity. Each manifestation type of the T81 boundary was influenced by at least one of the factors including paleo-geomorphology, tectonic movement, eustacy, and climatic change. Based on the correlation between carbon isotopes and sedimentary characteristics in different regions globally, we suggest that the δ13C weak negative shift that follows the strongly positive shift of the Late Cambrian SPICE event can serve as a reliable isotope stratigraphic marker for identifying the T81 boundary of the western Tarim Platform. The top of the Awatag Formation (the top of the Middle Cambrian) in the western Tarim Platform should thus be correlated to the top of the Mohershan Formation in the eastern Tarim Basin, corresponding to the top of the Miaolingian Stage. The absence of the SPICE event in the western part of the Tarim Basin may have been caused by a regional depositional hiatus associated with the T81 boundary.

Composition and Age of Rocks of the Provenance Areas for the Lower–Middle Cambrian (?) Terrigenous Sediments of the Ernichnaya Formation of the Argun Massif, Eastern Part of the Central Asian Foldbelt

Composition and Age of Rocks of the Provenance Areas for the Lower–Middle Cambrian (?) Terrigenous Sediments of the Ernichnaya Formation of the Argun Massif, Eastern Part of the Central Asian Foldbelt

Geochemical Facets of the Inikan Formation as Indicators of Stratification and Hydrochemistry of the Cambrian Paleobasin

Abstract —Geochemical studies of Inikan Formation rocks from the coastal outcrops of the Yudoma River were carried out. All analyzed lithological varieties are characterized by significant enrichment in B, Ni, Mo, Ag, Sb, U (redox-sensitive trace metals and elements associated with biodeposition). The source of the elements was sea water that leached them from the eolian material. It was found that the deposits accumulated in anoxic environments with periods of both euxinic (probably, in the bottom water mass) and suboxic conditions. Such anoxic and euxinic conditions developed due to the stable stratification of the ocean that existed in the early–middle Cambrian in this part of the basin. In addition, the long-term accumulation of organic-rich sediment under conditions of anoxia, periodic euxinia, and stable stratification indicates the presence of a geomorphologically isolated depression in this part of the basin. CIA-Kcorr ranges from 68 to 95. The low values of the CIA may be related to the height of the relief in the sedimentary source area.

Cambrian through Ordovician reef transitions in North and South China: Changes in reef construction and background geobiological environments

The Cambrian–Ordovician interval was an intriguing period of geologic time, when reef transitions occurred along with important biodiversification events. This study examines the secular reef transitions in North and South China during the Cambrian and Ordovician in terms of changes in reef construction and related geobiological changes. Microbial reefs in the Terreneuvian have not been well documented in South China. The Cambrian Series 2 interval saw the almost simultaneous appearance of reef-building skeletal archaeocyaths and calcimicrobes. The widely developed archaeocyath–calcimicrobe reefs had totally disappeared by the middle of Series 2 in South China and were followed by purely calcimicrobial reefs. During the Miaolingian and Furongian, skeletal-reef builders were commonly thought to have been inhibited owing to elevated sea-water temperature, which caused lowered oxygen. However, microbial reefs containing lithistid sponges and keratosan sponges were more common in this interval in North China than previously thought. The first skeletal-dominated reefs were established during the Early Ordovician, coincident with a decline in stromatolite reefs in the central Yangtze Platform. During the Middle and Late Ordovician interval, a variety of skeletal reef-builders with diversified calcimicrobes contributed markedly to reef construction in limited areas of both North and South China. The appearance and increase of skeletal-dominated reefs are considered to have been related to lowering global sea-water temperature. Recently reported Late Ordovician cement-rich reefs in both North and South China may contain further evidence of global cooling; however, from an ecological perspective, the novel niches and biological interactions created by early skeletal-dominated reefs may have promoted the further development of reef-building organisms. Cambrian and Ordovician reef successions in North and South China are distributed discontinuously; however, integrating information from both North and South China could yield improved understanding of reef evolution and background environments through these intervals.

A tectonic solution for the Early Cambrian palaeogeographical enigma

Abstract The Early Cambrian palaeogeographical enigma arises when tectonic reconstructions are made using palaeoclimatic v. palaeomagnetic data that result in possibly contradictory tropical, mid-latitude, and south polar locations for major continents. For example, NW Africa and Cadomia may have lain in a tropical zone (0° to ±30° latitude) based on the presence of archaeocyath reefs, minor evaporites, and carbonate platforms at c. 520 Ma ± 5 Ma or, alternatively, NW Africa and Cadomia may have lain in a south polar zone (90° to 60° south latitude) based on palaeomagnetic constraints. Greater Avalonia may have evolved independently from NW Africa if a dropstone constraint implying polar latitudes at c. 530 Ma and a palaeomagnetic constraint implying c. 50° latitude at c. 505 Ma are accommodated. We show here how counterclockwise rotation of Gondwana during the Cambrian about an interior axis may solve the enigma. Gondwanan apparent polar wander becomes consistent with tropical conditions inferred for NW Africa when adjusted to accommodate constraints placing the south pole near Peru for c. 540–520 Ma. Concurrent counterclockwise rotation of Baltica and Gondwana during the Middle Cambrian may have facilitated separation of Greater Avalonia from Baltica across dextral shear zones.

Seawater carbon and strontium isotope variations in the eastern Tarim Basin during the Cambrian: Implications for connection with the open ocean

Cambrian was a critical period in Earth's history, marked by rapid diversification of early animals and significant changes in oceanic conditions. Marine sediments, particularly carbonates, have been used to study variations in the global carbon cycle, seawater redox, weathering input, etc. In search of new archives of Cambrian paleo-seawater chemistry, we conducted an integrated study of Cambrian C and Sr isotope stratigraphy using the carbonates of the Xingdi section in the Kruqtagh region of the eastern Tarim Basin. Based on reported paleontological data, the C isotope record is comparable to the Cambrian global seawater C isotope variation, providing a feasible stratigraphic framework for the Cambrian strata in the eastern Tarim Basin. A sharp δ13C increase at the top of the Xidasan Formation probably corresponds to the recovery of the Redlichiid–Olenellid Extinction C isotope Excursion. Additionally, a negative C isotope excursion in the lower part of the Mohuosan Formation and a positive C isotope excursion at the bottom of the Tuersaketagh Formation likely correspond to DrumIan Carbon isotope Excursion and Steptoean Positive Carbon Isotope Excursion, respectively. However, the Sr isotope curve in the eastern Tarim Basin decoupled from global seawater Sr isotope variations during Cambrian Series 2–3 until Furongian, indicating that the eastern Tarim Basin was not well connected with the open ocean until Furongian. It also highlights a significant impact of paleogeographical connections of basins with the open ocean on their seawater Sr isotopic signal.

U-Pb zircon dates from North American and British Avalonia bracket the Lower–Middle Cambrian boundary interval, with evaluation of the Miaolingian Series as a global unit

AbstractHigh-precision U-Pb zircon ages on SE Newfoundland tuffs now bracket the Avalonian Lower–Middle Cambrian boundary. Upper Lower Cambrian Brigus Formation tuffs yield depositional ages of 507.91 ± 0.07 Ma (Callavia broeggeriZone) and 507.67 ± 0.08 Ma and 507.21 ± 0.13 Ma (Morocconus-Condylopyge eliAssemblage interval). Lower Middle Cambrian Chamberlain’s Brook Formation tuffs have depositional ages of 506.34 ± 0.21 Ma (Kiskinella cristataZone) and 506.25 ± 0.07 Ma (Eccaparadoxides bennettiZone). The composite unconformity separating the Brigus and Chamberlain’s Brook formations is constrained between these ages. An Avalonian Lower–Middle Cambrian boundary between 507.2 ± 0.1 and 506.3 ± 0.2 Ma is consistent with maximum depositional age constraints from southwest Laurentia, which indicate an age for the base of the Miaolingian Series, as locally interpreted, of ≤ 506.6 ± 0.3 Ma. The Miaolingian Series’ base is interpreted as correlative within ≤ 0.3 ± 0.3 Ma between Cambrian palaeocontinents, although its exact synchrony is questionable due to taxonomic problems with a possibleOryctocephalus indicus-plexus, invariable dysoxic lithofacies control ofO. indicusand diachronous occurrence ofO. indicusin temporally distinct δ13C chemozones in South China and SW Laurentia. The lowest occurrence ofO. indicusassemblages is linked to onlap (epeirogenic or eustatic) of dysoxic facies. A united Avalonia is shown by late Early Cambrian volcanics in SW New Brunswick; Cape Breton Island; SE Newfoundland; and the Wrekin area, England. The new U-Pb ages revise Avalonian geological evolution as they show rapid epeirogenic changes through depositional sequences 4a–6.

Continental Genesis of Sands of the Novolutsk Member, Sablinskaya Formation of the Middle Cambrian (Northwestern Flank of the Moscow Syneclise)

Despite the high degree of knowledge about Cambrian-Ordovician sandy sequence in the northwestern flank of the Moscow syneclise, some of its subdivisions are still poorly studied. For the first time the article provides a comprehensive lithological analysis of the textural and mineral-structural composition of the Novolutsk member sands, common in the west of the Leningrad region. This member conventionally belongs to the Sablinskaya Formation of the Middle Cambrian. Significant differences between the studied sands and the sands of other units of the formation are shown, and their analogues are found in the eastern sections of the Baltic-Ladoga glint. The lenticular occurrence, specific textural complex and coarse-medium-grained composition of the Novolutsk unit sands made it possible to prove their continental alluvial genesis and link them with the sedimentary environments of the earlier identified Putilovskaya member sands of the Sablinskaya Formation.

Composition and Age of Sources of the Lower–Middle Cambrian (?) Terrigenous Rocks from The Ernichny Formation in the Argun Massif, Eastern Part of the Central Asian Fold Belt

The paper presents the results of the mineralogical and geochemical studies of sandstones and siltstones of the Ernichny formation of the Argun series in the Argun continental massif, and the results of U–Pb (LA‑ICP-MS) dating for the detrital zircons from these rocks. It is established that the youngest detrital zircons from sandstone of the Ernichny Formation have age of 549–570 Ma. A maximum on the relative probability curve of zircon age corresponds to 566 Ma. These data determine the lower age limit of their accumulation on the border of the Ediacaran and Paleozoic age. According to the results of U–Pb dating of detrital zircon, the majority of detrital zircons from sandstones of Ernichny Formation are characterized by Neo- and Paleoproterozoic ages. The sources of zircons were Neo- and Paleoproterozoic igneous and metamorphic rocks that were widely developed within the Argun continental massif. In turn, the geochemical features of the terrigenous rocks of the Ernichny Formation, together with the presence of the poorly sorted and rounded clastic material in the studied samples, as well as the presence of interlayers of gravelstones, enabled us to establish that they were formed in a subduction-related setting.

Shell injuries, repair and malformation in the early Cambrian mollusc Helcionella antiqua from Scania, Sweden

ABSTRACT Three cases of repaired injuries and malformation in specimens of the helcionelloid mollusc Helcionella antiqua (Kiær, 1917) from the lower Cambrian (Cambrian Series 2, Stage 4) Gislöv Formation of southern Sweden document some of the oldest known durophagous attacks on Palaeozoic molluscs. Two of the injuries are developed as clefts, of which one had a severe effect on the continued growth of the shell. The third example is a large embayment removing large portions of the supra-apical part of the shell. A similar repaired injury is known in the slightly older mollusc Marocella mira Geyer, 1986. from Antarctica and Australia. The morphology of the injuries and the hydrodynamically quiet depositional setting suggests that the shell damage was caused by failed predatory attacks. The location of the repaired injuries suggests that the attacks may have targeted the head region of the molluscs, thus supporting an endogastrically coiled orientation of the shell in Helcionella. Only three repaired injuries in 252 Helcionella specimens were found, giving a shell repair frequency of 1.2%. All three examples occur in the larger size classes. The size-frequency distribution (N = 182) is strongly right skewed, which could suggest high input of juvenile specimens into the assemblage. The assemblage is interpreted as a time averaged and mixed death assemblage, albeit with good correspondence with the living shelly assemblage, due to a relatively thin, homogenous unit that may suggest within-habitat time averaging.

Helcionelloid molluscs from the lower Cambrian (Series 2, Stage 4) of southern Sweden

ABSTRACT Helcionelloid molluscs are described from the Gislöv Formation (Cambrian Series 2, Stage 4; Vergalian–Rausvian regional stages) in Scania, southern Sweden. Seven species are distinguished,including ‛Pelagiella’ sp., Helcionella antiqua, Davidonia puppis, Davidonia cf. rostrata, Latouchella cf. costata, Stenotheca norvegica, and an indeterminate helcionelloid. The c. 750 specimens were collected from a 30 cm interval with a diverse shelly fauna, encompassing the Ellipsostrenua spinosa Zone. The peak distribution of the molluscs is within the 2–3 mm size class, and there was limited sorting or transport prior to burial. Helcionella antiqua is characteristic of the helcionelloid assemblage, and together with Davidonia puppis, the only molluscs in the assemblage exceeding 10 mm in length. All species except ‘Pelagiella’ display full ontogenetic series providing verification of morphological transitions and direct ontogenetic ties between small helcionelloid juveniles or embryonic shells and their macro adults. Analysis of distributions indicates that the Gislöv assemblage is closely comparable to the Evjevik Member of the Mjøsa area, Norway, while sections in northern Sweden form a separate cluster. The distribution of the shelly assemblage in the E. spinosa Biozone is consistent with a temporal distribution of this fauna, rather than biofacies control.

The First Report of the Acrotretoid Brachiopod Hadrotreta from the Tsinghsutung Formation Cambrian (Series 2, Stage 4), Guizhou, South China.

Hadrotreta is a worldwide acrotretoid brachiopod reported from the Cambrian Series 2 to Miaolingian. Here, a number of well-preserved fossils of Hadrotreta, identified as Hadrotreta cf. H. timchristiorum, were found in the Protoryctocephalus arcticus Zone of the Tsinghsutung Formation of Cambrian Series 2, Stage 4 in Jianhe, Guizhou, south China. This is the first report of Hadrotreta in China, which enriches its global palaeogeographical distribution. Hadrotreta is very similar to acrotretoids such as Kostjubella, Vandalotreta, Linnarssonia, and Eohadrotreta. It differs from them with its well-developed ventral boss-like apical process, apical pits, and dorsal median sulcus. In view of the palaeogeography of Hadrotreta, this genus was mainly distributed in low-latitude regions. Hadrotreta was only found in south China and Laurentia during the Cambrian Age 4, then expanded its distribution to other regions such as Siberia, Baltica, the Kazakh Terranes, the Far East, and Gondwana Pange during the Miaolingian Epoch. Hadrotreta seems to have shifted from deeper water to shallow-water environments during the period from the Cambrian Series 2 to the Miaolingian.

A macroscopic free-swimming medusa from the middle Cambrian Burgess Shale.

Cnidarians are regarded as one of the earliest-diverging animal phyla. One of the hallmarks of the cnidarian body plan is the evolution of a free-swimming medusa in some medusozoan classes, but the origin of this innovation remains poorly constrained by the fossil record and molecular data. Previously described macrofossils, putatively representing medusa stages of crown-group medusozoans from the Cambrian of Utah and South China, are here reinterpreted as ctenophore-grade organisms. Other putative Ediacaran to Cambrian medusozoan fossils consist mainly of microfossils and tubular forms. Here we describe Burgessomedusa phasmiformis gen. et sp. nov., the oldest unequivocal macroscopic free-swimming medusa in the fossil record. Our study is based on 182 exceptionally preserved body fossils from the middle Cambrian Burgess Shale (Raymond Quarry, British Columbia, Canada). Burgessomedusa possesses a cuboidal umbrella up to 20 cm high and over 90 short, finger-like tentacles. Phylogenetic analysis supports a medusozoan affinity, most likely as a stem group to Cubozoa or Acraspeda (a group including Staurozoa, Cubozoa and Scyphozoa). Burgessomedusa demonstrates an ancient origin for the free-swimming medusa life stage and supports a growing number of studies showing an early evolutionary diversification of Medusozoa, including of the crown group, during the late Precambrian-Cambrian transition.

First confident evidence of moulting in eodiscid trilobites from the Cambrian Stage 3 of South China

Abstract Trilobite moulting behaviour has been extensively investigated. However, exuviae in eodiscid trilobites are poorly known. Here, we report two eodiscid trilobite specimens, Tsunyidiscus niutitangensis and Tsunyidiscus sp., showing Somersault configuration from the Niutitang Formation and Mingxinsi Formation of South China, respectively (Cambrian Series 2, Stage 3). The arrangements of the exoskeletons indicate that the two specimens are the slightly disturbed and undisturbed exuviae. The impression of the lower cephalic unit (LCU) displays the rostral plate in Tsunyidiscus niutitangensis. The exuviae showing the LCU inverted anteriorly under the trunk. The opening of the facial and rostral sutures would have allowed the emergence of the post-ecdysial trilobite with the partial enrolment of exoskeleton. Moreover, our discovery indicates a Somersault configuration which employed the facial and rostral sutures to create an anterior exuvial gape that also exists in eodiscid trilobites besides redlichiid trilobites, corynexochid trilobites and ptychopariid trilobites during the Cambrian.