Microfacies Associations Research Articles

Microfacies associations in Asbian carbonates: an example from the Urswick Limestone Formation of the southern Lake District, northern England

Abstract Semi-quantitative analysis of allochems from the Urswick Limestone Formation (Asbian) of the southern Lake District area of northern England has revealed a distinctive cyclicity of the microfacies. Cycle-top grainstone microfacies contain an algal flora comprising Koninckopora, Anatolipora and Polymorphocodium , with Girvanella filmaents and Ortonella lumps. Other allochems include intraclasts, large peloids and thick-shelled bivalves and gastropods. The middles of cycles are mostly packstones and micro-grainstones and contain allochems dominated by small peloids and the algae Kamaeena, Kamaenella and Epistacheoides , with the microproblematicum Ungdarella and relatively high abundances of micritic-walled formainifera such as endothyrids. Cycle bases contain a diverse algal assemblage including Coelosporella and Stacheoides , with other allochems represented by trilobites, ostracodes, Saccamminopsis , foraminifera such as Archaediscidae, the base late Asbian guide Howchinia , the base early Asbian guides Gigasbia gigas and Vissariotaxis , bivalves, small gastropods, bryozoans, sponge spicules and bored grains. Other allochems are found throughout most cycles, decreasing only in the very shallowest (intertidal) facies, or have an irregular distribution, and include brachiopod debris, crinoid ossicles and coral fragments. There are significant variations in allochem distribution according to palaeogeography. Close to the shelf margin there are higher abundances in the cycle top grainstones of the algae Koninckopora and Anatolipora , and also of the calcified filaments Girvanella and Ortonella , with thick-shelled gastropods, intraclasts and coarse peloids. At cycle bases, echinoderm arm plates and bryozoans are particularly abundant in packstone-wackestone textures. Platform interior facies are differentiated into a diverse open-marine type, with a high total abundance of bioclasts in the cycle base pack-wackestones including trilobites, Coelosporella, Stacheoides, Kamaena and bored grains, grainstones are dominated by small peloids, Kamaenella and Ungdarella ), and a more restricted cycle type, in which total bioclast abundances are low.

Trends in the development and cyclic patterns of middle and upper devonian buildups

A comprehensive review of Middle and Upper Devonian reefs, based on about 350 papers describing the reef development within 44 areas all over the world, resulted in the recognition of four reef megacycles (Upper Eifelian; Givetian; Uppermost Givetian to Frasnian; Upper Frasnian to Lower Famennian). These megacycles are characterized by the occurrence of typical reef types and by characteristic microfacies associations. Twelve microfacies associations can be distinguished using lithological, paleontological and microfacies criteria; each microfacies association includes several standard microfacies types.

Morphology, faunas and genesis of ordovician hardgrounds from Southern Ontario, Canada

We have used associations of different microfacies to define facies (or microfacies associations) which form reasonably well-defined sequences, which we infer, from analogies with recent and ancient carbonate environments, to have been deposited in a shelf environment characterized by small-scale topographic differentiation into shoal, slope and basinal environments. Shoal environments are characterized by typically cross-bedded, well-sorted bioclastic sands, with intershoal areas consisting of interbedded bioclastic sands and heavily bioturbated finer-grained carbonates. Slope and “basinal” environments are typically represented by “proximal” and “distal” cycles respectively. These we compare with deposits of carbonate ramp bypass channels, and with the more thoroughly studied deep-water clastic submarine fans. Many of the strong variations in environmental energy in these proximal and distal cycles can be attributed to migration of channels on the fans and the effect of funnelling of storm surges down the channels. Although hardground morphology and faunas are mostly related to local effects such as intensity of scouring, time of exposure, topographic differentiation of the surface and other factors, differing hardground types tend to be found in different environments. Smooth and rolling hardgrounds occur in the deeper distal environments, where the beds were subject to only slight scour and often limited exposure before renewed sedimentation. Hummocky and undercut hardgrounds are characteristic of the middle parts of proximal cycles, where they developed marginally to the main bypass channel, and in intershoal areas. Both these areas are sites of intermittent sedimentation and moderate turbulence, where cemented beds may be exposed for some time in environments optimal for attached benthos. These hardgrounds usually contain the most diverse hardground biotas. Pebbly and reworked hardgrounds occur in coarse, basal units of proximal cycles, which are interpreted as the grain-flow fillings of the central parts of bypass channels, though isolated examples occur in intershoal areas and in the higher parts of proximal channels. These hardgrounds contain low-diversity faunas, reflecting the stresses imposed by intermittent or constant abrasion; though some contain more diverse faunal assemblages formed after redeposition.

Hardground Petrography and Carbonate Microfacies: Paola Limestone (Upper Pennsylvanian), Southeastern Kansas: ABSTRACT

The Paola Limestone (Missourian) of the Mid-Continent region is the basal carbonate member of the Iola Formation (Kansas City Group). The Paola is a thin (1 to 3 ft; .3 to .9 m) massive layer of bioturbated, fossiliferous (algae, crinoids, and foraminifers) calcilutite containing abundant phosphatic nodules. This distinctive limestone is, according to previous investigators, correlative from Nebraska, southward, into northeastern Oklahoma. The Paola Limestone is overlain (in ascending order) End_Page 447------------------------------ by the Muncie Creek Shale and Raytown Limestone. In a NE-SW outcrop trend across Allen County, Kansas, the Paola Limestone forms the initial substrate on which a phylloid algal buildup developed within the Raytown Limestone. The Paola consists of three distinctive carbonate microfacies (described below). Microfacies 2 overlies microfacies 1; this microfacies association occurs only beneath the phylloid algal buildup. Both exhibit petrographic features indicative of submarine lithification. Northeastward, away from the phylloid algal buildup, microfacies 1 and 2 change abruptly into microfacies 3. Microfacies 1 is a moderately bioturbated pyritized calcilutite with Archaeolithophyllum crusts, Hikorocodium, Tetrataxis, Tuberitina, and low-spired gastropods. This microfacies has a highly irregular (scoured) upper surface that is encrusted by Nubecularia, Archaeolithophyllum lamellosum, and bryozoans and locally penetrated by borings. Microfacies 2 consists of profusely bioturbated, matrix-supported, crinoidal-fusulinid biocalcarenite. Large, bean-shaped, algaloid concretions of Nubecularia and Archaeolithophyllum lamellosum are common accessory components. The large, ramose burrow networks are infilled with microcrystalline dolomite and scattered phosphate nodules; small rugose corals also occur in the burrow fills. Microfacies 3 is a crinoidal-pelletoidal biocalcarenite containing Archaeolithophyllum crusts. Composita, oncolites, productid brachiopods, small gastropods, fenestrate bryozoans, brachiopod and echinoid spines, Nubecularia-encrusted bioclasts, ostracods, and neomorphosed pelecypods shells are accessory components. Baroque dolomite occurs as a filling within phylloid algal blades. Bioturbation textures are present, but sparse, relative to microfacies 1 and 2. Prior to lithification, the hardground (microfacies 1) was bioturbated; following lithification it was scoured, encrusted, and bored. The lithification of microfacies 1 is inferred to have occurred in a submarine environment because: (1) it contains a fauna of encrusting marine organisms and (2) petrographic features indicative of subaerial exposure are lacking. Microfacies 2 is interpreted as a firm ground. Microfacies 3 represents a normal, shallow marine subtidal environment. The recognition of ancient hardgrounds allows a more thorough understanding of the sedimentologic, paleoecologic, and diagenetic histories of carbonate sequences. Submarine diastems also have potential as chronostratigraphic markers. Because petroleum accumulations are commonly associated with diastems, an awareness of these features could provide insights for the location of some obscure hydrocarbon traps. Additionally, hardgrounds can create intraformational permeability barriers; the recognition of such reservoir heterogeneities is essential for optimum hydrocarbon recovery. Detailed petrographic analysis is a prerequisite to the location and understanding of ancient hardground sequences. End_of_Article - Last_Page 448------------

History of a Late Carboniferous phylloid algal bank complex in northeastern New Mexico

The growth history of a phylloid algal biohermal complex of Late Carboniferous (Westphalian) age, outcropping within the La Pasada Formation in northeastern New Mexico, U.S.A., is described in relationship to paleogeography, biostratigraphy, and microfacies associations. This phylloid algal biohermal complex occurs within typical Late Paleozoic cyclical sediments, in a paleogeographic setting along a narrow shelf margin bordering a deep geosynclinal trough to the northwest. Fusulinid foraminifers suggest that the bioherm is of Middle Pennsylvanian (lower Des Moines) age, whereas the overlying sediments are of middle Des Moines age. Both field and petrographic evidence suggest that the bioherm grew upward from relatively shallow water depths, into extremely shallow water where it probably was subaerially exposed. Biohermal growth appears to have been initiated on a ‘hard-ground’ surface on which lithified clasts and abundant clusters of robust linoproductid brachiopods served as a foundation. Four major microfacies have been identified from the biohermal complex. These are: (I) phylloid algal wackestones-packstones (mound proper), (2) marly limestones (sediments laterally adjacent to and overlying mound), (3) pelletal-foraminiferal wackestone (partially capping the mound), and (4) crinoidal packstones-grainstones (post-mound). Comparison is made with other surface and subsurface occurrences of phylloid algal complexes of similar age in the southwestern United States.

Stratigraphy and Microfacies of Type Magwa Formation of Kuwait, Arabia; Part 2: Mishrif Limestone Member

The upper part of the type Magwa is correlated with the type Mishrif Limestone of southern Iraq, whereas its basal part is equated with the type Rumaila. Therefore both the Mishrif and underlying Rumaila are included as two members within the Magwa Formation, inasmuch as these two rock units, although representing different limestone facies, grade into each other, interdigitate, or change laterally into one dominant facies. A thin-section study of the Mishrif Limestone Member of the type Magwa Formation has led to the recognition of two distinctive microfacies associations: a lower alveolinid, dicyclinid, algal clayey or marly limestone with intercalating shaly bands and some recrystallization, pyritization, or dolomitization; and an upper skeletal, micritic, chalky limestone (coralline, algal, bryozoan, rudistid) with a few benthonic foraminifers and very few planktonics. The latter assemblage leads us to postulate an agitated, littoral-to-supralittoral marine environment for the deposition of the Mishrif Limestone Member, followed by regression to near emergence. The rich collections of fossils prove the late Cenomanian age of the basal Mishrif and leave a possibility that the upper Mishrif can be earl Turonian. The data substantiate the existence of the Wasia/Aruma unconformity, separating the Magwa from the overlying Gudair Limestone.

Stratigraphy and Microfacies of Type Magwa Formation of Kuwait, Arabia; Part I: Rumaila Limestone Member

The upper part of the type Magwa is correlated with the type Mishrif Limestone of southern Iraq, whereas its basal part is equated with the type Rumaila. Therefore both the Mishrif and underlying Rumaila are included as two members within the Magwa Formation, inasmuch as these two rock units, although representing different limestone facies, grade into each other, interdigitate, or change laterally into one dominant facies. A thin-section study of the Mishrif Limestone Member of the type Magwa Formation has led to the recognition of two distinctive microfacies associations: a lower alveolinid, dicyclinid, algal clayey or marly limestone with intercalating shaly bands and some recrystallization, pyritization, or dolomitization; and an upper skeletal, micritic, chalky limestone (coralline, algal, bryozoan, rudistid) with a few benthonic foraminifers and very few planktonics. The latter assemblage leads us to postulate an agitated, littoral-to-supralittoral marine environment for the deposition of the Mishrif Limestone Member, followed by regression to near emergence. The rich collections of fossils prove the late Cenomanian age of the basal Mishrif and leave a possibility that the upper Mishrif can be earl Turonian. The data substantiate the existence of the Wasia/Aruma unconformity, separating the Magwa from the overlying Gudair Limestone.