Crinoidal Wackestone Research Articles

Palaeontological and sedimentological characteristics of the Changhsingian (Late Permian) platform−margin sponge reef and intraplatform skeletal mound in NE Sichuan Basin, South China: their palaeoenvironmental implications

Well-preserved fossil reef/mound-building communities and shallow microfacies have been recovered from Changhsingian platform−margin sponge reef at the Panlongdong Section and intraplatform sponge skeletal mound at the Yanggudong Section in NE Sichuan Basin, South China. The present contribution focuses on the palaeoenvironmental implication of the differentiated development processes on the platform−margin reef and intraplatform skeletal mound based on the palaeontology and sedimentology. Precise field assessments and macroscopic/microscopic qualitative observations led to the identification of five reef/mound-building community and 14 reef/mound-related microfacies in the 2 sections of Changxing Formation. In the Panlongdong platform−margin sponge reef, reef-building communities are recognized into Amblysiphonella Community, Preperonidella−Amblysiphonella Community, and Preperonidella–Sollasia Community in ascending order. The reef-related microfacies are mainly classified into calcisphere−pelletoid wackestone to packstone, crinoidal wackestone to packstone of shallow water bank environment, hypercalcified sponge bafflestone, calcareous algae−hypercalcified sponge binding framestone, hypercalcifed sponge−calcareous algae bindstone, and calcareous algae−hypercalcifed sponge rudstone of reef environment, and benthic foraminifera−calcareous algae wackestone to packstone of shallower water lagoon environment. Two mound-building communities can only be found in the Yanggudong intraplatform skeletal mound, including Preperonidella−Intrasporeocoelia Community of lower part and Preperonidella−Sollasia Community of upper part. The mound-related microfacies include calcisphere−calcareous algae wackestone to packstone, crinoidal wackestone to packstone and crinoidal packstone to grainstone of shallow water bank environment, hypercalcified sponge bafflestone and cyanobacteria−hypercalcified sponge binding bafflestone of skeletal mound environment, benthic foraminifera−calcareous algae packstone grainstone of shallower water bank environment and benthic foraminifera−calcareous algae wackestone to packstone of shallower water lagoon environment. These palaeontological and sedimentological analysis results have facilitated the classification of sponge reef and skeletal mound sections corresponding to a solitary shallow benthic zone into multiple microfacies attributed to variable environmental depositional conditions. The differentiated evolutional process of multiple reef/mound-building stages at the Panlongdong and Yanggudong sections clearly demonstrate a general shallowing-upward trend from an inner shelf setting to a supratidal and low energy lagoon environment.

Compositional changes in sigmoidal carbonate clinoforms (Late Tithonian, eastern Sardinia, Italy): insights from quantitative microfacies analyses

AbstractQuantitative analysis of sediment composition was performed on a kilometre wide section of Upper Tithonian low relief (up to 70 m), gently inclined (3° to 15°), sigmoidal carbonate clinoforms (eastern Sardinia) to identify changes in sediment composition along the slope and across the studied succession. These changes may reflect modifications of the carbonate factory and of processes responsible for sediment transport. Point‐count analysis of carbonate microfacies, Q‐mode/R‐mode cluster analysis and Spearman’s rank provided a composition‐based classification of microfacies and highlighted relationships among sediment components. The studied clinoforms are mainly composed of non‐skeletal grains (70%), such as peloids and lithoclasts, together with micrite and cements and only a limited contribution from coated grains (2%). Among skeletal grains (28%), the greatest contribution derives from a coral–stromatoporoid–encruster reef that provided 15% of the components. Crinoids, brachiopods and other along‐slope thriving biota provided nearly 5% of the allochems, whilst fragments of molluscs (gastropods, bivalves and diceratids) from the backreef sourced another 2%. The contribution of platform interior biota is negligible (1%). The association of composition‐based facies varies along the slope. The upper slope beds consist of coral‐stromatoporoid grainstone to rudstone; the middle slope deposits are dominated by encruster‐lithoclast grainstone and packstone. At the lower slope, peloidal lithoclastic packstone as well as brachiopod–crinoidal wackestone prevail. Also the association of skeletal grains changes along the slope. The encruster–frame builder association typifies the upper slope whilst encrusters characterize the middle slope sediments. In the lower slope encrusters are equally represented as the brachiopod–crinoid association. Along‐slope compositional changes evidence a scarce downslope transport of frame builders and a progressive enrichment in along‐slope thriving biota. Quantitative analysis of microfacies allowed the sigmoidal clinoforms to be grouped into six sets. Each set gathers sigmoids with a similar sediment composition. Coated grains are dominant in the first set whilst they are lacking in the overlying sets reflecting a change in the carbonate factory. Other major compositional changes among the sets concern the relative amounts of peloids, micrite, frame builders (corals and stromatoporoids) and encrusters. The contribution of peloids varies inversely to that of cements and micrite as evidenced in the third and fifth sets which, respectively, record the highest occurrence of peloids or cement and micrite. Variations in the amount of frame builders and encrusters are instead non‐linear. High percentages of both frame builders and encrusters, as recorded in the second and fifth sets, are related to low amounts of peloids and lithoclasts that probably reflect episodes of reduced background sedimentation. This study demonstrates that quantitative analysis of carbonate microfacies represents a powerful tool that can improve the reconstruction of the stacking pattern in carbonate slope successions both in outcrop and in subsurface settings.

Waulsortian-Type Bioherm Development and Response to Sea-Level Fluctuations: Upper Visean of Bechar Basin, Western Algeria

ABSTRACT An exceptionally well-developed and well-preserved upper Visean succession rich in sponge-bryozoan-crinoid mounds in the foreland Carboniferous Bechar Basin (northwestern Algerian Sahara) is described and discussed. The succession is composed of recurrent facies mosaics forming individual superposed members. An ideal member is made up of two distinct fades assemblages. The lower assemblage forms the actual mounds, and is composed of sponge bafflestone-wackestone at the base, overlain by massive sponge-fenestellid bafflestone-wackestone, and capped by massive crinoid wackestone with bedded flanks of lithoclastic wackestone. In contrast to massiveness of the mounds, the upper facies assemblage is composed of well-bedded crinoid packstone-grainstone and ooid grainstone, with local rugose coral and algal-foraminiferal banks. On the basis of benthic assemblages and nature of substrate, seven bathymetric zones are defined, and indicate that each member of the bioherm-rich formations is a shallowing-upward parasequence controlled by an asymmetrical transgressive-regressive cycle. The local curve of relative sea level for the late Visean shows 13 cycles interpreted as fourth-order eustatic sea-level changes, each cycle averaging half a million years in duration. Third-order cycles could not be identified. The platform model proposed is a distally steepened ramp, 15-20 km wide, that developed in a rapidly subsiding foreland basin. Lateral distribution of facies on the ramp depended strongly on local tectonic setting, whereas vertical development was controlled by sea-level fluctuation. The deep-water, mud-rich, sponge-bryozoan-crinoid mounds developed in 100-150 m water depths during phases of sea-level highstand, whereas deposition of shallow-water facies occurred on top of the mounds, in less than 70-80 m water depths during regressive phases. The Bechar mounds share similarities with the classical Lower Carboniferous Waulsortian mounds, but they differ in two aspects: the abundance of large sponges, which is unique, and their vertical zonation, wherein basal facies consist of sponge-dominated assemblages. The proposed model can serve as a tool in developing exploration strategies in the search for hydrocarbon reservoirs in the Bechar Basin, because the peculiar architecture of the platform may provide suitable plumbing systems for fluid migration, and the depositional signature of the mounds and associated facies is possibly recognizable on seismic profiles.

An Oil-Producing Stromatoporoid Patch Reef in the Famennian (Devonian) Wabamun Formation, Normandville Field, Alberta

The Normandville oilfield in the Peace River district of northcentral Alberta produces oil and gas from a stromatoporoid patch reef about 70 m below the top of the Wabamun Formation. Fracturing of the limestone associated with a set of northwestsoutheast striking faults has increased the porosity of the reservoir. The patch reef and its associated facies constitute the Normandville zone. The reef is 1.5 km2 in area and 10 m thick. The following lithofacies are distinguished in the zone: 1) the platform facies of nodular limestone that overlies and underlies the reef 2) the reef facies of stromatoporoid framestone and boundstone with minor rudstone, floatstone, and grainstone, 3) the reef-margin facies of stromatoporoid floatstone and rudstone with minor brachiopod and crinoid wackestone and packstone, 4) the cap facies of peloidal and skeletal grainstone, and 5) the off-reeffacies of peloidal and skeletal wackestones and packstones. The stromatoporoid fauna of five species is dominated by two species, one of Labechia and the other of Stylostroma. Stromatoporoids were reduced in numbers and diversity at the Frasniani Famennian extinction event but retained the ability to make reefs. Famennian faunas contain remnants of Frasnian genera and are marked by the resurgence of the order Labechiida. We review the world distribution of Famennian and Strunian stromatoporoids and revise generic assignments of many species. On a global scale, we find the faunas divided into three assemblages: 1) labechiids characterized by Stylostroma, 2) mixed labechiids and clathrodictyids with species of Atelodictyon, and 3) clathrodictyids without labechiids. The third assemblage is confined to western Europe and Kazakhstan in areas of the late Devonian Paleotethys ocean.

Depositional Facies of San Andres and Grayburg Formations and Their Control on Porosity Distribution and Production--Dune Field, Crane County, Texas: ABSTRACT

Dune field is located on the eastern margin of the Central Basin platform on the western side of the Midland basin. Major production is from Permian (lower Guadalupian) San Andres and Grayburg porous dolomites. The southern half of Dune field (University Land, block 30) was selected for study because of the large amount of data readily available at the University Lands Office and because a cooperative operator (Mobil) provided cores and core analyses. The study centered around the Mobil University Unit 15/16, where cores were obtained from nine wells, and logs from hundreds of wells. Cores from all wells show a similar upward-shoaling sequence with fusulinid wackestone and crinoid wackestone in the lower part, pellet and ooid grainstone near the top, and pisolite grainstone at the top of the approximately 300-ft thick section. Very thin (5 to 10-ft thick) siltstone beds occur commonly in the upper part of the section studied. Core and log correlations of the siltstones have permitted delineation of the lateral distribution of carbonate facies and recognition of a logical facies tract comprising arid island, tidal-flat, and shallow-water subtidal depositional environments. Production is highest in the eastern third of University Unit 15/16, where porosity is best developed in 20- and 30-ft-thick pellet grainstone reservoirs; porosity also occurs in the much thicker fusulinid wackestone facies. In contrast, production in the western part of the area is from a thick section of low porosity sponge and algal framestone, equivalent to the pellet grainstone and crinoid wackestone in the lower part of the eastern sequence. Consequently, the eastern and western areas essentially function as two distinct reservoirs that are located in different parts of the San Andres and Grayburg section and have considerably different performance characteristics. The nonporous siltstones probably form barriers to vertical migration of fluids throughout the field. End_of_Article - Last_Page 141------------

New Biostratigraphic and Paleotectonic Interpretation of Devonian and Mississippian Rocks in Southwestern Montana Thrust Belt: ABSTRACT

The structurally complex area of previously undifferentiated Mississippian rocks below the Meramecian Kibbey Sandstone in the southwestern Montana thrust belt of the northern Tendoy Range actually comprises an unusual combination of Devonian and Mississippian basinal, slope, platform, and nearshore deposits. The lower part of this sequence comprised two widespread western Montana formations: the Devonian part of the Three Forks Formation over 65 m (213 ft) thick, containing all three members, and the basinal Kinderhookian and Osagean Paine Limestone, 228 m (748 ft) thick. However, the Paine is succeeded by an eastern tongue of the older, Osagean part of the lower-slope Middle Canyon Formation, 266 m (872 ft) thick. This tongue consists of clinoform cherty micrite and bedd d chert, with some encrinite debris flows that increase upward in number and thickness. The overlying Osagean and Meramecian Mission Canyon Limestone comprises 102 m (335 ft) of an upper-slope encrinite lower member and 74 m (243 ft) of a mainly shelf-margin wackestone and encrinite upper member. Thus, the Mission Canyon here represents the distal part of a broad carbonate platform. Succeeding the Mission Canyon is 140 m (459 ft) of the newly named Meramecian McKenzie Canyon Limestone, which comprises a sabkha, back-mound, and lagoonal sequence of evaporite-solution breccia, micrite, dismicrite, and pelmicrite, with interbeds of crinoidal wackestone, encrinite, and bio-oopelsparite. This formation represents beds absent at an unconformity elsewhere in western Montana and western Wyoming. Thus, the sequence from the Paine through the McKenzie Canyon, constituting the newly named Tendoy Group is 810 m (2,657 ft) thick and represents one of the most complete records of Kinderhookian to Meramecian carbonate deposition in the northwestern United States, displayed in the predominantly upward-shallowing part of a eustatic megacycle. The reconstructed detailed biostratigraphy aids new structural interpretations and provides several new plays for petroleum exploration in the Montana thrust belt. End_of_Article - Last_Page 865------------

Upper Silurian lithistid sponge reefs on Somerset Island, Arctic Canada

ABSTRACTThe Upper Ludlow Douro Formation contains the first reported Silurian sponge reefs. These relatively small (5–35 m diameter), mound‐shaped structures contain, on average, 35% lithistid demosponges. Reefs are surrounded by irregular haloes of crinoid debris; abundance and diversity of all fossil groups decreases away from the reefs. Each reef is underlain by a lens of crinoid wackestone to grainstone rich in crinoid holdfasts; trepostomate bryozoans, solenoporacean algae and rhynchonellid brachiopods are locally common. The bulk of each reef consists of lime mudstone with abundant lithistid sponges. This is capped by a thin layer of wackestone with abundant tabulate and rugose corals and fewer lithistid sponges, calcareous algae, trepostomate bryozoans and stromatoporoids. This zonation, in which a sponge colonization community was replaced by a coral diversification community, is similar to that reported from some Middle Ordovician, Upper Jurassic and Holocene sponge reefs.The Douro sponge reefs were relatively low structures, with about 3 m maximum topographic relief. They grew on a broad carbonate platform, probably in warm, tranquil, turbid waters of normal or near‐normal marine salinity. Periodic influxes of terrigenous mud adversely affected reef size, and caused biotic changes. Some of the reef lime mud was derived from non‐reef sources, but significant quantities were also produced on the reefs. Reefs underwent synsedimentary lithification, bioerosion and minor storm erosion. Fabrics and compositions of sparry calcite in cavities record three generations of meteoric cementation. Originally siliceous spicules of the lithistid sponges were dissolved and the moulds later filled with sparry calcite. Early dissolution of siliceous spicules is common in reef environments, and may have caused fossil sponges to be under‐represented in ancient reefs.

Eustatic Sea-Level Control of Silurian (Niagaran) Reefs, Michigan Basin: ABSTRACT

Eustatic sea-level changes controlled Niagaran reef and off-reef facies and eogenesis both in the Michigan basin and on the adjacent platform, as shown by surface (Thornton, northeast Illinois; Pipe Creek Jr., central Indiana) and subsurface reef studies (Onandaga, south Michigan). We recognize four stages of development defined by alternating highstands and lowstands of sea level. (1) During Llandoverian-Wenlockian time, a highstand resulted in growth of reefs with 10s to 100 m depositional relief with a basal stromatactis mudstone facies capped by volumetrically dominant crinoidal wackestone to grainstone-coral boundstone facies. Reef growth was below wave base and was characterized by extensive submarine cementation. (2) A relative fall of sea level in the late Wenlock an caused a saline brine to develop in the restricted Michigan basin, halting pinnacle reef growth and resulting in A-1 Evaporite deposition and anhydrite replacement of reef fossils and sediment. This fall of sea level did not expose the shelf or bring reef tops above wave base. It may be expressed in the surface reefs as distal megabreccias containing normal marine stromatoporoid-coral-Renalcis fauna and in the subsurface reefs (basin) by a hiatal break. (3) A Ludlovian-Pridolian highstand resulted in basinal reef rejuvenation (stromatoporoid-algal boundstone facies and followed by the stromatolite facies) and dissolution of replacement anhydrite. The deep-water basinal A-1 Carbonate was deposited at this time. (4) A subsequent lowstand (Pridolian?) resulted in basinal hypersalinity, c ssation of pinnacle reef growth, and A-2 Evaporite deposition. End_of_Article - Last_Page 604------------

Resedimented Deposits and Evolution of Thornton Reef (Niagaran) Northeastern Illinois: ABSTRACT

The significance of the Thornton reef, long established as a major reef model by paleoecologic studies, is enhanced by the recent recognition that significant volumes of the reef were deposited by submarine sediment-gravity flows and that these resedimented deposits document a distinct stage of reef growth. The Thornton reef (2 km diameter) is an erosionally truncated carbonate buildup that consists mostly of flank beds that dip steeply (30 to 40°) and radially away from the reef's center. The flank beds are thin, even, continuous, and consist of syndepositionally cemented crinoid wackestone and packstone. They were deposited by in-place sediment accumulation. In contrast, the resedimented conglomerates, which comprise 10 to 20% of the reef, occur as wedges, tongues, and sheets 0.1 to 25 m thick of lithoclast/skeletal wackestone/packstone. Lithoclasts range from 1 mm to 20 m across, and are made up of wackestones and boundstones. Debris flows were probably responsible for the thick boulder-bearing tongues and sheets in the interreef facies. A more prevalent and enigmatic resedimented deposit consists of steeply dipping (to 15°) wackestone/packstone conglomerate wedges that mantle the reef's outer margin. These are interpreted as the product of grain cascades down the reef flank. Clasts in the resedimented deposits, derived from the now-eroded part of the reef are more stromatoporoid-rich and faunally diverse than the contemporaneous flank. This difference is attributed to a more energetic environment in the eroded part of the reef. There is no evidence in these clasts for lagoonal restriction, emergence, or shallow-water surf action. End_of_Article - Last_Page 957------------

Porosity Evolution of Niagaran Thornton Reef, Northeastern Illinois: ABSTRACT

The Thornton Reef has long been a model of reef sedimentology, but as thick tar occurs in the upper 22 m of the porous and deeply eroded buildup, Thornton is also a well-exposed fossil oil field. Thornton Reef is about 2 km in diameter and bowl-shaped in cross section. The reef consists of radially and steeply dipping flank beds of dolomitized crinoidal wackestone and minor coral boundstone. Reef porosity (5 to 10%) is dominantly secondary, consisting of fossil molds, vugs, intercrystal voids, and fractures. At time of deposition porosity was probably high (50 to 70%), and consisted largely of intraparticle and interparticle pores of all sizes. Abundant hardgrounds and palisade-cemented grainstones suggest major reduction of depositional porosity by syndepositional submarine and marine phreatic carbonate cementation. Secondary dolomite preferentially replaced abundant fine-grained carbonate sediment but only partly replaced fossils. Leaching removed the remaining calcitic part of the fossils, slightly enhancing porosity. Extensive fracturing of the reef began at deposition and continued throughout the reef's geologic history, producing fractures that may extend hundreds of meters laterally. The fractures may be open or filled with syndepositional carbonate or younger terrigenous sediment. Thornton Reef's superb exposures and reservoir scale assure its importance to geologists studying reef facies and porosity. End_of_Article - Last_Page 748------------