Search

The Esquias Formation of central Honduras is a thick sequence of carbonate rocks conformable within the upper part of the Valle de Angeles Group. The usage of the term "Esquias" in stratigraphic literature on Central America has been ambiguous and confusing. Some workers even have doubted that the Esquias exists. Recent studies in and near the type area of Esquias, Honduras, have clarified the nature, age, and stratigraphic relation of the Esquias Formation and constitute a basis of formal redefinition. The Upper Cretaceous Valle de Angeles Group is dominantly a succession of red, mixed terrigenous and marine strata that conformably overlie carbonate rocks of the Lower Cretaceous Yojoa Group. These red strata are about 3,000 m thick and are distributed widely throughout Honduras. The only named formation within the Valle de Angeles Group is the Esquias Formation, which essentially divides the Group into upper and lower redbed sequences. At its type locality the Esquias Formation is a 470-m thick sequence of calcilutite, marlstone, and calcarenite. Lithically it is similar to, although somewhat more argillaceous than, limestones of the Yojoa Group exposed in the same area. Collections of molluscan and echinoid faunas indicate that the Esquias Formation is probably Cenomanian in age. > Interpretations of stratigraphic relation, petrology, biofabric, and faunal assemblage suggest that the Esquias was deposited under shallow, nearshore marine conditions, probably within lagoons and marine embayments that encroached across a complex paleogeographic surface. The region simultaneously was being subjected to early phases of Laramide orogenesis. The Esquias Formation seems to be an enigmatic synorogenic transgressive-marine sequence, and thus reflects the Mesozoic tectonic evolution of northern Central America.

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

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.

Small hills, 10-20 km. across, rise 150-350 m. above the Sigsbee abyssal plain in the Gulf of Mexico The abyssal plain received a thick layer of silty gray lutite during the Wisconsin sea-level lowering where as the knolls are covered primarily by foraminiferal ooze and calcilutite; thus they stood above the abyssal plain for the period of time exceeding that required to deposit their sediment column. By extrapolation beyond the maximum C l4 date in cores from the top of the knolls, at least 200,000 years is estimated as required for deposition of the top-capping sediments. At least 20 structures are present, called the Sigsbee Domes, and as investigated by the seismic reflection profiler, most do not reach the ocean floor, and are judged to be piercement salt domes. Each of the buried domes underlie small knolls on the sea floor; the observed rate of distortion calculated above for a knoll 200 m. high is about 1 m. per 1000 years or 1 mm. per year.

An unusual succession of fine clastic, feldspathic coarse clastic and cherty strata, and correlative coarse clastic rocks in a 170-mile-long band along the northern front of the Brooks Range in northern Alaska is named the Nuka Formation. The succession is more than 6,500 feet thick. It is composed of 13 members that are characterized by light-colored arkosic to calcarenitic layers, by dark-colored layers of mud shale to calcilutite, or by varicolored chert and cherty shale. Rhythmic repetition of the three main rock types may represent depositional cyclicity. The succession is overlain by Jurassic (?) graywacke-type strata; the base, though not exposed, may be gradational with the Lisburne Group (Mississippian). Fossils in the upper half are Permian in age and those in t e lower third are Late Mississippian; no diagnostic Pennsylvanian fossils have been recognized. Away from the type area, the Nuka consists predominantly of arkosic and calcarenitic rocks that contain fossils of Permian age. The feldspathic rocks indicate that a granitic provenance in northern Alaska spread over a considerable area during Permian time and was at least periodically exposed locally for the estimated 80 million years of Late Mississippian to Permian time. Granitic detritus and rocks deposited under varied and active conditions, rather than in a stable shelf environment, differentiate the Nuka Formation from other upper Paleozoic units in the region. These contrasting facies are now juxtaposed, probably as a result of large-scale structural dislocations. The Nuka may extend into the subsurface toward the north and have characteristics favorable to the accumulation of petroleum.

The Recent (last 10,000-11,000 years) geologic history of the northeastern corner of the Gulf of Mexico, i.e., western and southern continental shelves of peninsular Florida, is recorded by the character and stratigraphy of outer-shelf and nearshore deposits. These deposits chiefly reflect the interplay of a generally rising sea level and the proximity of sources of terrigenous detritus, especially detrital quartz. For example, seaward of west-central Florida the outer shelf is essentially a bedrock surface overlain by a thin veneer of bioclastic sediment and biogenic reef growths that initially formed in a shallow nearshore environment. In contrast, the inner part of the shelf is flooded with shelly quartz sand or silt. Some of this detrital debris has been transferred t the shore to form prisms of quartzose beach sand, tracks of prograding beach ridges, and high coastal dunes. The quartz is chiefly derived from reworking of residual shelf and terrace deposits and drowned coastal plain sediments of Pleistocene age. Sources of detrital quartz disappear toward the south; consequently the inner belt of quartzose deposits narrows and becomes increasingly mixed with shell debris and finer calcilutaceous components in this direction. As an important constituent of shelf sediments, detrital quartz essentially vanishes by the latitude of Cape Sable (25°15^primeN). Attesting to this, the carbonate content of unconsolidated sediment in Florida Bay (just south of the cape) averages nearly 90 percent. This sediment is primarily composed of comminuted molluscan foraminiferal, and algal debris, 80-85 percent of which consists of "metastable" aragonite and high-magnesian calcite. The calcarenitic and calcilutaceous deposits of Florida Bay are as much as 4 m thick and overlie a thin stratum of fresh-water peaty and calcareous sediment resting on a karsted bedrock surface of Pleistocene age. The basal fresh-water deposits have a radiocarbon age of approximately 4,000 years, which implies that sea level at this time was about 4 m lower than its present position. Also beginning about 4,000 years ago marine water slowly inundated the western margin of the fresh-water swamps of the Everglades, thereby providing the necessary paralic environment for the growth of the magnificent coastal mangrove forest and swamps of southwestern Florida. Strata underlying submerged waterways, intra-forest bays, and tidal channels of the swamps form a simple transgressive sequence con isting of a basal fresh-water unit of peat and calcitic mud, a middle unit of paralic and brackish-water peat, and an upper marine unit of organic-rich quartzose sediment or shell debris. Deposits underlying the floor of the mangrove forest, or associated salt-grass marshes, range from peaty and calcareous quartzose sand and silt to compact, fibrous autochthonous peat. These organic-rich units also indicate approximately 4 m of marine submergence during the last 4,000 years. Concomitant with this submergence a complex sequence of peaty and calcareous sediments accumulated along the western margin of the Everglades. If lithified, the modern shelf and coastal deposits of the northeastern corner of the Gulf of Mexico would be mapped as a somewhat discontinuous and slightly time-transgressive stratigraphic sequence consisting of a variety of shallow-water facies composed of mixtures of three lithologic end members: (1) calcarenite and calcilutite, (2) quartzose sandstone and siltstone, and (3) coal. These facies, and their stratigraphic relations, duplicate some of the essential aspects of Paleozoic cyclothems. End_of_Article - Last_Page 2170

In the Duhamel area of central Alberta, the Upper Devonian section under study comprises in ascending order: (1) Cooking Lake formation, (2) basal calcilutite of the Duvernay formation, (3) Leduc reef, and (4) Ireton formation. In off-reef sections, the Duvernay consists of basal calcilutites and argillaceous beds overlain by bioclastic limestones and an upper sequence of brown and black shales and calcilutites, overlain by the Ireton formation. The Cooking Lake formation is composed mainly of pelletoid (pseudo-oolitic) calcarenites and calcilutites formed probably by precipitation on a widespread bank. Calcarenites were concentrated along the western side of an interpreted shoal, the margin of which trended north to north-northeast approximately along the locus of later Leduc reef growth at Duhamel. The basal Duvernay calcilutite unit is less than 10 feet thick above the Cooking Lake shoal, but increases in thickness abruptly to 50 feet directly west of the shoal margin. It is interpreted that these facies and thickness changes occurred along a tectonic hinge line caused by possible fault movement in the Precambrian basement that determined the locus of later Leduc reef growth. Leduc reef growth at Duhamel commenced essentially after the basal Duvernay calcilutite was deposited. Earliest or incipient patch-reef growth was confined to the relatively positive eastern side of the hinge line during latest Cooking Lake sedimentation. However, with differential subsidence along the aforementioned hinge line, the reefs grew laterally westward, overlapping the thickest section of basal Duvernay calcilutites, and the subsequent main reef buildup occurred on the interpreted downwarped, western side of the hinge line. The bioclastic limestone unit of the Duvernay increases in thickness toward reefs, suggesting a derivasion by erosion of actively growing reefs. The thickness distribution of bioclastic deposits indicates dominant currents from the northeast that transported a greater amount of debris to the southwestern (leeward) side of the reefs; however, thicker deposits at the southeast may have resulted from longshore current transport southward along the fore-reef side. During upper Duvernay deposition, increased subsidence resulted in mainly vertical reef growth and reduction in amount of reef erosion; only calcium carbonate and argillaceous muds accumulated more than one mile away from the reef locus. The muds were deposited in relatively deep water, resulting in dark bituminous and partly laminated rocks with a sparse benthonic fauna. Brown and black shales accumulated in greater amounts in the more protected and stagnant waters southwest (leeward) of the reefs, and only in minor amounts as far as 10 miles north and east of the Duhamel reef front. In lower Ireton time, further increase in subsidence rate removed the restricting effects of the biohermal reefs and gray-brown and green-gray calcareous shales and calcilutites were deposited in off-reef areas. Reef growth was probably terminated during lower or middle Ireton deposition. Facies and isolith maps of pertinent units in the Cooking Lake and Duvernay formations may be used as an effective new tool in the search for petroleum-bearing Leduc reefs in central Alberta.

Continuous seismic-reflection profiles on the Blake Plateau are correlated with sediment cores, seismic-refraction data, and logs of wells on Florida to study the structural relations of the Florida peninsula and continental shelf with the plateau. Results indicate that the entire Blake-Bahama-Florida area was the site of shallow-water carbonate deposition behind a barrier reef until late in the Mesozoic. Death of the reef along the Blake Plateau margin and continued subsidence of the entire area created the present submerged plateau. Florida and the Bahama banks continued to build throughout the Tertiary and have maintained an elevation near sea-level. Four strong reflectors are observed in the plateau sediments, the deepest apparently representing an interface within the Upper Cretaceous and correlating with the top of a 4.5 km./sec. refracting layer. The overlying beds probably are composed of bank-derived calcarenite and calcilutite; the reflectors correspond to major changes in sediment types or rates of deposition. The Tertiary sediments on the plateau essentially form a wedge, about 1,200 m. thick on the west side and 200-300 m. thick near the escarpment on the east. The extension of the Cape Fear arch southward onto the plateau apparently diverts and restricts the flow of the Gulf Stream. This has resulted in extensive erosion of much of the surface of the plateau. In places the erosion has uncovered beds of Eocene and Paleocene age. The material eroded from the plateau by the stream, as well as that which stayed in suspension, has b een swept off the plateau and has formed the Blake-Bahama outer ridge.

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.

There is a net flow of Gulf of Mexico high salinity water into Terminos Lagoon through the eastern inlet, mixing with runoff from three large rivers, and a net flow of low-salinity lagoon water out through the western inlet. The average circulation is reflected in distribution patterns of salinity, sediment size distributions, forams, percent of calcium carbonate in the sediments, and water turbidity. Relatively high organic productivity is indicated by numerous large standing stocks of forams. The lagoon is on the boundary between biogenic carbonate sediments on the east and detrital river-borne sediments on the west. The modern lagoon barrier began during the Holocene on a foundation of calcilutite when sea level was at about -- 10 m. When it reached about its present level the inlets were east of the present ones. Predominant flow of lagoon water out through the western inlet as at present is indicated by river-borne detritus under a surface cap of biogenic carbonate on the western end of the barrier. The seaward side of the barrier is being eroded, except on the western end, where there is deposition. The lagoon is essentially filled with sediment, and most detritus being delivered by rivers is bypassing the lagoon. It is suggested that deposits similar to those in Terminos Lagoon can be identified in ancient sediments.

Carboniferous rock units on the eastern flank of the Klamath Mountains, northern California, include the upper Mississippian Bragdon Formation, upper Mississippian and Pennsylvanian Baird Formation, and possibly the basal part of the predominantly lower Permian McCloud Limestone. The Bragdon is dominantly argillite and wacke, with local limestone and calcilutite containing two kinds of brachiopod associations and a coral association. The Baird is a mixture of argillite, siltstone, tuff, volcanic-rich siltstone and arenite, and numerous minor rock types. It contains in its lower part one low- and one high-diversity mollusk association, and one coral association. The Hirz Mountain Limestone Member (new name) of the Baird Formation, of late Pennsylvanian or early Permian age contains an additional brachiopod association and a coral association, and is approximately correlative with the oldest part of the McCloud Limestone, which contains a brachiopod-fusulinid association. Brachiopod associations indicate shallow marine waters of relatively constant depth and salinity from late Mississippian through early Permian times; sedimentary deposition was complex and highly variable.

A phylloid algal-mound complex is a local to subregional thickening of limestone attributed chiefly to the presence of a distinctive suite of rock types containing leaflike or phylloid algae. Twenty-three such mound complexes are present at or near the southern ends of most limestone units in Missourian and lower Virgilian (Upper Pennsylvanian) rocks exposed in eastern Kansas, northeastern Oklahoma, and northwestern Missouri. Mound complexes are composed of two facies, (1) the mound, consisting primarily of massive algal calcilutite to algal sparite, and (2) mound-associated facies, consisting primarily of thin- and cross-bedded skeletal and oolitic calcarenite capping and flanking the mound. Overlying shale beds thin across the tops of mound complexes. Most mound complex s grade northward into thinner, more diversely fossiliferous, open marine limestone beds, and grade abruptly southward into thin limestone beds and lenses and thick terrigenous clastic strata. Pennsylvanian phylloid algae are comparable with Holocene calcareous codiacean green algae and coralline red algae, and flourished in shallow sunlit water where they were sediment suppliers and stabilizers. Mounds probably began on topographic highs favorably situated between a region of great clastic influx and the open sea, and grew as the algae proliferated and produced sufficient sediment to compensate for subsidence. Mound growth allowed the algae to continue to flourish in their optimum sunlit environment. Stacking of mound complexes may reflect positive topographic influence of underlying mounds on the sea bottom, and shifts in the stackings probably resulted from shifts in northward extent of the clastic influx.

The Burnt Bluff Group of Wisconsin consists of approximately 100 feet of completely dolomitized, generally thin-bedded non-fossiliferous strata, which, when analyzed petrographically, reveal that, despite thorough dolomitization, their original textures and components could be recognized, allowing subdivision of the rocks studied into distinct microfacies. More than 600 thin sections were studied from three locations (Nasbro, Sturgeon Bay, and Chilton) at which mounds or bioherms had been described in the previous literature. The indices of frequency and(or) clasticity were determined for algal fragments, algal pellets, algal balls, leperditids, other ostracods, oolites, lithic fragments, detrital quartz, and feldspar, and curves for these parameters were plotted. A crystallinity curve showing the maximum crystal diameters of important components was also compiled, and the presence of algal mats, stromatoporoids, corals, Stromatactis, brachiopods, and large and extremely well rounded (St. Peter type) quartz grains was recorded. The petrographic data, general appearance, and field characteristics of the samples were used in subdividing them into eight microfacies, all deposited in water probably not exceeding a few tens of feet in depth. Microfacies 1, a coarsely crystalline dolomite matrix enclosing leperditid carapaces, clay aggregates, and pyrite, is a dolomitized calcilutite which was deposited below wave base. Microfacies 2 represents a shallower environment and is transitional between microfacies 1 and 3, and consists of interlayered dolomitized calcilutite and calcisiltite. Microfacies 3 is a dolomitized calcisiltite, in some places fossiliferous or oolitic. Layers of dolomitized oolitic limestone with an original clear calcite cement make up microfacies 4, which developed in fairly shallow water. Micr facies 5 is a dolomitized algal fragment breccia-conglomerate which was deposited in an extremely shallow, agitated environment. Microfacies 6 forms algal bioherms and is laterally equivalent to microfacies 1, 2, and 3. Microfacies 7 is a dolomitized biocalcarenite to calcisiltite containing large quartz grains, rounded lithic debris, and leperditid fragments, and is interpreted as a lateral equivalent of microfacies 4. Microfacies 8 is represented by algal, stromatoporoidal or coral biostromes which are laterally equivalent to microfacies 5. The investigated bioherms are small (reaching 8 feet in height and 30 feet in width) and display simple forms. They developed initially on coarser layers within a calcilutite unit or on slight positive irregularities. In a normal marine environment (Sturgeon Bay), algal growth was slow and poorly developed, whereas in a penesaline environment (Nasbro), growth was rapid, and numerous, well developed bioherms formed. The simple shapes of the bioherms are an expression of the nature of the constructing algae and of the relatively quiet, non-subsiding environment in which they developed, and to which they did not contribute any debris. Their associated sedimentary sequences exhibit textural changes expressing a general shallowing. In all respects, the Burnt Bluff bioherms are entirely dif erent from the classical Niagaran reefs of the Great Lakes area. The principal textural effect of dolomitization has been an increase of the crystal size of the components of the rock with maximum effect upon the originally fine-grained part of the sediments.

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.