Lower Ordovician Limestone Research Articles

In situ LA-ICPMS U[sbnd]Pb dating and geochemical characterization of fault-zone calcite in the central Tarim Basin, northwest China: Implications for fluid circulation and fault reactivation

Direct evidence for fault reactivation is crucial for understanding long-term fault behavior and reconstruction of fluid circulation and hydrocarbon migration and accumulation during active tectonics. However, absolute dating of fault reactivations and fluid circulation is still challenging due to dissolution and recrystallization of fault zone materials and obliteration of previous events by the latest deformation/hydrothermal event. The lower Ordovician limestone strata at 3.8 km depth in the Central Uplift of the Tarim Basin, northwest China experienced multi-stage regional tectonic activities that developed a series of NW–SE striking reverse faults and NE–SW oriented strike-slip faults. In this study, we performed in situ laser-ablation ICP-MS UPb dating and trace element characterization of calcites collected from borehole cores within one of the main fault zones in the Central Uplift of the Tarim Basin. A new in-house calcite standard AHX-1A (209.8 ± 1.3 Ma), collected from the Tarim Basin, calibrated against well-calibrated WC-1 and ASH-15 calcite standards, is used for mass-bias correction. We obtained in situ UPb ages of 9 calcite samples from two wells within a Paleozoic fault zone in the Tarim Basin. They are 456 ± 11 Ma, 454.7 ± 7.2 Ma, 450.4 ± 6.2 Ma, 435.2 ± 9.7 Ma, 328.0 ± 9.2 Ma and 307.6 ± 7.1 Ma from well TZ2, and 371 ± 18 Ma, 390.6 ± 6.0 Ma and 395 ± 14 Ma from well TZ4 (all reported uncertainties are 2σ). Combined with petrographic evidence, these results define at least six generations of secondary calcite precipitations in the fault zone, that occurred during 456 ± 11 to 450.4 ± 6.2 Ma (weighted mean 452.8 ± 4.3 Ma), 435.2 ± 9.7 Ma, 395 ± 14 to 390.6 ± 6.0 Ma (weighted mean 391.3 ± 5.5 Ma), 371 ± 18 Ma, 328.0 ± 9.2 Ma and 307.6 ± 7.1 Ma. Chondrite-normalized rare earth element (REE) + yttrium (Y) patterns of these calcite samples show prominent positive Gd and Y anomalies, lack of negative Ce anomaly, and enrichment in light REE but depletion in heavy REE, similar to the shallow marine platform host-rock limestone. The results suggest multi-stage fluid circulation under shallow burial, low temperature, and non-oxidizing environments along the fault zone, with the secondary calcite precipitating from a similar fluid source that originated from dissolution of host-rock carbonate or early-stage diagenesis. The detail petrographical and mineralogical analysis of this study, combined with the new interpretation of high-resolution seismic data, constrains two main episodes of faulting and related uplift during 456–435 Ma and 395–371 Ma, as well as a younger faulting episode with weaker intensity during 328–308 Ma. This study demonstrates that the central fault zone of the Tarim Basin experienced ~150 Ma of episodical reactivation during the Paleozoic, which has important implications for hydrocarbon exploration.

The source of Fengjia and Langxi barite–fluorite deposits in southeastern Sichuan, China: evidence from rare earth elements and S, Sr, and Sm–Nd isotopic data

The barite–fluorite metallogenic belt in southeastern Sichuan is one of the most important barite–fluorite ore‐concentrated areas in China. In order to find the ore‐forming source and the mineralization age of the Fengjia and Langxi barite–fluorite deposit, we systematically evaluated the rare earth element (REE) contents and isotopic characteristics of S, Sr, and Sm–Nd. The results show that the δ34S values of barites and pyrites are high and similar to S isotopic values of gypsum in the Cambrian Qingxudong Formation (∈1q), suggesting that the sulphur source was from Cambrian evaporite strata. The initial 87Sr/86Sr ratios of fluorites and barites range from 0.708800 to 0.712999, and these values are similar to Cambrian carbonate rocks and Lower Ordovician limestone. Except for black shales of the Lower Cambrian Niutitang Formation, the Ba content of other strata was generally low. The REE characteristics of barite and fluorite have close relationships, in regard to the source of ore‐forming materials, to the Lower Cambrian Niutitang Formation black shales. Hence, we have concluded that the Cambrian carbonate rocks and Lower Ordovician limestone provide the source of calcium for mineralization and that the source of barium comes from the Lower Cambrian Niutitang Formation black shales. The fluorine contents of the Upper Sinian Doushantuo Formation and the Lower Cambrian Mingxinshi Formation are much higher than other strata, and these formations could provide the source for fluorine. The Sm–Nd isochron age of fluorites is 104 ± 11 Ma, which demonstrates that the mineralization mainly occurred during the late Yanshanian. Copyright © 2016 John Wiley & Sons, Ltd.

The Sub-Cambrian Peneplain in southern Norway: its geological significance and its implications for post-Caledonian faulting, uplift and denudation

The Sub-Cambrian Peneplain in southern Norway formed in the Cryogenian–early Cambrian. It was transgressed in the Cambrian–early Ordovician, and subsequently broken up by Caledonian thrusting, post-Caledonian normal faulting and regional uplift. In southern Norway the Sub-Cambrian Peneplain is generally expressed as an angular unconformity or nonconformity surface with no apparent deep weathering. In some localities the basement is characterized by a weathered top. Furthermore, the basement may have a mineralized top in some cases. The peneplain is commonly overlain by in situ regolith, and/or Cryogenian glacial diamictite, Lower–Middle Cambrian siliciclastic sediments, Lower–Upper Cambrian black shale or Lower Ordovician limestone. It has commonly mildly tectonized contacts, but occasionally displays contacts that have been completely destroyed by Caledonian thrusting and reworked into fault rocks. Tracing the peneplain across southern Norway reveals a fault-affected and fault-delineated plateau with a sloping eastern margin. Onshore cumulative offsets record vertical movements of > 2000 m. A 1–5° dip below Caledonian nappes suggests a flexural component related to tectonic overburden. As the oldest identifiable and dateable surface covering much of Scandinavia the peneplain constitutes the reference surface for studies of the younger regional morphotectonic evolution. Its shape supports the interpretation that Cenozoic uplift combined with faulting, not smooth flexural doming, caused the Southern Scandes high in South Norway.

Fractured - Vuggy Genesis Model of Middle - Lower Ordovician Carbonate Rocks in Tahe Oilfield of Trim Basin, Western China

The fracturedvuggy characteristics of carbonate rocks of Middle Lower Ordovician limestone in Tahe Oilfield was studied in the article. By analysis of the affecting factors of reservoir space include sedimentary diagenesis, tectonism and karstification, we found deposition determined the Formation and the number of primary porosity, diagenesis affected the pore destruction and construction, tectonism controled the reservoir burial history and the Formation of structure cracks and weathering fissure, karstification affected the latter change of the previous diagenetic porosity and remodeled the large hole-cave reservoir in Tahe Oilfield. Then the fissure-cave porosity evolution is studied. And we found that the Lower or dovician carbonate rocks in Tahe area experienced shallow marine diagenetic environment, shallow surface meteoric diagenetic environment, early shallow burial diagenetic environment, uplift exposed supergene meteoric diagenetic environment, shallow deep burial diagenetic environment. As a result, the Tahe Oilfield Ordovician fissure-cave porosity evolution model is established.

Mesogenetic dissolution of the middle Ordovician limestone in the Tahe oilfield of Tarim basin, NW China

The mesogenetic dissolution is well developed in the middle Ordovician Yijianfang formation (O 2yj) limestone, and the dissolution pores are very important for petroleum accumulation in the south slope area of the Tahe oilfield which lies in the north of the Tarim basin, northwestern China. Mottled, dotted or laminar dissolution can be observed in the O 2yj limestone. Under microscope, the grains, lime matrix and all stages of calcite cements (including oil-inclusion-bearing blocky calcite cements) can all be found dissolved ubiquitously. The stylolites in the limestone were enlarged and rounded because of dissolution. Some dolomite rhombs, precipitated along stylolites in burial environment, were found dissolved as well. The dissolution of the blocky calcite cements and dolomite rhombs and the enlarging of stylolites demonstrate that the dissolution took place in the mesogenetic environment. Concentration of trace elements, including REEs, of the eroded part of the O 2yj limestone is intermediate between that of the uneroded part and that of the underlying lower Ordovician limestone hydrocarbon source rocks. Both δ 13C PDB and δ 18O PDB values of the eroded part are less than those of the uneroded part, respectively. The geochemical characteristics indicate that the eroding fluids are hydrocarbon-bearing fluids coming from the underlying hydrocarbon source rocks.

Bioerosional innovation for living in carbonate hardgrounds in the Early Ordovician of Sweden

Some of the world's oldest macroborings occur in hardgrounds in lower Ordovician (Arenig) limestones exposed on the island of Öland, southern Sweden. The trace fossils, which are described here as Gastrochaenolites oelandicus isp. nov., appear to be dwelling structures excavated in the indurated substrate by invertebrates of unknown taxonomic affinity. They are the oldest examples of this ichnogenus. The appearance of a macroboring life habit at this early time represents a revolutionary new adaptive strategy for inhabiting carbonate hardgrounds. However, this innovative strategy apparently was not successful for the long term, because this particular macroboring taxon seems to have disappeared shortly after its early Ordovician appearance.

The Lower Palaeozoic of the probable impact crater of Hummeln, Sweden

A 1.2 km wide, sub-circular hole in Lake Hummeln (57°22′N, 16°15′E) has been suggested to be an impact crater. The lake has furthermore been suspected to contain an outlier of Cambrian and Ordovician sedimentary rocks. A shallow seismic survey, followed by core drilling to 164.25 m below the lake surface, demonstrated the presence of a strongly slumped succession consisting, from below, of shattered crystalline basement rock, Lower and Middle Cambrian sandstone and claystone, Lower Ordovician limestone, and Middle Cambrian clay-stone. The lithogenesis and original stratigraphy are closely similar to coeval beds on Öland 40 km to the east. Dating was done through acritarchs in the Cambrian, and through conodonts and chitinozoans in the Ordovician. An impact is postulated although no quartz with PDF has been identified in the available lithologies. The drilling had to stop, apparently without reaching the crater floor. If dated by the oldest sediments present, the impact would be Early Cambrian. A late Early or early Middle Ordovician age, however, is more likely because slumping after impact filled the crater with sediments aged Lower Cambrian through Lower Ordovician, and because the slumped sediments bear evidence of not having been originally deposited in a crater.

Palaeomagnetism of the Lower Ordovician Orthoceras Limestone, St. Petersburg, and a revised drift history for Baltica in the early Palaeozoic

Palaeomagnetic investigation of Lower Ordovician limestone in the vicinity of St. Petersburg yields a pole position at latitude 34.7°N, longitude 59.1°E (dp/dm=5.7°/6.4°). A probable primary remanence origin is supported by the presence of a field reversal. The limestone carries one other remanent magnetization component associated with a Mesozoic remagnetization event. An apparent polar wander path is compiled for Baltica including the new result, ranging in age from Vendian to Cretaceous. Ages of the published Lower to mid-Palaeozoic palaeomagnetic pole positions are adjusted in accordance with the timescale of Tucker & McKerrow (1995). The new Arenig result is the oldest of a series of Ordovician and Silurian palaeomagnetic pole positions from limestones in the Baltic region. There are no data to constrain apparent polar wander for the Tremadoc, Cambrian and latest Vendian. If the Fen Complex results, previously taken to be Vendian in age (c. 565 Ma), are reinterpreted as Permian remagnetizations, an Early Ordovician—Cambrian—Vendian cusp in the polar wander path for Baltica is eliminated. The apparent polar wander curve might then traverse directly from poles for Vendian dykes on the Kola peninsula (c. 580 Ma) towards our new Arenig pole (c. 480 Ma). The consequence of this change in terms of the motion of Baltica in Cambrian times is to reduce significantly a rotational component of movement. The new Arenig pole extends knowledge of Ordovician apparent polar wander an increment back in time and confirms the palaeolatitude and orientation of Baltica in some published palaeogeographies. Exclusion of the Fen Complex result places Baltica in mid- to high southerly latitudes at the dawn of the Palaeozoic, consistent with faunal and sedimentological evidence but at variance with some earlier palaeomagnetic reconstructions.

Accretion rates of meteorites and cosmic dust in the Early Ordovician.

Abundant fossil meteorites in marine, condensed Lower Ordovician limestones from Kinnekulle, Sweden, indicate that accretion rates of meteorites were one to two orders of magnitude higher during an interval of the Early Ordovician than at present. Osmium isotope and iridium analyses of whole-rock limestone indicate a coeval enhancement of one order of magnitude in the influx rate of cosmic dust. Enhanced accretion of cosmic matter may be related to the disruption of the L chondrite parent body around 500 million years ago.

Seismic correlation of the Cambrian sequence between Gotland and Hiiumaa in the Baltic Sea

Seismic reflection profiling in the Baltic Sea between Gotland and Hiiumaa reveals that the lower boundary of the Cambrian mainly coincides with the crystalline sub‐Cambrian peneplain here. In the Gotska Sandön area, the Cambrian sequence rests partially on Proterozoic sedimentary bedrock, whereas Vendian rocks may underlie the Cambrian at the Estonian coast. The sandy‐argillaceous Cambrian sequence is covered by Lower Ordovician limestones in the western part, and by an up to 10 m thick layer of Lower Ordovician siliciclastic rocks in the eastern part. At the end of Lontova time, the early Cambrian depocenter shifted westwards from the Moscow Basin to the Baltic Basin, prohibiting any detailed Trans‐Baltic seismic correlation. Two Cambrian reflectors extend westwards from Hiiumaa, and seven reflectors eastwards from Gotska Sandön. Only one reflector extends across the area. It is located near the base of the Cambrian sequence within the När Member shale unit in the Gotska Sandön drill core, and at the boundary between the Irbeni and Soela Formations near the top of the Cambrian sequence on Hiiumaa. Some 55 km NE of Gotska Sandön, an area of “stone‐pillar” structures divides the investigated area. These hardrock structures cross vertically through the Cambrian softrock sequence. The “stone‐pillars” are interpreted as due to secondary cementation of the Cambrian sandstone by gases and fluids percolating through the sequence along tectonic faults. A similar formation and age is proposed for the so‐called “Precambrian sandstone” of the Gotska Sandön drill core. Tuuling, I., Flodén, T. & Sjöberg, J., 1997: Seismic correlation of the Cambrian sequence between Gotland and Hiiumaa in the Baltic Sea. GFF, Vol. 119 (Pt. 1, March), pp. 45–54. Stockholm. ISSN 1103–5897.

Trilobite systematics, biostratigraphy and palaeoecology of the lower ordovician komstad limestone and huk formations, southern Scandinavia

Trilobite systematics, biostratigraphy and palaeoecology of the lower ordovician komstad limestone and huk formations, southern Scandinavia

Endolithic trace‐fossil assemblage in Lower Ordovician limestones from northern Estonia

Abstract An assemblage of microbial endoliths is described from echinoderm and trilobite skeletal grains in limestones from the Lower Ordovician (Arenig‐Llanvirn) of northern Estonia. The different morphologies indicate that the largest structures, both straight and branched (2–8 μn in diameter), belong to autotrophic organisms, most probably cyanobacteria or algae, whilst thinner forms belong to saprotrophic fungi. The occurrence of described endolithic autotrophic assemblage suggests photic conditions at the sediment‐water interface and may confirm low sedimentation rate or even nondeposition at the transition between Lower and Middle Ordovician in northern Estonia. Podhalanska, T. & N[otilde]lvak, J., 1995: Endolithic trace‐fossil assemblage in Lower Ordovician limestones from northern Estonia. GFF, Vol. 117 (Pt. 4, December), pp. 225–231. Stockholm. ISSN 1103–5897.

The Lower-Middle Ordovician transition in south-central Sweden: Phosphorite and iron ooid formation induced by volcanic ash

Abstract The transition between the Lower and Middle Ordovician in south-central Sweden marks a change from rather stable sedimentological conditions to a period with several dramatic events. The transition is accompanied by a pronounced change in the macrofauna and there are indications of a eustatic regression leading to an hiatus of about 1 My, and traces of explosive volcanism have been detected. It is suggested that elements released by the devitrification of volcanic ash caused the formation of the chamosite ooids and iron impregnations that occur at the boundary. The volcanic ash may also indirectly have caused the formation of a thin, organic-rich phosphorite resting unconformably on the Lower Ordovician limestone. The phosphorite formation, and the enrichment of organic matter, is believed to have occurred when benthic communities were buried by the ash, and reducing conditions favoured precipitation of phosphate. The phosphate could also have been derived from volcanic gases.

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

Lateral facies changes in the Lower Ordovician of northern Öland, Sweden

Abstract A method of presence/absence analysis of insoluble residues, in combination with correspondence analysis, was used in order to establish a microlithological zonation for the Lower Ordovician limestone sequence at Hälludden, northern Öland. The results from the correspondence analysis were applied to five neighbouring profiles in which similar presence/absence analyses were made. The procedure allowed unbiased comparisons between the profiles. The microlithological zones distinguished in the profile at Hälludden, were recognized also in the other profiles, and available data regarding the depositional environments at Hälludden could thus be applied to a larger area. Based on this analysis, an environmental interpretation for the Lower Ordovician of northern Öland was made. The regression in the lower part of the Hälludden sequence affected the whole of northern Öland, and the sea-floor was situated slightly higher in the northwest than in the east and south. This difference in sea-floor elevation did not, however, have any major influence on the general facies pattern produced by the regression. The relief may have been of the order of a few meters, while the regression may have been of a magnitude of up to 150 m.

Genesis of phosphatic hardgrounds in the Lower Ordovician of northern Öland, Sweden

Abstract The Lower Ordovician limestone sequence at Hälludden, northern Öland, contains phosphatic hardgrounds and phosphatic constituents such as clasts, envelopes, ooids and moulds. The phosphate accumulated in a shallow epicontinental sea with extremely low relief, and is thus difficult to explain genetically by any actualistic model, e.g. the upwelling model. An alternative model, proposed for Cambrian phosphate deposits (which accumulated under similar conditions) in northern Queensland, Australia, is applicable also to the sequence at Hälludder. The model involves the deposition of blankets of organic material on semi-emergent to emergent pavements. Diagenetic processes in the organic material resulted in the release and precipitation of phosphate. Based on this model for phosphogenesis, it is possible to infer a cyclic sequence of events which could have produced the phosphatic surfaces at Hälludden. These cycles reflect small fluctuations in sea level, with conditions ranging from submergent to semi-emergent or emergent.

Lithostratigraphy and sedimentology of a Lower Ordovician limestone sequence at Hälludden, Öland, Sweden

Abstract In order to achieve a useful lithological zonation of the Lower Ordovician (Arenig-Llanvirn) limestone sequence at Hälludden, N. Öland, a method of presence/absence analysis was used in combination with correspondence analysis. Also conventional modal analysis was tried, but proved less effective for zonation purposes. The zonation achieved at Hälludden is valid at least locally, as indicated by the results from a preliminary correlation analysis with a corresponding sequence on northern Öland (Hagudden). Based on the presence/absence analysis, the section at Hälludden is subdivided into seven more or less distinct zones representing different depositional environments. The four lowermost zones (I-IV) represent shallowing conditions, reaching subaerial conditions in zone IV, while the sediments of the overlying zones (V-VII) were laid down in a shallow to very shallow, mainly low-energy environment. There is a scarcity of readily interpretable structures and constituents in the sequence. Presence of spar, grain-support and polished glauconite grains are some of the high-energy criteria used in the study, while shallow-water conditions are recognized mainly through the composition of the biota. The sediments of Zone IV (the “ooid horizon”) display a complexity in composition and texture indicating a mixing of sediments with different affinities. Evidence of selective leaching indicates early diagenesis under subaerial conditions. The distribution of different morphologies of glauconite grains in the sequence reflects their polygenetic origin and, in the case of internal molds, probably also faunal changes. Differences in the Fe2+/Fe3+ ratio cause the color of the sediment to shift from red to grey in the lower part of the sequence. This is mainly a synsedimentary feature, related to time of exposure at the sea-bottom. The environmental models discussed assume deposition in a wide, shallow sea with very limited tidal movements.

The cyanophytic alga Girvanella in the Lower Ordovician of Hälludden, northern Öland, Sweden

Abstract Traces of microscopic filaments of the cyanophytic alga Girvanella have been found in the inferred regressive Lower Ordovician limestone sequence at Halludden, N. Oland. The algae and their mode of occurrence, as aggregates or as encrustations, are described. Their vertical range is discussed in relation to water depth and water energy. A correlation between the presence of Girvanella and limited water depth is demonstrated. The distribution of Girvanella thus supports the idea of a shallowing in the A. expansus zone in the upper part of the section. An observed association between sponge spiculae and encrusting blue-green algae, is discussed briefly.

Calcitized Tephra, Sedimentary Sills and Micro-vents, in Lower Ordovician Pelagic-type Limestone, Sweden

ABSTRACT A lower Arenigian limestone bed contains the first Lower Ordovician tephra layer to be found in northern Europe outside of the Caledonian fold-belt. The tephra is unusual insofar as the volcaniclastic fragments are preserved as calcite pseudomorphs surrounded by a thin skin of mixed-layer illite/beidellite. Structures occurring in the limestone beneath the tephra are interpreted as due to percolation of gas. These structures include forcibly injected sedimentary sills and pipes. The pipes functioned as micro-vents after sedimentation of the tephra layer and incipient sedimentation had stopped free percolation of gas through the calcareous sediment. Spar filled cavities contain evidence of early cementation. The succession of events has created structures some of which might incorrectl be regarded as intertidal.

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.