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Homogenites are homogeneous and structureless, fine-grained rocks. They have been studied extensively in marine settings and are typically interpreted as being related to catastrophic events such as earthquakes. However, studies on lacustrine homogenite-like rocks are limited. New core data from the Shulu Sag in China reveal that thick massive calcilutites exist in the lower Shahejie 3 Formation, which formed during the middle Eocene rifting stage. Based on petrographic, geochemical, and mineralogical data, these massive calcilutites are interpreted as homogenites and grouped into two types. Type A homogenite is composed of intrabasinal materials characterized by high calcite, total organic carbon (TOC) content, and δ13C value. Type A homogenite has two facies successions, the first of which is characterized by homogeneous appearance capped by an erosional interface and the second is characterized by repeated massive-laminated calcilutite successions. Type B homogenite contains abundant allochthonous materials displaying relatively high dolomite content and low calcite, TOC, and δ13C content. Type B homogenite represents the topmost finer portion of a graded megabed which includes, from base to top, grain supported carbonate conglomerates, slumped deformed portions, and debrites. The whole unit can be considered as a megaturbidite. Earthquakes are the most reasonable triggers of homogenite formation in the Shulu Sag. Type A homogenites are thought to be derived by fluidification, resuspension, and decantation of lacustrine fines due to shock waves generated by an earthquake. Type B homogenites might be deposits of distal turbidity currents and turbid clouds transformed from other gravity flows. The results of this study provide a better understanding of the formation of homogenites caused by earthquakes in a confined lacustrine basin.

Tight oil has been discovered in calcilutite and carbonate conglomerate reservoirs in the lower part of the third member of the Eocene Shahejie Formation of the Shulu Sag, Bohai Bay Basin, Northern China. Understanding the nature of lithofacies is essential to evaluating the quality of tight reservoirs. This study analyzed the sedimentary characteristics, geochemistry, and reservoir characteristics of each lithofacies based on thin-section and field-emission-scanning electron microscope observations combined with X-ray diffraction, geochemical parameter, and physical property analysis. Eight lithofacies were identified: (1) varve-like laminated calcilutite, (2) non-regular laminated calcilutite, (3) interlaminated calcisiltite-calcilutite, (4) massive calcilutite, (5) massive calcisiltite-calcarenite, (6) grain-supported carbonate nconglomerate, (7) matrix-supported carbonate conglomerate, and (8) mixed-source carbonate conglomerate. The varve-like laminated calcilutites have intrabasinal organic matter with a low Pr/Ph ratio (0.50–0.63) and high gammacerane/C31 homohopane (0.13–0.29), indicating that they are normal sediments and have good hydrocarbon generation potential. The other lithofacies are event sediments containing a certain amount of terrestrial organic matter. Laminated, organic-rich lithofacies develop organic matter pores and other storage spaces modified by organic acid. Their reservoir quality is better than that of massive or organic matter-lean lithofacies. The conglomerates containing less matrices have better reservoir quality than conglomerates with high matrix contents. The hydrocarbon generation ability and reservoir quality of lithofacies as well as the fracture density are the key factors that control the formation of tight oil reservoirs in the Shulu Sag.

Red pelagic sediments are relatively common in the Phanerozoic. They are often interpreted as products of sea-bottom oxidation during greenhouse climate showing a conspicuous alternation with black shales and thus carrying important palaeoceanographic information. The Lower Devonian (Pragian) carbonate strata of the Prague Basin, Czech Republic (Praha Formation) contain a marked band of red pelagic carbonate, up to ~ 15 m thick, which can be correlated for several tens of km. We investigated seven sections (17 to 255 m thick) of the Prague Basin using the methods of facies analysis, outcrop gamma-ray logging, diffuse reflectance spectroscopy, optical microscopy, element geochemistry, magneto-mineralogy and electron microprobe analysis. The aim was to find the mineral carriers of the red colour, investigate the stratigraphic context of the red carbonates and evaluate the local and global prerequisites for their formation. The red pigmentation represents enrichment by hematite with respect to goethite. Approximately 31% of the total reflectance falling in the red colour band represents a threshold for red coloration. The red pigmentation is carried by submicronic hematite dispersed in argillaceous pelagic calcilutite and/or inside skeletal allochems. Gamma-ray log correlation indicates that the red carbonate band developed in stratigraphic levels with low sedimentation rates, typically from 1 to 7.1 mm/kyr, which are comparable to the Mesozoic Rosso Ammonitico facies. The red beds and the whole Praha Formation (Pragian to early Emsian) are characterized by low TOC values (

Fine-grained carbonate rocks, which extensively occur in the Eocene strata in the Shulu sag, Bohai Bay Basin, North China, represent an unconventional, fine-grained carbonate reservoir. However, previous studies have ignored the complexity of the lithofacies components and their formation mechanisms. Fine-grained carbonate rocks are typical reservoirs in which hydrocarbons form and gather. A better understanding of the nature of these rocks is extremely important for evaluating the quality of unconventional, fine-grained carbonate reservoirs. Various lithofacies components were discriminated in this study with a combination of petrographic observations and carbon isotope analyses. These finegrained carbonate rocks comprise terrigenous, biogenic and diagenetic materials. Terrigenous input and biologically induced precipitation are the main sources of the materials in the lake. Five lithofacies were identified based on the observations of sedimentary features (core and thin section) and mineralogical data: (1) varve-like laminated calcilutite, (2) graded laminated calcilutite, (3) interlaminated calcisiltitecalcilutite, (4) massive calcilutite, and (5) massive calcisiltite-calcarenite. Their origins were recorded by various lithofacies components, which are controlled by the interactions of physical, chemical and biological processes. This study indicated that the lithology of the bedrocks was the key factor controlling carbonate accumulation. The tectonics and climate can influence the weathering and types of lithofacies. Primary productivity controlled the precipitation of the endogenic calcite. These factors jointly determined the abundant fine-grained carbonate rocks that have accumulated in the Shulu sag.

Although extensive studies have been conducted on unconventional mudstone (shales) reservoirs in recent years, little work has been performed on unconventional tight organic matter-rich, fine-grained carbonate reservoirs. The Shulu Sag is located in the southwestern corner of the Jizhong Depression in the Bohai Bay Basin and filled with 400–1000 m of Eocene lacustrine organic matter-rich carbonates. The study of the organic matter-rich calcilutite in the Shulu Sag will provide a good opportunity to improve our knowledge of unconventional tight oil in North China. The dominant minerals of calcilutite rocks in the Shulu Sag are carbonates (including calcite and dolomite), with an average of 61.5 wt.%. The carbonate particles are predominantly in the clay to silt size range. Three lithofacies were identified: laminated calcilutite, massive calcilutite, and calcisiltite–calcilutite. The calcilutite rocks (including all the three lithofacies) in the third unit of the Shahejie Formation in the Eocene (Es3) have total organic carbon (TOC) values ranging from 0.12 to 7.97 wt.%, with an average of 1.66 wt.%. Most of the analyzed samples have good, very good or excellent hydrocarbon potential. The organic matter in the Shulu samples is predominantly of Type I to Type II kerogen, with minor amounts of Type III kerogen. The temperature of maximum yield of pyrolysate (Tmax) values range from 424 to 452 °C (with an average of 444 °C) indicating most of samples are thermally mature with respect to oil generation. The calcilutite samples have the free hydrocarbons (S1) values from 0.03 to 2.32 mg HC/g rock, with an average of 0.5 mg HC/g rock, the hydrocarbons cracked from kerogen (S2) yield values in the range of 0.08–57.08 mg HC/g rock, with an average of 9.06 mg HC/g rock, and hydrogen index (HI) values in the range of 55–749 mg HC/g TOC, with an average of 464 mg HC/g TOC. The organic-rich calcilutite of the Shulu Sag has very good source rock generative potential and have obtained thermal maturity levels equivalent to the oil window. The pores in the Shulu calcilutite are of various types and sizes and were divided into three types: (1) pores within organic matter, (2) interparticle pores between detrital or authigenic particles, and (3) intraparticle pores within detrital grains or crystals. Fractures in the Shulu calcilutite are parallel to bedding, high angle, and vertical, having a significant effect on hydrocarbon migration and production. The organic matter and dolomite contents are the main factors that control calcilutite reservoir quality in the Shulu Sag.

V kamnolomu Crnotice je bila izkopana s sedimenti in sigo z zapolnjena jama. Analiziran je bil 1,75 m visok del zapolnitve. Profil je sestavljen iz pasovite karbonatne kamnine z interkalacijami rdecih ilovic. Te so se odložile na erozijsko/korozijsko povrsino starejse, že mocno prekristaljene sige. Pasovita in laminirana karbonatna kamnina profila je sestavljena iz prekristaljenega kalcilutita ter je podobna sladkovodnemu apnencu. Znacilna laminiranost nakazuje nastanek iz bogatih organskih filmov. Rdece ilovice sestavljajo kremen, smektit, vermikulit, gibbsit, pM kaolinit, goethit, atanas, rutil, haematit, kalcit, sljude in glinenec. Vsebujejo tudi pelete z vec Mn hidroksioksidov. Rdece ilovice so ostanki preperevanja, odloženi v z vodo zasicenem okolju. Za v profilu vzete vzorce so znacilne srednje do visoke vrednosti Jn kar je posledica velike kolicine železovih mineralov. Srednja paleomagnetna smer je za skupino normalnih paleomagnetnih polaritet enaka D = 10.6°; I = 55.0°, za skupino reverznih polaritet pa D = 173.0°; I = -31.3°. Zgornji del profila kaže reverzno paleomagnetno smer, ki jo prekinjata dve normalni magnetozoni. Sodec po razporeditvi posameznih magnetozon sklepamo, da je vrh najvisje normalno polarizirane cone primerljiv z dogodkom Olduvai (1.76/1.79 Ma) kot najmlajso možnostjo, ostali del profila pa je lahko le starejsi. A fossil cave, filled with cave sediments was open in the Crnotice Quarry. An about 1.75 m high section was analysed. Profile consists of banded carbonate rocks intercalated by red clays which was deposited on corroded/ eroded surface of older speleothems. Banded and laminated carbonate rocks are composed of recrystallized calcilutite resemble freshwater limestones. Characteristics of lamination could indicate its origin from organic-rich films. Red clays are composed of quartz, smectite, vermiculite, gibbsite, pM kaolinite, goethite, anatase, rutile, haematite, calcite, micas and feldspar. They contain pellets with Mn hydroxyoxides. That red clays are weathering products redeposited in water-saturated environment. Samples are characterised by intermediate up to high magnetic values of Jn which is caused by the presence of high amount of Fe-minerals. Mean palaeomagnetic directions are for the group of normal palaeomagnetic polarity equal to D = 10.6°; I = 55.0°, and for the group of reverse polarity than D = 173.0°; I = -31.3°. The top part of the profile shows reverse palaeomagnetic direction interrupted by two normal magnetised zones. According to the arrangement of individual magnetozones we assume, that the top of the highest normal polarised magnetozone could be correlated with the Olduvai event (1.76/1.79 Ma) as the youngest possibility, and therefore the rest of profile must be older.

The Karoo Supergroup outcropst in the mid-Zambezi Valley, southern Zambia. It is underlain by the Sinakumbe Group of Ordovician to Devonian age. The Lower Karoo Group (Late Carboniferous to Permian age) consists of the basal Siankondobo Sandstone Formation, which comprises three facies, overlain by the Gwembe Coal Formation with its economically important coal deposits, in turn overlain by the Madumabisa Mudstone Formation which consists of lacustrine mudstone, calcilutite, sandstone, and concretionary calcareous beds. The Upper Karoo Group (Triassic to Early Jurassic) is sub-divided into the coarsely arenaceous Escarpment Grit, overlain by the fining upwards Interbedded Sandstone and Mudstone, Red Sandstone; and Batoka Basalt Formations. Palynomorph assemblages suggest that the Siankondobo Sandstone Formation is Late Carboniferous (Gzhelian) to Early Permian (Asselian to Early Sakmarian) in age, the Gwembe Coal Formation Early Permian (Artinskian to Kungurian), the Madumabisa Mudstone Late Permian (Tatarian), and the Interbedded Sandstone and Mudstone Early or Middle Triassic (Late Scythian or Anisian). The marked quantitative variations in the assemblages are due partly to age differences, but they also reflect vegetational differences resulting from different paleoclimates and different facies. The low thermal maturity of the formations (Thermal Alteration Index 2) suggests that the rocks are oil prone. However, the general scarcity of amorphous kerogen, such as the alga Botryococcus sp., and the low proportion of exinous material, indicates a low potential for liquid hydrocarbons. Gas may have been generated, particularly in the coal seams of the Gwembe Coal Formation, that are more deeply buried.

The Dorud Formation (Lower Permian) in the Alborz Mountains, Iran, was sampled on the basis of changes in lithology and 185 vertically oriented thin sections and acetate peels were prepared and subjected to detailed petrographic study. Statistical sequence analysis using embedded Markov chains, combined with microfacies techniques, led to recognition of an ideal shallowing-upward sequence, which, in turn, was converted into a horizontal depositional model. This new combined approach revealed that the Dorud Formation consists of two distinct, but related, groups of microfacies; designated as carbonates and siliciclastics. The first group represents marine, subtidal to intertidal environments and includes the following microfacies: (1) a mud-supported calcarenite with calcilutite matrix; (2) a grain-supported biocalcarenite with calcilutite matrix containing Pseudoschwagerina sp.; (3) a grain-supported biocalcarenite with sparite cement; (4) a grain-supported oncolitic calcirudite with sparite cement; (5) a dolomitized arenaceous calcarenite with sparite and dolosparite cements; and (6) a grain-supported pelletoidal calcarenite with sparite cement. The second group represents a marine to fluviodeltaic environment and consists mainly of sandstone (ranging from quartz-arenite to chert-arenite) and siltstone. The following diagenetic events occurred in the carbonate microfacies: dissolution of high-Mg calcite and aragonitic shells; precipitation of syntaxial and bladed calcite cement; compaction; precipitation of blocky sparite cement; fracturing; dolomitization; a second phase of bladed calcite cementation; a second phase of blocky calcite cementation; and stylolitization. The diagenetic features observed petrographically in the siliciclastic microfacies represent changes which took place under shallow to deep burial conditions.

The Cow Head Group is an Early Palaeozoic base-of-slope sediment apron composed of carbonate and shale. Whereas coarse-grained conglomerate and calcarenite are readily interpreted as debris-flow and turbidite deposits, calcilutite (lime mudstone), calcisiltite, and shale combine to form three distinct lithofacies whose present attributes are a function of both sedimentation and early diagenesis. Shale is the most common lithology. Black, green, and red shale colour variations reflect the abundance of organic matter in the source area and oxygenation conditions of the sea bottom. In black and green shale, millimetre- to centimetre-thick, alternating dark and light laminations represent terrigenous mud turbidites and hemipelagites, respectively. The calcisiltite/shale facies is uncommon and is composed of numerous graded carbonate-shale sequences (GCSS) deposited from waning carbonate turbidites and fall-out of terrigenous muds. Some of the characteristics of ribbon and parted lime mudstones in the calcilutite/shale facies can be explained by deposition of carbonate mud from dilute turbidity currents or hemipelagic settling. Other features are diagenetic in origin. The lack of micrite in GCSS and in the interbedded shales of the calcilutite/shale facies is interpreted to reflect early dissolution of the finer carbonate from these sediments. This remobilized carbonate was precipitated locally to: lithify lime mudstone turbidites or hemipelagites; form diagenetic lime mudstone beds and nodules; cement calcisiltites; and form dolomite. Many of the calcisiltites and calcilutites were, therefore, carbonate enriched at the expense of adjacent argillaceous sediments. These attributes characterize not only fine-grained sediments of the Cow Head Group but many other Early Palaeozoic slope carbonates as well, suggesting that the model proposed here for depositionl diagenesis has wider application.

The first results on the periodicity in the carbonate/insoluble residue ratio of a (hemi-) pelagic calcilutite-marl succession 77.5 m thick, and late Berriasian, are presented. The succession forms part of the Miravetes Formation, near Caravaca, SE Spain. It starts at the base of the Calpionellopsis simplex Zone (D1) and terminates at the top of the Calpionellopsis oblonga Zone (D2). Depending on the time scale used a time interval between 1.1 and 2.5 ma is involved. The succession, comprising 360 layers, has been surveyed and sampled in detail (1275 samples). The ratio of carbonate and insoluble residue content is used as a crude measure reflecting the combined effect of carbonate productivity and terrigenous dilution. The variations in this measure are stationary in the wide sense. Spectral analysis demonstrates the presence of a dozen cyclicities. Diagenesis is not the primary cause of the cyclicities, although it may have enhanced the contrasts between individual layers. After transformation into time, several high powers of the discrete periodogram can be matched with the quasi-periodicities of the Milankovitch model. The highest peak, located at 646.4 cm, is interpreted as the short cycle of eccentricity, the peaks at 133.7, 115.8, 106.3 and 97.0 cm are associated with precession and the period of 2585.5 cm can be matched with the long eccentricity cycle. Finally, a period at 3878.2 cm within the range of low resolution matches a cyclicity between 600 and 750 Ka. The absence of significant powers at periods matching obliquity is explained by the low palaeolatitude of the area of deposition. From this interpretation of the spectrum, the time-span of the zones D1 plus D2 is estimated to fall between 1.2 and 1.5 Ma.

SUMMARY Approximately 1,200 ft. of Devonian limestone, 900 ft. belonging to the Arrow Canyon Formation and 280 ft. to the Crystal Pass Limestone, were sampled at 2 ft. intervals. A thin-section was prepared for each sample and examined petrographically. Polished specimens were examined also and a selected group of these slabs were investigated by X-ray diffraction analysis to determine their gross mineralogy. The petrographic examination involved the measurement of the particle size and determination of the frequency of occurrence of all the inorganic particles such as pellets and quartz grains. The organic elements such as Amphipora, stroniatopo- roids, calcispheres, bryozoans, ostracods and crinoids were also counted. On the basis of these measurements the samples were divided into ten microfacies, which are calcilutite, fossiliferous calcilutite, calcilutite with pocket-like structures, breccia- conglomerate, poorly sorted calcarenite, arenaceous poorly sorted calcarenite, well sorted calcarenite, arenaceous well sorted calcarenite, sandstone and bioconstructed Amphipora-stromatoporoid limestone. The raw data were subjected to a multiple correlation analysis in order to determine the inter-relationships of the microfacies. This information also made it possible to order the microfacies in such a way that the microfacies from the highest environmental energy was one end-member, and the microfacies formed in the lowest environmental energy was the other end-member. These two end-members were deposited in the deepest and the shallowest parts respectively, of the carbonate environment. The variation in relative depth was plotted against the lithological column to give the bathymetric curve. Tangents drawn to the maxima and minima of this curve defined the degree of Juctunfion of the environmental energy. The mean of these maxima and minima values was defined as the energy index. This was also plotted against the lithological column in order to trace the evolution of the environmental energy. Near the base of the measured section the degree of fluctuation is large and the energy index also has a great range in intensity. As sedimentation progressed, the degree of fluctuation lessened and the environmental energy attained a uniformly high value. The periodicity, that is apparent in the lower portion of the section for both the degree of fluctuation and the energy index, becomes less pronounced and then disappears completely in the Crystal Pass Limestone. The increasing environ- mental energy and the decreasing fluctuation in the bathymetry have been interpreted as being due to the carbonate basin being slowly infilled with sediments. The deposits were subjected to prolonged periods of reworking and occasional violent disturbances such as severe storms.

Several Waulsortian-type carbonate banks nearly 200 feet thick and about 1/4 mile long occur in the Mississippian Lodgepole formation in the Big Snowy Mountains of central Montana. Compositional layering extends through all parts of the banks, with depositional attitudes up to about 35°. The layering results from variations in the amount of skeletal material and sparry calcite diluting the calcilutite matrix. Although depositional relief was as much as 150 feet on steep slopes of lime mud and silt, no biotic constituent yet found was responsible for growth and development of the banks. Fenestrate bryozoans and crinoids are the most abundant faunal constituents, and there are minor quanities of brachiopods, mollusks, and Stromatactis. Development of the banks may have begun from separate growth centers, and in one bank unit an early bank generation was subsequently engulfed by a later stage. Major postdepositional events included the formation and filling of large dilatational fissures and of irregular cav...

All dates presented in this list are of deep-sea core samples collected from the Tongue of the Ocean, Bahamas, B. W. I. The material used for dating consisted of specific bulk calcilutite (silt- and clay-sized calcium carbonate) horizons which could provide 4 kinds of basic information: 1) rates of accumulation of bulk sediment, 2) periodicity of turbidite deposition, 3) estimates of the carbonate supply rate from the Bahama Banks to the Tongue of the Ocean, and 4) insight into the anomalous behavior of Ra (super 226) distribution in the sediments. All 28 samples were measured twice with at least a 1000-min counting period each and a minimum of 14 days between counting periods. The ages are calculated as usual by comparison to the NBS standard, but in the case of marine carbonate material, it has been taken into account the fact that sea-surface carbonate exhibits an apparent age of ca. 400 yr, or a - value of -50 + or - 12%,(where + or - 12 is the standard deviation for a single sample).

SUMMARY A Lower Devonian algal reef complex, with a basinal algal turbidite facies, offers excellently preserved autochthonous and allochthonous algal textures and structures of the following types: algal calcilutite biolithites of dense, grumous, cellular, tubular, “pelletoid” and “granuloid” textures, algal calcilutite crusts and algal-bound detritus (e.g., stromatolites), algal pisolites, oolites and circumcrusts, algal pellets and grains (in contrast to faecal and bahamite types), colonial and stem fragments, algal filaments and cells, stromatactis and birdseyes. Encrusting Foraminifera and stromatoporoid colonies were of very subordinate importance in forming the turbulent-water reefs. Protected pools, crevices, and local lagoons offered a favourable niche for calm-water Codiaceae, crinoids, polyzoans, Amphipora and some molluscs. Corals occur rarely. The wave-resistant calcilutite reefs as well as the high percentage of subsequently introduced fine “internal sediments” within the limestone-frameworks, indicate that grain-matrix-cement ratios do not necessarily reflect degree of turbulence and winnowing. The algal reefs have no effective primary porosity. Channels and cavities, predominantly horizontally oriented, were filled with internal sediments and early-formed sparite cement. Fibrous calcite cement is confined to limestones that seem to have formed in a littoral environment whereas granular sparite appears to be an intra-stratal product of a later stage.

Quantitative data on the carbonate constituents of the early Viruan (‘Middle Ordovician’) Segerstad Limestone of Sweden are given, based on modal ana lyses of thin sections. The grain size of the sediment transgressed the boundary between carbonate mud mixed with skeletal sand (micritic calcarenite), and skeletal sand mixed with carbonate mud (calcarenitic calcilutite). Increase in grain size is largely due to the increase in the abundance of echinoderm particles. The term ostraconzass is introduced for the total mass of the skeletal material produced by the organisms in a given area at a given time. The composition of the skeletal sand is completely different from that of the macrofossils in the same beds. The originally aragonitic grains constitute only one to ten per cent of the skeletal sand. This supports suggestions that during the early Palaeozoic the aragonite-calcite ratio of the ostracomass was low. Attention is drawn to some observations which indicate that the aragonite-calcite ratio of skeletal sand in the modern temperate marine shallow-water environment may be considerably lower than in tropical and subtropical seas.

Restudy of the Middle Ordovician Plattin formation of southeast Missouri to determine the mode of its northward thinning has led to the establishment of the Plattin group of four formations. The group consists of the following units, from older to younger. 1. Bloomsdale formation--oolitic and carbonate pebble-bearing calcilutite, commonly with green shale layers and beds of dolomite rock 2. Beckett formation--fucoidal fine-textured calcite limestone with many layers of intraformational conglomerate in the upper member 3. Hager formation--mainly of fine-textured calcite rock which grades northward into calcarenite and calcilutite 4. Macy formation--fucoidal calcite rock and succeeding fossiliferous calcite rock, the latter with shaly partings and a widespread metabentonite near the top Northward thinning of the Plattin group, from approximately 400 feet at Cape Girardeau to approximately 125 feet in southern Lincoln County, is by convergence within units and offlap and overlap of units. The Beckett formation overlaps the oolite-bearing Bloomsdale. The Hager formation disconformably offlaps the Beckett. The Macy formation thins by the reduction of the fucoidal part; the upper member is essentially plate-like. The Plattin has been classified as Bolarian, Black River in age, and correlated with the Platteville of the upper Mississippi valley. Faunules from the upper Macy suggest that those beds are lower Trentonian. Lower formations resemble the Bolarian in central Kentucky and central Pennsylvania.

The basal siltstones of the Edwardsville Formation of the Borden Group in southern Indiana contain almost tabular bodies (2 miles long by 60 feet thick) of crinoidal limestones showing abrupt lateral contacts with the contemporaneous and surrounding siltstones. Microscopic investigation of these bodies, which are thought to have developed in shallow depressions of the sea bottom, shows that they consist essentially of numerous alternations of coarse bioaccumulated crinoidal limestones and partially dolomitized calcilutites almost entirely devoid of crinoids. The coarse beds, deposited in place, were protected from any clastic influx by the screening effect of crinoidal growth, whereas the calcilutites, interpreted as accumulations of algal dust generated by phytoplankton corresponded o definite encroachments of the surrounding clastics. The temporary proliferation of the phytoplankton could result from the increased CO2 concentratration produced by the metabolism of the crinoids in combination with reduced circulation created by their growth. The algal precipitation of calcium carbonate would then make the environment lethal to the crinoids, and the related disappearance of the screening effect would allow an encroachment of the clastics, restricting the calcilutite area available for the next cycle of crinoidal growth. Repetition of this mechanism would lead to the gradual disappearance of the crinoids and to the burial of the carbonate body in the clastic sediments.

A detailed description is given of a band of peculiar calcareous concretions in the Jet Rock of the Yorkshire coast. Two separate phases of segregation and interstitial crystallization of CaCO3 in bituminous shale have produced two distinct sets of calcite siltstone concretions. The earlier concretions are highly irregular in shape and are associated with small bodies of calcilutite, which exhibit a variety of structures including veining, colloform banding and microbreccias. A complex diagenetic history is deduced.

The Key Largo Limestone is a Pleistocene reef limestone that forms the upper Florida Keys. It consists of an organic framework of coral colonies and an interstitial skeletal calcarenite, both of which are homogeneous throughout the upper part of the formation. The Pleistocene reef was formed chiefly by massive and rounded coral heads, the principal frame builder being Montastrea annularis. No organic zonation is visible perpendicular to the long axis of the reef. Both a well-sorted and a poorly sorted calcarenite facies can be distinguished. The poorly sorted facies, which predominates, surrounds coral heads and is characterized by its high calcilutite content. The well-sorted facies forms channel deposits between coral buildups. In organic composition, the ancient Key Largo reef is roughly equivalent to the deep-water Montastrea annularis zones of living West Indian bank reefs and to West Indian patch reefs. It grew under semi-restricted conditions, possibly as a series of coalescing patch reefs, but more probably below the surf zone in op n water, where it spread laterally over large areas of the sea floor. The constituent composition of the two calcarenite facies indicates that aragonite and high-magnesium calcite originally were the chief mineralogic components. Fossils that initially were high-magnesium calcite have been altered in place to low-magnesium calcite through the removal of excess magnesium by fresh water in the meteoric zone; such fossils have otherwise remained intact. Aragonit, on the other hand, currently is being dissolved in the meteoric zone, and the dissolved CaCO3 is being redeposited near the site of dissolution as low-magnesium calcite cement. Many fossils that originally were aragonite now remain only as molds of sparry calcite. The rate of aragonite dissolution and calcite cementation is accelerated by artificial exposure. The diagenetic fabrics exhi ited by the Key Largo Limestone should be useful in recognizing ancient limestone units that were subjected to subaerial exposure soon after deposition.

The Jurassic Reynolds oolite is the upper member of the Jurassic Smackover Formation in southern Arkansas. This carbonate unit is widespread, relatively uniform in thickness, and has definite patterns of regional and local lithic changes. The patterns are analogous with those of carbonate sediments now being deposited in the Bahama Islands. Lithically, the Reynolds and Bahamian oolites are similar. In southern Arkansas the Reynolds oolite member is shown to contain five facies: (1) calcarenite-carbonate mudstone facies composed of fine-grained to silt-size carbonate particles in a microcrystalline (carbonate mud) matrix; (2) interbedded facies characterized by a predominance of calcilutite interbedded with lesser amounts of oolitic limestone; (3) grapestone facies compose of pellets and oolites aggregated or encrusted into lumps resembling bunches of grapes, in a matrix of calcarenite-carbonate mudstone; (4) modified grapestone facies composed of essentially the same rocks as the grapestone facies but with a pitted or honeycombed appearance resulting from solution of the oolites and pellets; and (5) oolite facies composed predominantly of oolites and pisolites of various sizes cemented by crystalline calcite. The Buckner Formation overlies the Smackover Formation and is largely evaporitic and nonmarine. Lateral changes in Buckner lithology parallel facies changes in the Reynolds oolite member.

The Scurry reef of Scurry County, Texas, is a subsurface feature 23 miles long and 4-8 miles wide that is of Permian and Carboniferous age. The reef is a low mound composed chiefly of clastic organic limestone with minor amounts of black bituminous claystone. The reef limestone consists of three main textural types: calcilutite, calcarenite, and calcirudite. The calcilutite is composed mainly of particles too fine to be resolved under the petrographic microscope. Part of this fine material appears to be comminuted organic debris; the remainder may be chemically precipitated ooze. The calcarenite forms a gradational series between sorted, rounded debris in clear crystalline calcite cement and unsorted, angular debris in a matrix of lithified lime mud. The calcirudite is composed of angular, granule- to boulder-sized fragments in a matrix of lithified lime mud or lime sand. The fragments consist of organic debris and pre-existing reef limestone and claystone. The limestone contains minor amounts of secondary calcite, dolomite, and chert. Characteristic structures in the limestone are stylolites, joints, irregular fractures, and slump structures. The vast bulk of observable porosity was formed by leaching solutions. This condition may have been accomplished principally by the enlargement of primary openings through the removal of the least stable constituents. The Scurry reef was formed from the hard parts of organisms that lived, died, and were buried in essentially one locality. The two major faunal elements that could be identified in the reef are the Crinoidea and the Foraminifera, principally Fusulinidae. Corals constitute a minor element and are not numerous enough to be important here as reef-forming organisms. It is suggested that the welding of lime mud and the constant precipitation of calcite cement rigidified the accumulating material and thus acted as a substitute for the rigid organic framework that characterizes many reefs.

Massive pure calcilutite mounds in the Tully Limestone contain the distinctive carbonate spar-filled, smooth-bottomed, unsheltered cavities known as stromatactis, for which this report offers a possible inorganic origin. Contrasting with continual uniform compaction observed in more argillaceous sediments, compaction in the Tully mounds apparently was nonuniform, local, and occurred only during initial unmixing of water from an originally partly water-supported accumulation of pure lime mud. Sediment collapse in local areas of loosely packed lime mud formed cavities bounded along the top by irregular surfaces that separated more tightly packed coherent mud above, from loosely packed incoherent mud below. The collapsed mud flowed until supported, either in place or through a network of connected voids, and left water-filled smooth-bottomed cavities that are called stromatactis when filled with spar. Shapes of loosely packed zones, positions of larger grains and so forth, contributed to differences apparent in stromatactis. Some differences in size, associated structures, and spar mineralogy are related to stage of dewatering of the mound in which the cavity was preserved. Many small cavities were filled early by various forms of calcite spar. Large connected cavities containing pebbles of more coherent mud and fossil fragments served as channels for later water movement; these and even later desiccation cracks joining them were filled subsequently by ferroan dolomite spar. This hypothesis of stromatactis formation may explain the occurrence of similar features in other pure calcilutite mounds, and obviates the need for postulating either total obliteration of rigid skeletons or decay of soft organisms as a cause where no definite evidence exists.

Field examination of the Carboniferous Limestone at Nant y Gamar, southeast of Llandudno on the coast of North Wales, indicates that fenestrate bryozoa are the major organic components of a shoal or knoll-like structure in limestone of S2 age. The limestone is a bryozoan calcilutite consisting of large, intact or occasionally broken fenestrate Bryozoa in a matrix of dark brown microcrystalline calcite containing fine grained crinoid and brachiopod fragments, Foraminifera and calcispheres (Biomicrudite, Folk, 1959). The bryozoan limestone is flanked by crinoidal calcirudites, slightly quartzose bioclastic calcarenites with matrices of microcrystalline calcite (respectively Biomicrudite and Biomicrite, Folk, 1959), and intraclastic calcirudites with calcite cement (Intrasparudite, Folk, 1959). In view of the structure and lateral variation of facies, it is tentatively proposed that the bryozoan rock unit represents a shoal or possibly a fore-reef deposit of a small reef-knoll. The bryozoa do not appear to be in a growth position, or to form a rigid framework, but seem to represent a loose mesh of zoaria derived by transportation or scavenging action. It is assumed that they were continually derived from a nearby reef or broken in situ, and entrapped sediment transported by currents in shallow water, thus forming a prominence. The prominence grew by accumulation, and resulted in a shoal or knoll-like structure.

Zones of breccia within the Madison Group of southwest Montana consist of very poorly sorted angular fragments of calcilutite or silty limestone, which are set in a fine grained matrix of crushed "rock-flour" and cemented by calcite. In places where the matrix was silty or sandy it weathers yellow-brown and contains abundant authigenic quartz crystals. The breccias are confined stratigraphically to zones which may be correlated with evaporites in subsurface sections. The upper contacts of the breccia zones are indistinct but the lower contacts are well defined; in some localities, brecciation occurred in place with little displacement of the fragments. The brecciation resulted from the collapse and crushing of non-soluble strata after the solution by ground water of beds of evaporite.

End_Page 2168------------------------------The modern oolite, coral reefs, calcarenite, calcilutite, and beach rock are cemented with a CaCO3 cement which consists of crystals of aragonite. Experimental evidence supports the view that the crystals of aragonite are formed by organisms which are thought to be actinomycetes. The Floridan and Puerto Rican beach rocks are formed in tropical and subtropical climates in littoral and supralittoral environments and are confined to high-energy areas where there is daily agitation of water and a supply of fresh nutrients. The Floridan beach rock occurs as an intermittent shallow shelf on the Atlantic coast from Key West to Key Largo and along the east coast from West Palm Beach to Jupiter. However, the Puerto Rican beach rock is primarily confined to the north side of the island where the formation of beach rock may be related to the path of the currents which bring nutrients to the surface. The aragonite in these sediments is most likely of a biogenetic origin. However, the partial to complete alteration of the cementing agent to calcite is by penecontemporaneous solution and redeposition. In the alteration of aragonite to calcite, some of the CaCO3 is removed in solution. Consequently, voids are formed within the matrix of the rock and the resulting rock can be very porous because of the development of microporosity. End_of_Article - Last_Page 2169

The Rundle Group (Mississippian) of Western Alberta consists of a predominantly clastic carbonate sequence exhibiting variations in texture and composition which, when analyzed by petrographic methods, reveal rhythmical changes of environments of deposition and provide means for fine correlation. More than 800 thin sections were studied from three sections in the front ranges of the Rocky Mountains of the Ram River area. The indices of clasticity and frequency were determined for crinoid fragments, pellets, oolites, and quartz grains, and curves for each of the parameters were plotted. The presence of arenaceous foraminifera, bryozoan fragments, algae, sponge spicules, and echinoid spines was recorded. The statistical data, combined with the distribution of the detrital matrix and biochemical cement, reveal five microfacies which represent different environments of deposition, ranging from a shallow lagoonal to a deep open environment. Microfacies 1 is the deepest and consists predominantly of small crinoid fragments and sponge spicules in a fine bioclastic and argillaceous matrix. Microfacies 2 represents a more shallow environment which is transitional between microfacies 1 and 3, the latter consisting of oolites and well rounded, well sorted crinoid fragments and pellets in a clear calcite cement. Microfacies 4 consists almost entirely of interlocking crinoid fragments, whereas microfacies 5 is characterized by calcilutite with scattered crinoid fragments and oolites. Locally thi microfacies may be entirely pelletoidal. The variations of the parameters of the measured components in relation to each other produce a consistent rhythmic pattern of curves when the microfacies are superposed in a succession from 1 to 5 or 5 to 1, indicating that such a succession expresses an ideal cycle, which has been interpreted laterally as a lagoonal environment (microfacies 5) grading into a zone of crinoid colonies and agitated water (microfacies 4) which in turn grades into a succession of environments of gradually deepening water in which mechanical distribution of the particles is predominant (microfacies 3 to 1). The ideal cycle was used for the analysis and construction of a relative bathymetrical curve for each section. General trend curves were drawn to stress the over-all evolution of the sequences, on the basis of which correlations were made, which afford more detail than megascopic field correlations and are in general agreement with them. A comparison of corresponding microfacies of the three sections indicates an alternation of two conditions of the sea floor: either a slope in which the eastern part was consistently more shallow than the western or uniform depth conditions.