Trilobite Fragments Research Articles

First Report of Small Skeletal Fossils from the Upper Guojiaba Formation (Series 2, Cambrian), Southern Shaanxi, South China.

A small skeletal fossil assemblage is described for the first time from the bioclastic limestone interbeds of the siltstone-dominated Guojiaba Formation, southern Shaanxi, China. The carbonate-hosted fossils include brachiopods (Eohadrotreta zhujiahensis, Eohadrotreta zhenbaensis, Spinobolus sp., Kuangshanotreta malungensis, Kyrshabaktella sp., Lingulellotreta yuanshanensis, Eoobolus incipiens, and Eoobolus sp.), sphenothallids (Sphenothallus sp.), archaeocyaths (Robustocyathus sp. and Yukonocyathus sp.), bradoriids (Kunmingella douvillei), chancelloriids sclerites (Onychia sp., Allonnia sp., Diminia sp., Archiasterella pentactina, and Chancelloria cf. eros), echinoderm plates, fragments of trilobites (Eoredlichia sp.), and hyolithelminths. The discovery of archaeocyaths in the Guojiaba Formation significantly extends their stratigraphic range in South China from the early Tsanglangpuian at least to the late Chiungchussuan. Thus, the Guojiaba Formation now represents the lowest known stratigraphic horizon where archaeocyath fossils have been found in the southern Shaanxi area. The overall assemblage is most comparable, in terms of composition, to Small skeletal fossil (SSF) assemblages from the early Cambrian Chengjiang fauna recovered from the Yu'anshan Formation in eastern Yunnan Province. The existing position that the Guojiaba Formation is correlated with Stage 3 in Cambrian Series 2 is strongly upheld based on the fossil assemblage recovered in this study.

Sedimentary facies, lithofacies paleogeography, and an evaluation of the Ordovician sequences in the Sichuan Basin, southwest China

Ordovician strata in the Sichuan Basin occur in formations of varying thicknesses. Deposited in intracratonic and craton margin depressions of the Yangtze Block, these strata are rarely exposed in the basin with poorly described facies types and multiple controversies in sequence stratigraphy, which can lead to inaccurate interpretations of depositional settings and paleogeography. We studied outcrops, samples, and thin sections, and conducted interpretations of the environment of deposition and sequence stratigraphy as well as the lithofacies paleogeographic characteristics of different depositional stages of Ordovician units. Macroscopic facies (08) and their associated microfacies (14) were identified from outcrops and thin sections. The depositional environments of the carbonate rocks were interpreted using the standard microfacies types (SMF) of Flügel (2010) as a reference. The Ordovician units in the Sichuan Basin were divided into three 2nd-order sequences, including two complete cycles (SSQ1 and SSQ2) and one semi-cycle (SSQ3), and further subdivided into ten 3rd-order sequences (SQ1-SQ10). Ordovician units were partitioned into seven formations: the Tongzi, Honghuayuan, Meitan, Shizipu, Baota-Linxiang, and Wufeng formations. Based on the variation characteristics of the carbon-13 isotope (δ13C) curve, two large cycles of sea-level rise and fall were detected in the study area. An extensional rifting event occurred in the basin during the first cycle (Lower Tongzi Formation to Early/Middle Meitan Formation). The basin was in a compressive stage during the second cycle (Late Meitan Formation to Late Wufeng Formation). The Early Ordovician Tongzi and Honghuayuan formations were deposited in shallow water. During the deposition of these units, shoal facies occurred in the platform margin. Afterward, rimmed carbonate platforms occurred in the platform margin with a fauna mainly composed of brachiopods, echinoderms, trilobites, bryozoans, and algae. The Middle Ordovician Meitan Formation is characterized by increased clastic components deposited in continental shelf environments with graptolite and trilobite fossils. The Late Ordovician Baota Formation is marked by the development of slope facies deposited in carbonate ramp environments with cephalopod and trilobite fragments. The Late Ordovician Wufeng Formation was deposited in deep water shelf settings characterized by graptolite fossils. Tectonic activities and relative sea-level change mainly controlled the paleogeographic pattern and the evolution of the Sichuan Basin. The results presented in this study provide valuable insights and fill a significant gap in facies analysis, sequence stratigraphic division, and paleogeography of Ordovician units in the Sichuan Basin.

Cambrian (Series 2 to Miaolingian) platform facies from central Sonora, Mexico and the regional correlation

This study provides new insights about depositional paleoenvironments through siliciclastic microfacies, carbonate microfacies, and biofacies analysis from sedimentary formations of the lower and middle Cambrian (Stage 4–Wuliuan), exposed in central Sonora, northern Mexico. Results of the petrographic analysis of 48 samples revealed the following lithologies: quartzarenite, oncolytic rudstone, grainstone-packstone, wackestone, mudstone, and to a lesser extent sandy limestone. Two siliciclastic microfacies were identified: (A) quartzarenite with cross-bedded and horizontal stratification deposited in an intertidal and supratidal environment; and (B) massive quartzarenite with Skolithos ichnofacies deposited in subtidal and intertidal environments. Four carbonate microfacies were identified: microfacies 1 is a sandy limestone with trilobite fragments; microfacies 2 is a grainstone with intraclasts, salterellids, hyolithids, trilobites, and echinoderms plates; microfacies 3 is an oncolytic rudstone consisting of microbes and abundant echinoderms plates; and microfacies 4 is a packstone-grainstone with abundant ooids, trilobite fragments, and echinoderm plates. Two biofacies were identified: Agnostid-polymeroid biofacies with predominance of the trilobites Pentagnostus, Bathyuriscus, Oryctocephalites, and Elrathina; and Pagetia biofacies with abundant trilobites of the genera Pentagnostus, Pagetia and Elrathina. It is concluded by the sedimentation model that changes in sea level is the most important parameter in determining the siliciclastic microfacies, carbonate microfacies and biofacies; as well as the depositional environments that vary from the coastline (subtidal to supratidal) to shallow-water open circulation marine platform with low and high energy waters. The Cambrian deposits of northern Mexico are correlated with the deposits of California and Nevada (USA), as well as to the Precordillera (Argentina), where the species in common show a strong affinity.

Cambrian stratigraphy (Series 2 to Miaolingian) of the El Sahuaral area in central Sonora, Mexico: Biostratigraphic implications

The lower and middle Cambrian strata exposed in the state of Sonora, Mexico, consists of sequences of quartzarenite, limestone, shale and dolomite exposed in several localities, mainly in the northwestern and central regions of Sonora. Approximately 800 m interval of Cambrian marine sedimentary rocks are exposed in the El Sahuaral area, a new Cambrian locality in central Sonora. The sedimentary succession of these marine facies is divided into five lithostratigraphic units previously documented in Mexico: Proveedora, Buelna, Cerro Prieto, El Gavilán and El Tren formations. The Proveedora Formation is the oldest sequence of Cambrian strata in the region and consists of a 168 m thick sequence of bioturbated, fine-grained quartzarenite. The Buelna Formation conformably overlies the Proveedora Formation in the El Sahuaral and adjacent areas and consists of 70 m of interbedded limestone and shale, with abundant fragments of olenellid trilobites from either the Bristolia mohavensis or Bristolia insolens zones (Series 2, Stage 4). The overlying Cerro Prieto Formation is composed of a 70 m thick sequence of recrystallized oolitic limestone with possible fragments of Amecephalus arrojosensis, suggesting the Amecephalus arrojosensis Zone (uppermost Series 2, Stage 4). The El Gavilán Formation conformably overlies the Cerro Prieto Formation with a thickness of 225 m; it is composed of interbedded limestone and shale deposited in a marine open shelf environment, with abundant fauna of the upper Ehmaniella Zone (Altiocculus Subzone), Ptychagnostus praecurrens - Ptychagnostus gibbus Zone (Miaolingian Series, Wuliuan Stage). The El Tren Formation, reported by the first time in central Sonora, conformably overlies the El Gavilán Formation and consists of 250 m thick sequence of dolomitic limestone with unidentified trilobite fragments. The Cambrian deposits of central Sonora present marine facies that vary from the littoral zone to open marine shelf, accumulated in the western part of the North American craton and contains an abundant and diverse biota comprised by cyanobacteria, poriferans, brachiopods, trilobites, and echinoderms.

Precise early Cambrian U–Pb zircon dates bracket the oldest trilobites and archaeocyaths in Moroccan West Gondwana

AbstractNew U–Pb radioisotopic ages on early Cambrian volcanic zircons condition a high-resolution Bayesian age model that constrains the first occurrences and zonations of West Gondwanan archaeocyaths and trilobites in southern Morocco. The oldest archaeocyaths in the Tiout Member of the Igoudine Formation (519.71 + 0.26/− 0.35 Ma) are c. 6 Ma younger than the oldest Siberian archaeocyaths. The oldest Moroccan trilobite fragments, from the lower member of the Igoudine, are constrained to 519.95 + 0.43/− 0.40 Ma. The succeeding Issendalenian Stage (i.e. Hupetina antique – Eofallotaspis tioutensis – Fallotaspis plana – Choubertella – Daguinaspis trilobite zones) spans c. 1.5 Ma (519.78 + 0.26/− 0.37 Ma to 518.43 + 0.25/− 0.69 Ma). Identifiable Moroccan fallotaspidids and bigotinids, among Earth’s oldest trilobites, occur above a positive δ13C excursion dated with our age model at 520.27 + 0.59/− 0.57 Ma, and correlated with the IV excursion peak within the lower range of Siberian Atdabanian Stage trilobites (Repinaella Zone). This excursion is the best standard for a Cambrian Series 2 base. The oldest West Gondwana trilobite fragments are c. 1 Ma younger than those in Siberia and c. 0.5 Ma older than the oldest Avalonian trilobites (Callavia Zone). This diachrony means a trilobite first appearance datum is an inappropriate chronostratigraphic base for Cambrian Series 2. Taxonomic differences in the oldest trilobites between Cambrian palaeocontinents are in accordance with trace fossil evidence for the group’s appearance possibly as late as c. 530 Ma in the Cambrian Evolutionary Radiation. Coeval 519–517 Ma dates from Avalonia (cool-water siliciclastic shelf) and West Gondwana (tropical carbonate platform) sections with distinct macrofaunas emphasize these successions were latitudinally separate by the late Ediacaran Period.

Dating the Cambrian Purley Shale Formation, Midland Microcraton, England

AbstractZircons from a bentonite near the base of the Purley Shale Formation in the Nuneaton area, Warwickshire, yield a 206Pb/238U age of 517.22 ± 0.31 Ma. Based on the fauna of small shelly fossils and the brachiopod Micromitra phillipsii in the underlying Home Farm Member of the Hartshill Sandstone Formation, trilobite fragments that are questionably referred to Callavia from the basal Purley Shale Formation, and the presence of trilobites diagnostic of the sabulosa Biozone 66 m above the base of the Purley Shale Formation, the bentonite likely dates an horizon within Cambrian Stage 3, at about the level of the Fallotaspis or basal Callavia Biozone. This is consistent with bentonite ages from other localities in southern Britain, which constrain the age of the lower and uppermost parts of Cambrian Stage 3. The new date provides additional chronological control on the earliest occurrence of trilobites in the Midland Microcraton, a date for the marine transgression at the base of the Purley Shale Formation, and is the first radiometric age from the Cambrian succession of Warwickshire.

New anatomical information on Anomalocaris from the Cambrian Emu Bay Shale of South Australia and a reassessment of its inferred predatory habits

AbstractTwo species of Anomalocaris co‐occur in the Emu Bay Shale (Cambrian Series 2, Stage 4) at Big Gully, Kangaroo Island. Frontal appendages of Anomalocaris briggsi Nedin, 1995, are more common than those of Anomalocaris cf. canadensis Whiteaves, 1892, at a quarry inland of the wave‐cut platform site from which these species were originally described. An oral cone has the three large, node‐bearing plates recently documented for Anomalocaris canadensis, confirming that Anomalocaris lacks a tetraradial ‘Peytoia’ oral cone and strengthening the case for the identity of the Australian specimens as Anomalocaris. Disarticulated anomalocaridid body flaps are more numerous in the Emu Bay Shale than in other localities, and they preserve anatomical details not recognized elsewhere. Transverse lines on the anterior part of the flaps, interpreted as strengthening rays or veins in previous descriptions of anomalocaridids, are associated with internal structures consisting of a series of well‐bounded, striated blocks or bars. Their structure is consistent with a structural function imparting strength to the body flaps. Setal structures consisting of a series of lanceolate blades are similar to those of other anomalocaridids and are found in isolation or associated with body flaps. A single specimen also preserves putative gut diverticula. The morphology of the appendages, oral cone, gut diverticula and compound eyes of Anomalocaris, along with its large size, suggests that it was an active predator, and specimens of coprolites containing trilobite fragments and trilobites with prominent injuries have been cited as evidence of anomalocaridid predation on trilobites. Based on frontal appendage morphology, Anomalocaris briggsi is inferred to have been a predator of soft‐bodied animals exclusively and only Anomalocaris cf. canadensis may have been capable of durophagous predation on trilobites, although predation (including possible cannibalism) by Redlichia could also explain the coprolites and damage to trilobite exoskeletons found in the Emu Bay Shale.

An extensive erosion surface of a strongly deformed limestone bed in the Gushan and Chaomidian formations (late Middle Cambrian to Furongian), Shandong Province, China: Sequence–stratigraphic implications

In order to understand sequence development and sea-level fluctuations during the late Middle Cambrian to early Furongian on the North China epeiric platform, the present study focuses on a unique, subtle erosion surface of an extensive (approx. 100 km), strongly deformed limestone bed in the uppermost part of the Gushan Formation, Shandong Province, China. The Gushan Formation and the overlying Chaomidian Formation consist mainly of shales and a variety of carbonates that were deposited in subtidal environments (e.g., deep subtidal, shallow subtidal, shoreface/shoal, subtidal microbial flat, and restricted platform interior). Three third-order depositional sequences (S1–3) are identified, each of which comprises a thin transgressive systems tract (TST) and a relatively thick highstand systems tract (HST). Each sequence is bounded by a drowning unconformity (SB1), a subaerial unconformity (SB2), or a surface of submarine erosion (SB3). The upper sequence boundary (SB2) of sequence 1 (S1) is represented by a subtle erosion surface of an extensive, deformed limestone bed with a wide variety of soft-sediment deformation structures (e.g., lime mudstone breccias, chaotic wacke-packstone laminae and fragments, homogenized oolites, and clastic dykes), and is overlain by small sporadic microbial buildups and an extensive bioclastic grainstone bed. The deformed limestone was formed during early diagenesis by differential deformation processes (brecciation, liquefaction/fluidization, and injection) which were most likely induced by pore-water overpressure during the period of rapid sea-level fall. Despite the lack of subaerial exposure features (e.g., paleokarst, paleosol, etc.), the characteristics of the erosion surface (cutting well-lithified sediment below), the missing of a significant geological record (the Prochuangia biozone), and the worldwide correlatable positive carbon isotope excursion collectively indicate that the erosion surface developed under conditions of subaerial exposure after contemporaneous marine cementation of the deformed sediment. The missing of the Prochuangia biozone is most likely due to non-deposition at a subaerial hiatal surface. The erosion surface was submerged as a result of subsequent rise in sea level, where sporadic microbial buildups formed under suitable conditions. Freshly deposited, winnowed, shell-dominated transgressive lag deposits (containing Chuangia trilobite fragments, brachiopod shells, and abundant glauconite grains) formed with continued rise in sea level, which became, in turn, overlain by shale-dominated facies. The unique combination of the subtle erosion surface ( sensu stricto a subaerial unconformity) and the underlying deformed limestone bed provides an important criterion for recognizing the subtle changes in relative sea level on shallow epeiric platforms.

The Oldest Isorophid Edrioasteroid (Echinodermata) and the Evolution of Attachment Strategies in Cambrian Edrioasteroids

Two new middle Cambrian edrioasteroid (Echinodermata), Protorophus hispanicus gen. et sp. nov., and Isorophida gen. et sp. indet., are described from the early middle Cambrian (Cambrian Series 3, Stage 5) of Spain. These are the oldest and probably the most primitive isorophids, a clade previously known from the upper Cambrian onwards. Specimens are attached to trilobite fragments indicating that edrioasteroids had by this time separated into two lineages each with different strategies for attachment, sediment attachers and hard substrate attachers. The single U-shaped ambulacral flooring plates of Protorophus are unique while Isorophida gen. et sp. indet. shares the presence of spines in common with some pyrgocystitid isorophids. The shift from facultative soft-bottom attachment to obligate hard-ground attachment in edrioasteroids involved the retention of a juvenile trait into adulthood and was already underway by the middle Cambrian.

Settling strategy of stalked echinoderms from the Kaili Biota (middle Cambrian), Guizhou Province, South China

A total of 124 articulated echinoderms, including Sinoeocrinus lui ( n = 30), “ S. globus” ( n = 92), one indeterminate gogiid, and one indeterminate edrioasteroid, from the Kaili Formation (Middle Cambrian), eastern Guizhou, China was examined. Among them, 73% of gogiid echinoderms are preserved attached to skeletal substrates. Suitable skeletal substrates for gogiid holdfasts in the Kaili Biota are organophosphatic brachiopods; large trilobite fragments, including cranidia, free cheeks, thoracic segments, and pygidia; hyoliths; and Scenella shells. The high frequency of “ S. globus” attached to organophosphatic brachiopods is due to the high richness of organophosphatic brachiopods rather than a host-specific association in the Kaili Biota. However, the possible discrimination against attachment to small trilobites, such as Pagetia, requires further investigation. Based on 11 hand samples, echinoderm population density (EPD) varies from 0.3 to 4.5 echinoderms per 50 grind points (0.06 to 0.96 echinoderms per cm 2), depending on the percent coverage of skeletal debris. Crowding effect is evident when EPD is approximately 0.32 echinoderms per cm 2 and the skeletal coverage is less than 20%.

Geology of the summit limestone of Mount Qomolangma (Everest) and cooling history of the Yellow Band under the Qomolangma detachment

Abstract Newly discovered peloidal limestone from the summit of Mount Qomolangma (Mount Everest) contains skeletal fragments of trilobites, ostracods and crinoids. They are small pebble‐sized debris interbedded in micritic bedded limestone of the Qomolangma Formation, and are interpreted to have been derived from a bank margin and redeposited in peri‐platform environments. An exposure of the Qomolangma detachment at the base of the first step (8520 m), on the northern slope of Mount Qomolangma was also found. Non‐metamorphosed, strongly fractured Ordovician limestone is separated from underlying metamorphosed Yellow Band by a sharp fault with a breccia zone. The 40Ar–39Ar ages of muscovite from the Yellow Band show two‐phase metamorphic events of approximately 33.3 and 24.5 Ma. The older age represents the peak of a Barrovian‐type Eo‐Himalayan metamorphic event and the younger age records a decompressional high‐temperature Neo‐Himalayan metamorphic event. A muscovite whole‐rock 87Rb–86Sr isochron of the Yellow Band yielded 40.06 ± 0.81 Ma, which suggests a Pre‐Himalayan metamorphism, probably caused by tectonic stacking of the Tibetan Tethys sediments in the leading margin of the Indian subcontinent. Zircon and apatite grains, separated from the Yellow Band, gave pooled fission‐track ages of 14.4 ± 0.9 and 14.4 ± 1.4 Ma, respectively. These new chronologic data indicate rapid cooling of the hanging wall of the Qomolangma detachment from approximately 350°C to 130°C during a short period (15.5–14.4 Ma).

Late Cambrian cabbage-head stromatolites from Saratoga Springs, New York, USA

Saratoga Springs, New York, is the site of one of the finest examples of domed stromatolites to be seen anywhere in ancient rocks and is significant in the history of geology as the area where stromatolites were first described and interpreted. These cabbage-head structures, which are part of the Hoyt Limestone of Late Cambrian (Late Franconian to Early Trempeleauan) age, were described by James Hall as early as 1847. Glaciated surfaces expose horizontal sections of the cabbage-shaped heads composed of vertically stacked, hemispherical stromatolites. The microbial heads are discrete domal structures built of hemispheroidal and bulbous stromatolites expanding upward from a base. The heads, many of them compound, are circular in horizontal section, and range in diameter from a few centimeters to a meter. Between the heads are ooids, skeletal fragments of trilobites, brachiopods, pelecypods, and quartz-sand particles.

Composite Stratigraphy of the Sailinhudong Group and Ore–bearing Micrite Mounds in the Bayan Obo Deposits, Inner Mongolia, China

AbstractBased on studies of sequence stratigraphy, event stratigraphy, biostratigraphy and lithostratigraphy, it is concluded that the Sailinhudong Group is a part of the Bayan Obo Group. Some trilobite fragments are first found in thin sections of the rock from the lower part of the Sailinhudong Group and some Ordovician acritarchs and chitinozoans are also found in this group. A formationa unit of carbonate seismites is first recognized in the upper part and a huge micrite mound is first identified at the top. Dolomite, the host rock of the super giant Bayan Obo Fe–Nb–REE deposits, is neither an igneous carbonatite nor a common bedded sedimentary carbonate, but a huge micrite mound. It has the same macroscopic characters as the micrite mounds at the top of the Sailinhudong Group, which suggests that they should be of the same horizon. According to the fossils, the Sailinhudong and Bayan Obo Groups should be of the Early Palaeozoic rather than the Middle Proterozoic. The new discovery and new idea will throw light on the explanation of the genesis of the supergiant Bayan Obo Fe–Nb–REE deposits.

Trace Fossils and Trolls: Report of the First International Ichnofabric Workshop in Norway

Ichnofabric is a relatively new concept in paleontology and sedimentary geology, having been introduced in the literature less than a decade ago. Ichnofabric is an important component of ichnology, as it deals with how organisms affect sediment properties, produce preservable sedimentary structures, and create biogenic sedimentary fabrics. Ichnologic research traditionally has centered on identifiable trace fossils, although the processes of bioturbation and bioerosion are quite important and have received significant attention by some workers. The advent of the ichnofabric concept during the 1980's has added a new dimension to the field of ichnology, because it provides a more comprehensive picture of the original depositional environment by emphasizing biogenic aspects of the fabric of sediments. In September of 1991, a very enthusiastic group of ichnofabric researchers from all over the world gathered in Oslo, Norway, for the First International Ichnofabric Workshop, which was sponsored by Norsk Hydro and several other oil companies that operate in the North Sea region. The meeting was organized by Fredrik Bockelie (Norsk Hydro, Stabekk, Norway) and Tony Ekdale (University of Utah, Salt Lake City). Other participants in the workshop included David Bottjer (University of Southern California, Los Angeles), Richard Bromley (Institute for historisk Geologi, Copenhagen, Denmark), Allen Curran (Smith College, Northampton, Massachusetts), Mary Droser (University of California at Riverside), Christian Gaillard (University Claude Bernard, Lyon, France), Roland Goldring (Sedimentology Institute, Reading, England), Niels-Martin Hanken (University of Tromso, Norway), Chris Maples (Kansas Geological Survey, Lawrence), Tom Mason (University of Natal Durban, South Africa), Molly Miller (Vanderbilt University, Nashville, Tennessee), John Pollard (University of Manchester, England), Charles Savrda (Auburn University, Auburn, Alabama), Ronald Steel (University of Bergen, Norway), Andrew Taylor (University Manchester, England), Guanzhong Wang (Sedimentology Institute, Reading, England), and Andreas Wetzel (University of Basel, Switzerland). The purpose of the workshop was to examine the concept of ichnofabric, to come to a consensus regarding its terminology, and to develop applications of ichnofabric in facies recognition, event correlation, reservoir prediction, sedimentologic modeling, and paleontologic reconstruction. Major themes that were addressed at the conference included the manner in which the tiered structure of infaunal organism communities is reflected in composite ichnofabrics, the application of empirical ichnofabric indices in assessing the degree of bioturbation of sediment packages, the utility of ichnograms in basin wide reconstructions, the recognition and correlation of recurrent ichnofabrics and strata in both the subsurface and outcrop sections, and the potentially widespread use of ichnofabric correlation in petroleum exploration and reservoir prediction. The workshop began in the Vaekero Research Center of Norsk Hydro Oil Company in Oslo. A long first day of oral presentations and free-wheeling discussions by all participants set the stage and established the major themes of the conference. On the second and third days, a field trip from Oslo to Bergen allowed the group to examine a variety of lower Paleozoic ichnofabrics in outcrop settings. The group went to Bygdoy Peninsula southwest of Oslo to examine Lower Ordovician limestone and shale sequences, where the Huk Formation contains three suites of trace fossils at successive omission surfaces. Complex, composite ichnofabrics are well displayed in these rocks. A pre-omission episode of biogenic homogenization caused chaotic orientation of trilobite fragments. This event was followed by development of an omission surface, including an omission suite of Thalassinoides-like firmground burrows, upon which a postomission set of light-colored burrows penetrating the tops of the firmground and the omission suite burrow fills was emplaced. A second stop was at Kalvoya, near Sandvika, where the group observed Upper Ordovician nodular limestones in the Husberoya Formation. Abundant trace fossils occur in positive hyporelief, and there are three main trace fossil associations occurring in distinct ichnofabrics representing mid to upper-shoreface, lower to mid-shoreface, and offshore facies. At Kolsas Hill along Tyri Fjord the participants observed rippled sandstones of Silurian age that contained epichnial arthropod trackways (Steinfjordichnus). At Langodden on the shores of Lake Mjosa, the group examined ichnofabrics in Lower Cambrian sandstones of the Ringsaker Quartzite. Trace fossils are patchy but locally well displayed, and the Skolithos Ichnofacies is exemplified by Skolithos, Monocraterion and Diplocraterion. Discussions on the outcrop

Lower Cambrian bioconstructions in Northwestern Mexico (Sonora). Depositional setting, paleoecology and systematics of Archaeocyaths

Investigations on Lower Cambrian carbonates of Caborca (NW Mexico) indicate that in the Puerto Blanco Formation the following two types of archaeocyathan-algal buildups occur: o I -Framework reefs occurring at the base of the carbonate interval of the Puerto Blanco Formation. These consist of an archaeocyathan framework with encrusting renalcids and abundant fibrous cements. Cavities are filled with mudstone and contain rare sponge spicules. Cements are well developed upon internal sediments. The archaeocyathan fauna is dominated by colonial Paranacyathus along with Batenevia and Claruscoscinus with taeniae and curved tabulae. A few regular archaeocyaths are present, including Palmericyatbus and Aulocricus. Lateral to the reef facies the lime mud sediment contains abundant skeletal fragments of trilobites, echinoderms, Chancelloria, hyoliths, and archaeocyaths. These buildups are interpreted to have formed as wave-resistant, framework reefs in a tidally dominated, shelf-margin setting seaward of a carbonate barrier. II - Girvanella-dominated buildups located in the upper part of the carbonate interval of the Puerto Blanco Formation and associated with oolites. These are built by stacked Girvanella crusts, with an important contribution of Renalcis and a less important presence of Epiphyton. Shelter cavities are filled by mudstone often rich in siltsize quartz. Cements are poorly developed and confined to the intraskeletal cavities. Archaeocyaths are scattered in the Girvanella frame either as isolated individuals or as clusters of solitary or colonial forms. Regular archaeocyaths are here abundant, represented by several taxa with an ethmophylloid inner wall, including Palmericyatbus and Cordilleracyathus. The dominant genus is Aulocricus. Colonial species of Arcbaeocyathus and Arrythmocricus are well developed. These latter two genera are now recorded from Mexico to northern Canada. The internal and lateral sediment contains a rich

Invertebrate paleoecology of the Upper Cliff coal interval (Pennsylvanian), Plateau Coal Field, northern Alabama

The Upper Cliff coal interval (Early Pennsylvanian) of northern Alabama consists of sandstone, siltstone, shale, and coal deposited within a southwestward prograding deltaic complex as previously defined using paleobotanical and sedimentological evidence. The paleoecology of two invertebrate-bearing lithofacies was studied within this context. A lower shaley-siltstone lithofacies records the inundation of the Upper Cliff #1 peat-accumulating swamp/marsh by fresh-water influenced brackish to restricted marine deposits. The fauna is dominated by the inarticulate Orbiculoidea and the trace fossil Planolites. As inundation continued, an interdistributary bay developed. Diversity and abundance of taxa increased with the establishment of a molluscan dominated Pteronites-Pianolites assemblage. The assemblage consists of a low diversity and low abundance fauna of bivalves and trace fossils that suggest soft substrates with abundant organics.The overlying sandstone lithofacies consists of a basal shell-bed (Schizophoria zone) composed of rare indigenous Pteronites and Wilkingia and a transported component of open marine epifaunal brachiopods, gastropods, and trilobite fragments. The Schizophoria zone thins to the northeast, suggesting open marine conditions to the southwest. Directly above this bed, the fauna of the sandstone lithofacies is composed entirely of Zoophycos. Higher in the section, rare Pteronites and Wilkingia occur, thus this lithofacies preserves a Wilkingia-Pteronites-Zoophycos assemblage of low abundance and diversity reflecting mobile organic-poor substrates. The sandstone lithofacies is interpreted as a migrating sand body with a basal shell-bed, initially formed probably as a result of storm activity.

Cambrian Pisolites as Paleoenvironment and Paleotectonic Stress Indicators, Rattlesnake Mountain, Wyoming: ABSTRACT

Pisolitic-rich carbonates occur within the uppermost 0.5 m of the Meagher Limestone member of the lower Gros Ventre formation in exposures near Cody, Wyoming. The Meagher Limestone is overlain by 51 m, and underlain by 63 m of dark gray Gros Ventre shale. Pisolites range in size from 2.0 to 18 mm in diameter and occur in lime grainstones associated with trilobite fragments, peloids, glauconite, fine-grained subangular quartz, and minor oolites. Girvanella grainstones 15-20 cm thick directly underlie the pisolite strata and have contributed to some of the carbonate material within pisolite nuclei. Dolomite and ankerite may occur within pisolitic rocks as finely crystalline irregular patches. Pisoliths commonly show an oblate ellipsoid shape, with maximum flattening perpendicular to bedding. Long-axis to short-axis ratios of these grains in fracture planes perpendicular to bedding average between 2.5 to 3.5, with the long axis parallel or subparallel to bedding. Grains observed in bedding planes have ratios averaging between 1.5 to 2.0. A paleostress state has produced a strain ellipsoid with long-axis ratios ranging from 1.7 to over 3.0. There appears to be little or no tectonic strain on the bedding plane, so the strain can be described as uniaxial, with maximum compression perpendicularmore » to bedding. The majority of carbonate rocks in the Meagher Limestone were deposited in a normal marine subtidal setting, while ooid and pisolitic grain types are suggestive of subtidal-peritidal conditions. Because of the strain deformed pisoliths, a subaqueous versus subaerial environment of pisolite genesis is difficult to assess. A siliciclastic sandstone, 0.6 m thick with low-angle tabular crossbedding, is present immediately beneath the Meagher Limestone. The sandstone is composed of 94% fine to medium sand-size subangular quartz grains and is associated with glauconite, minor biotite, zircon, and ilmenite.« less

Depositional Framework for Lower Member of Metaline Formation (Cambrian), Northeastern Washington: ABSTRACT

The Metaline Formation is a Cambrian unit that crops out in Stevens and Pend Oreille Counties, northeastern Washington. The 985 to 1,250-m (3,200 to 4,100-ft) thick formation has been divided into lower, middle, and upper members, although structural complications and numerous covered intervals have made measurement of complete stratigraphic sections impossible. Based mainly on trilobite studies, early workers assigned a Middle Cambrian age to the 120 to 290-m (390 to 950-ft) thick member. The present study divides the member into the following five lithofacies: (1) gray mudstone, (2) ooid-arenite, (3) gray packstone-wackestone, (4) black packstone-wackestone, and (5) black mudstone facies. The gray mudstone facies grades upward from underlying fine-grained siliciclastics, and it is composed of bioturbated lime mudstones with a small admixture of thin-shelled trilobite fragments, chitinophosphatic inarticulate brachiopods, and disarticulate echnioderms. The ooid-arenite facies occurs as pods within the gray mudstone facies, and is composed of spherical quartz grains and ooids developed around quartz cores in a neomorphic spar matrix. The facies is extremely well-sorted. The gray packstone-wackestone facies is composed of ooid-oncoid packstones alternating with argillaceous, fossiliferous wackestones. Rounded intraclasts and peloids are secondary components. Fossil allochems include robust trilobite fragments, echinoderm plates, and rare (?) Epiphyton algae. The black pac stone-wackestone facies contains the same components as underlying rocks but is characterized by an increase in carbonaceous material and pyrite causing the black color. Fossils are less robust than in the underlying facies. The black mudstone facies is characterized by bioturbated lime mudstones with rare fossil fragments. The top of the member is marked by a red-stained, well-cemented zone below a disconformable contact with the middle member. The member of the Metaline Formation represents the first carbonates deposited on a ramp-type shelf margin during a major Middle Cambrian transgression. The observed lithologies suggest deposition under conditions of changing water depth, agitation, and oxygenation in the shallow subtidal zone. The mudstones and wackestones in all facies were deposited under low-energy conditions. Shallowing allowed increased agitation, and oxygenation suitable for the local development of ooids and oncoids. Periodic storms produced high-energy packstone deposits composed of concentrations of the components found in the typically low-energy subtidal zone. Along with the transgression of marine environments, oxygen-starved waters migrated shoreward from an offshore, shelf basin. This transgression of the pycnocline caused dysaerobic and anaerobic conditions in shallow subtidal waters of the upper shelf. Aerobic and dysaerobic conditions produced gray rocks, and anaerobic conditions produced black rocks rich in organic material. Biofacies were also affected by the low oxygen levels. Thin-shelled and chitinophosphatic forms dominated the epifauna when dysaerobic conditions occurred. With decreasing oxygen content, the epifauna was progressively excluded from the shallow subtidal zone. The result of maximum anoxia was the cessation of carbonate sedimentation and the formation of a submarine hardground at the end of lower Metaline de osition. End_of_Article - Last_Page 1337------------

Probable sponge borings in Lower Ordovician limestone of Sweden

Endolithic borings attributable to sponges, hitherto unknown before Devonian time, are common in trilobite fragments in Scandinavian Lower to Middle Ordovician limestones. Most borings consist of branching, anastomosing tubes, approximately 25 µm in diameter, and closed end tubes, 50 to 150 µm long and 20 to 40 µm in diameter. The tube network may close and thereby form major chambers. The small borers were probably most successful in breaking down carbonate particles where rates of bio-genie carbonate sedimentation were low.

Environmental setting of fossiliferous rocks from the uppermost proterozoic—lower Cambrian of central England

The Hartshill Formation of Warwickshire in England provides a section through the initial stages of the “Cambrian marine transgression”. The sea encroached an irregular land surface of weathered volcanic and plutonic rocks which was probably fringed by sand dunes. The first transgressive deposits reflect this provenance but later sedimentation can be interpreted as comprising a gradual retreat of the shoreline and an increase in water depth; superimposed on this trend are several short periods of regression. the transgressive coastal deposits contain meandering trails, Planolites and Arenicolites traces rather than the Skolithos and Diplocraterion assemblages found elsewhere in similar facies of Cambrian (Tommotian and younger) age. The episodes of deeper water, sediment-starved deposition correspond approximately to an early “ Cruziana facies” in laminated sands and muds, to a shelly and stromatolitic limestone facies with a Tommotian fauna and finally to monotonous offshore muds bearing the first local trilobite fragments.