Crinoidal Limestones Research Articles

Speculations on the genesis of crinoidal limestones in the Tethyan Jurassic

Crinoidal limestones in the Tethyan Jurassic often occur capping reef or carbonate-platform successions, and these bioclastic deposits commonly exhibit a lensoid form. They may also occur in tectonic fissures or as turbidites in basinal sequences. Their age is commonly Liassic, though they may be younger.

Stratigraphic and Sedimentologic Analysis of Mississippian Madison Formation in Southwestern Saskatchewan: ABSTRACT

Rocks of Mississippian age in southwestern Saskatchewan consist of a sequence of relatively similar carbonate rocks ranging from 340 ft to 1,030 ft thick. A vertical change in lithologic character not generally reflected on a geophysical log permits an informal division of these rocks into a lower, generally sparsely fossiliferous argillaceous or bituminous limestone unit, and an upper, commonly fossiliferous nonargillaceous limestone unit. The lower unit shows some lateral variations from locally developed bituminous limestone to argillaceous limestone. Red ferric oxide coloration is common in the argillaceous rock, as is glauconite, particularly in the basal part. The upper unit is characterized by a preponderance of crinoid remains generally in the form of alternations of particle-supported crinoidal limestones and micritic limestones with varying percentages of crinoid columnals. Bryozoa, brachiopods, and solitary corals also are concentrated in this sequence of rocks. The variable thickness of Mississippian rocks is probably related to the effects of regional erosion on the post-Mississippian-pre-Jurassic erosion surface which have resulted in removal of progressively older beds in a northwesterly direction. A paleotopography having a distinct northeast-southwest grain was developed on the erosion surface. Local paleotopographic relief is in the order of 100 ft and regional relief is approximately 400 ft from west to east. At present oil in commercial quantities is obtained from only 1 well in the study area. This oil accumulated in a dolomitized crinoidal limestone that subcrops on the western slope of a paleotopographic ridge. The ridge is on the easterly plunging Battle Creek anticline. The prospects of discovering additional hydrocarbon accumulations in the Mississippian rocks of southwestern Saskatchewan are enhanced greatly by the presence of excellent local source rocks as represented by the bituminous limestones of the lower carbonate unit. End_of_Article - Last_Page 538------------

Aspects of the depositional environment and palaeoecology of crinoidal limestones

Synopsis The paper summarises the limited literature on crinoid ecology and palaeoecology and attempts to interpret the depositional environment of certain British crinoidal sediments in the light of experimental observations. Three types of crinoidal limestone are recognised: muddy limestones deposited in shallow, sheltered waters; poorly washed limestones formed in deeper, open conditions; and clean-washed limestones which may derive in part from the reworking of one or other of the first two types.

Scientific organizations in the earth sciences in Czechoslovakia

The Pieniny Klippen Belt (boundary between the Central and Outer Western Carpathians) is a tectonic melange of various Mesozoic sedimentary units. In the shallowest unit, a Czorsztyn Succession, syntectonic Middle Jurassic (Bajocian) sedimentary breccia to megabreccia was discovered. It resulted from extensional tectonic movements tied to Jurassic Neotethys rifting. The breccia was investigated by standard petrographic methods, cathodoluminescence, microfacies analysis and isotope analysis (C, O). It represents the whole spectrum from crackled breccia, through mosaic breccia to mixed rubble breccia. All of them consist mostly of crinoidal limestone clasts. The crackled breccias are penetrated by a network of radiaxial fibrous calcite; the mosaic breccias have interstitial pores mostly filled with a younger generation of crinoidal limestone. The rubble breccia usually possesses the most complex filling. As a rule, the clasts are coated by at least one generation of stromatolite and then cemented by radiaxial fibrous calcite. The stromatolites may appear at various levels of the breccia diagenesis (most of them are cryptic stromatolites). The remaining void filling starts with the crinoidal detritus which indicates that the breccia was formed due to synsedimentary tectonics occurring during the deposition of crinoidal limestones (Bajocian). The rest was filled by micritic limestone with “filamentous” microfacies (Bathonian–Callovian), which also contained local planktonic foraminifers, Globuligerina sp. The latest sedimentary filling is represented by sterile micrite containing a few allochems and fauna of cavity-dwelling ostracods Pokornyopsis sp. The latest cement is represented by clear blocky calcite filling veinlets cutting the whole rock. Numerous instances of disturbance, resedimentation and recementation are evident in the breccia. The isotope composition of some early generations of stromatolites, local etching of the clasts and early cements, together with some earlier generations of blocky-calcite veinlets, possibly indicate a presence of fresh-water diagenesis.

Cache Field--A Pennsylvanian Algal Reservoir in Southwestern Colorado

Oil and gas accumulation in the Cache field in southwestern Colorado is chiefly in reservoirs developed in a northwest-trending group of stacked algal carbonate mounds and related carbonate facies in the Ismay zone (Des Moines age) of the Paradox Formation. Cache field is the most productive Pennsylvanian field in the Colorado part of the Paradox basin. Characteristic cyclic deposition which is developed in the zones of the Paradox Formation is shown on a smaller scale within the Ismay of the Cache field. The Ismay has an average thickness of 180 ft and consists of nine carbonate lithofacies, anhydrite, siltstone, and black shale, which are recognized in one or more of the three cycles in the Ismay. The carbonate lithofacies are: (1) algal calcirudite breccia; (2) calcirudite breccia; (3) foraminiferal-pelletal limestone; (4) laminated boundstone; (5) crinoidal limestone; (6) earthy dolomite; (7) carbonate mudstone; (8) shelly mudstone; and (9) dolomite. The depositional environment of the normal-marine buildup cycle was dominated by the formation of algal mounds in relatively shallow water where phylloid Ivanovia algae grew in profusion on broad banks. In slightly deeper water flanking the mounds, and in channelways, fossiliferous carbonate mudstone was deposited. One facies (boundstone) probably formed in intertidal conditions. The evaporitic cycle which followed the normal-marine buildup cycle probably was begun by the lowering of sea level. During the evaporitic cycle, dolomite and massive anhydrite (gypsum) were deposited in an intertidal-supratidal environment. Favorable zones of porosity and permeability are present within the algal calcirudite breccia and the dolomitized shelly mudstone. Porosity is predominantly secondary (early diagenetic) and is the result of leaching of the algae and solution brecciation (vugs) in the algal limestone, and leaching and recrystallization to dolomite in the dolomitized limestone.

A lower carboniferous S 2 inlier near Hartington, Derbyshire

Carboniferous Limestone beds yielding a characteristic S 2 faunal assemblage are exposed in a broad anticlinal flexure north-east of Hartington. They are shown to comprise a lower group of predominantly crinoidal limestones (Vincent House Beds) and an upper group of interbedded calcarenites, fine-grained recrystallised limestones and calcite mudstones (Hand Dale Beds). The latter are unconformably overlain by D 1 limestones. The S 2 beds appear to have been deposited in shallow waters which were generally fairly turbulent but which at times became relatively calm. The axis of the anticlinal flexure trends NNE.-SSW. and the fold plunges to the SSW. The western limb of the flexure is modified by two subsidiary NE.-SW. folds and several faults with a general WNW.-ESE. trend traverse the area. Some of the faults are mineralised whilst others bear no mineral deposits, and the latter may be associated with a later tectonic episode. The succession shows marked similarities to S 2 beds in the Harpurhill area (near Buxton), near Peak Forest and in the Via Gellia. Only the upper calcite mudstone group in the Wye Valley can, however, be compared with beds at Hartington.

Alteration of host rock limestone adjacent to zinc-lead ore deposits in the Tri-State District, Missouri, Kansas, Oklahoma

The nature and lateral variation of a host rock limestone in the Tri-State zinc-lead district were petrographically studied within a single horizon, the M bed. This bed is a massive, cherty, crinoidal limestone in the Tri-State district of southwestern Missouri, northeastern Oklahoma and southeastern Kansas and is a part of the Mississippian Keokuk Formation.One hundred and twenty specimens from this bed were collected under-ground from six pull drifts in five mines: West Side, Grace B, Big John, Little Greenback, and Mudd Mines. The specimens were collected at regular intervals, 10 to 50 feet, beginning at the edge of an ore body and progressing away from the zone of mineralization and ore. Ninety-four thin sections prepared from the limestone specimens were described and classified according to Folk's classification of carbonate rocks. According to this classification M bed limestone is predominantly a sparry allochem.Distally from ore bodies the M bed limestone consists predominantly of allochems, mainly crinoids, and microcrystalline calcite, sparry calcite and microspar are much less abundant constituents. Toward ore bodies, the constituents of the M bed limestone are altered by the solutions that emplaced the ores. The allochem content progressively decreases, whereas the sparry calcite content increases. This gradual change in the limestone constituents toward the margin of ore bodies generally begins at a distance of 40 to 80 feet from the ore bodies. Microcrystalline calcite and microspar exhibit no significant lateral variations.

Formations eruptive antejurassiques en Eubee moyenne (Grece)

Abstract In addition to the well-known upper Jurassic or lower Cretaceous lava flow in the central part of Euboea island, Greece, an older flow occurs between a Permian formation with neritic fossils and an upper Triassic algal limestone. A Ladenian-Carnian (Triassic) age is tentatively assigned to the flow. Between this and the Jurassic flow, spilites and related rocks overlie shales interbedded with crinoidal and fusulinid limestones and overlain by algal limestones. Outcrops at Tharounia expose the flows and the intermediate beds.

Contribution a l'etude stratigraphique du Lias des Dolomites italiennes (Italie du nord)

Abstract The gray limestone facies of the Italian Dolomites is confined to the lower Liassic and rests unconformably on the last Triasina-bearing beds of pre-Liassic age. Middle Liassic crinoidal limestones overlie the gray limestone and are overlain by upper Liassic chemical sediments. The only evidence for upper Liassic age strata is the presence of the ammonites Harpoceras discodes and Hammatoceras insigne. For the most part, the upper Liassic was a period of nondeposition.

Sedimentary Basins of South-Central and Southwestern New Mexico

Major sedimentary basins in this, the eastern part of the Basin-and-Range province, are the Orogrande and Pedregosa basins of Mississippian, Pennsylvanian, and Wolfcampian age, the San Mateo and Lucero basins of Pennsylvanian age, the Carrizozo and Quemado-Cuchillo evaporite basins of Leonardian age, the Early Cretaceous basin near the Hatchet Mountains, and the continental basins containing much volcanic debris of Late Cretaceous age in central Sierra County and near Steeple Rock. Numerous Cenozoic intermontane grabens occur in the region, including the southern part of a long north-south string of interconnected grabens now followed by the Rio Grande and called the Rio Grande structural depression. Sediments filling the Cenozoic basins are mainly of Miocene, Pliocene, a d Pleistocene ages. Pre-Devonian strata, the Cambrian-Ordovician Bliss Sandstone, the Ordovician El Paso Limestone and Montoya Dolomite, and the Silurian Fusselman Dolomite, thin northward and westward mostly because of periodic erosion but the thinning is partly depositional. The Devonian shaly beds are relatively uniform in thickness, although marking the first large-scale influx of clay and silt. As with all older Paleozoic sediments, they appear to have been deposited in shallow epicontinental seas. The Pedregosa basin was autogeosynclinal, receiving thick deposits of Middle Mississippian crinoidal limestones, Late Mississippian arenaceous calcarenites, Pennsylvanian limestones, and Wolfcampian interbeds of limestone, black shale, and redbeds. The Orogrande basin began as a poorly defined autogeosyncline in which siliceous Middle Mississippian limestones were deposited, then became zeugogeosynclinal during late Pennsylvanian time as detritus was swept westward from the Pedernal landmass, and during Wolfcampian time was filled by limestone and shale that grade northward into redbeds. The San Mateo and Lucero basins were small autogeosynclines that connected the Pennsylvanian seas northward with the San Juan and Paradox basins. Source beds and possible petroleum reservoir rocks occur in the Paleozoic and Mesozoic sequences. The reservoir beds include porous sandstone, bioclastic calcarenite, dolomite, reef masses, porous lenses beneath truncating unconformities or amid intertongued redbeds and marine limestones.

Sedimentary Basins of Central and Southwestern New Mexico: ABSTRACT

Major sedimentary basins in this, the eastern part of the Basin and Range province, are the Orogrande and Pedregosa basins of Mississippian, Pennsylvanian, and Wolfcampian age, the San Mateo-Lucero and Estancia basins of Pennsylvanian age, the Carrizozo and Quemado-Cuchillo (Foster, 1957) evaporite basins of Leonardian age, the Early Cretaceous basin near the Hatchet Mountains, and the continental basins of vols canic piles of Late Cretaceous age near Elephant Butte and Steeple Rock. Numerous Cenozoic intermontane graben basins dot the region, with the southern part of a long north-south string of interconnected grabens now followed by the Rio Grande and called the Rio Grande structural depression. Sediments filling the Cenozoic basins are mainly of Late Miocene. Pliocene and Pleistocene in age. Pre-Mississippian Paleozoic rocks remain only south of about 33°45^prime N. Lat. The basal Paleozoic unit, the Cambrian-Ordovician Bliss Sandstone, thickens depositionally southward and southwestward. The Early Ordovician El Paso Limestone thins northward due to intra-Ordovician erosion whereas the Middle and Late Ordovician Montoya Dolomite is relatively uniform in thickness where overlain by Silurian rocks. The Silurian Fusselman Dolomite thins northward partly due to erosion during Late Silurian and Early Devonian time. The Devonian shaly strata are relatively uniform in thickness, although marking the first large scale influx of clay and silt; as with all older Paleozoic rocks they appear to have been deposited in shallow epicontinental seas. The Pedregosa basin was autogeosynclinal, receiving thick deposits of Middle Mississippian crinoidal limestones, Late Mississippian arenaceous calcarenites, Pennsylvanian limestones, and Wolfcampian interbeds of limestone, black shale, and redbeds. The Orogrande basin began as a poorly defined autogeosyncline in which siliceous Middle Mississippian limestones and Late Mississippian arenaceous calcarenites were deposited, then became zeugogeosynclinal during Late Pennsylvanian time as detritus was swept westward from the Pedernal landmass, and during Wolfcampian time was filled by limestones and shales that grade northward into redbeds. The Estancia basin was a small Pennsylvanian zeugogeosyncline, and the San Mateo-Lucero basins were autogeosynclines connecting the Pennsylvanian seas orthward toward the San Juan and Paradox basins. End_of_Article - Last_Page 1879------------

Application du 'tamisage virtuel' aux calcaires detritiques de la region de Nantua (Ain)

Abstract Interpretation of simple and cumulative curves constructed from granulometric analyses of thin sections of Bajocian-Barremian detrital limestones from Nantua (France) makes it possible to determine the sedimentological and oceanographic conditions of the deposition. The presence in the Bajocian limestones of Cancellophycus which lives at 200-1000 m depths points out the contrast between the deep-water facies and the coastal facies, represented by crinoidal limestones with Echinus and foraminifera. The Bajocian transgression, gradual at first, with horizontal facies variation, increased in the upper Bajocian with deposition of a fine limestone ooze. The Bathonian transgression attained its maximum in the upper Bathonian with deposition of argillaceous limestones in unfossiliferous beds. The Callovian nacreous flagstone is overlain by material reworked by strong, sometimes turbulent currents. The reduced thickness of Oxfordian deposits represents the transition between the organic sediments of the Dogger and the upper Jurassic oozes. The upper Jurassic transgression, most pronounced in the Argovian (Lusitanian), began to recede in the Portlandian depositing reworked oolitic limestones. A pisolitic layer marks the base of the Sequanian. The fine lacustrine limestones of the Purbeckian are followed by transgressive lower Cretaceous deposits.

Permian Facies Relationships in Eastern Nevada: ABSTRACT

Lower and Middle Permian formations between Confusion Range, Millard Co., Utah, and Dry Mountain, White Pine Co., Nevada, have many distinct facies, but all pre-Kaibab Permian rocks may be referred to five formations. The stratigraphic succession at Illipah Gorge comprises Reipe Spring limestone, Reipetown sandstone, Pequop formation, and Loray formation. In the Confusion Range, Butte Mountains, and Ely area, Pequop and Loray formations are generally recognizable, but no distinctive boundary for mapping purposes can be recognized. In these areas, the Arcturus formation is used to designate the entire section between Reipetown sandstone and Kaibab limestone. Reipe Spring limestone is rather uniform in its physical characteristics, although it becomes somewhat arenaceous at Dry Mountain and silty in the Butte Mountains. A limestone-rich facies near Divide Spring, and mudstone rich facies at Dry Mountain represent the Reipetown sandstone. The lower Pequop formation consists of interbedded calcarenite and sandstone at Illipah Gorge, but at Dry Mountain an almost entirely mudstone facies is present. In the Ely area and Confusion Range this member is represented by interbedded limestones and sandstones of the lower Arcturus formation. Massive, crinoidal, and fusulinid-rich limestones of the upper Pequop formation at Moorman Ranch are represented in the Ely area by interbedded thin limestones and sandstones of the middle Arcturus formation. The upper Arcturus formation in the Ely area, Butte Mountains, and Confusion Range comprises thin interbedded red and yello sandstones and dark gray limestones similar to those of the Loray formation in the northeast part of the Illipah Quadrangle. End_of_Article - Last_Page 1776------------

New graptolite localities in New Zealand

Abstract Three new graptolite localities are described: (1) Alfred River, near Springs Junction in south Nelson; (2) Stony Creek, two miles north of Baton River in north-west Nelson; and (3) Takaka River, two miles from Upper Takaka, also in north-west Nelson. Only at the first locality are graptolites well enough preserved to be identifiable, and only at the third locality were they found in place. Crinoidal limestone occurs at two of the new localities; and attention is drawn to the possibility of finding graptolites at the apparently anomalous crinoidal limestone locality at Kakahu in South Canterbury.

Petrography of Mississippian (Borden) Crinoidal Limestones at Stobo, Indiana

ABSTRACT 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.

Lower Mississippian Bioherms of Southwestern Missouri and Northwestern Arkansas

ABSTRACT Massive calcium carbonate mud mounds are present within the St. Joe Limestone (Lower Mississippian) of southwestern Missouri and northwestern Arkansas. These lenticles are up to 30 feet thick and 300 feet long. Thin-bedded crinoidal limestones enclose the bioherms. The enclosing limestones onlap the bioherms and by successive overlap extend over the mounds. Point-count analyses of thin sections from nine localities were made. The bioherms consist mainly of calcite-mudstone (about 80 percent) with only a small percentage (approximately 13 percent) of skeletal remains and 5 percent sparry calcite. Fenestrate bryozoans are the most abundant skeletal constituents, making up about 8 percent of the rock. Echinoderm fragments comprise only 2 percent of the mounds. Other known skeletal compon nts are present in a total amount generally less than 3 percent. The interbiohermal beds contain 47 percent skeletal grains including 13 percent fenestrate bryozoans and 25 percent echinoderm remains, 51 percent calcite mud and about 1 percent sparry calcite. The most acceptable hypotheses for the origin of the bioherms are: (1) the interaction of physical environmental factors and bathymetry; (2) the sediment-trapping and-stabilizing activities of fenestrate bryozoans, and; (3) the sediment-trapping and -stabilizing activities of plants, not preserved. Although none of the three hypotheses can be completely rejected, it is possible that fenestrate bryozoans alone may have caused the growth and accumulation of these Missouri bioherms. It is also possible that the bryozoans were at least ssisted by plants of which there are no preserved remains.

The Lower Carboniferous rocks of the Carrick-on-Shannon syncline

Summary Carboniferous limestone of Viséan age occupies much of the district around Carrick-on-Shannon in north-central Ireland. The limestone succession, nearly 3000 feet thick, is composed of alternating groups of thin-bedded limestones-with-shales and massive, crinoidal limestones. Lenticular sheet reefs, composed largely of unbedded calcite-mudstone, are interbedded with the uppermost massive limestones. The limestones are ascribed zonal positions ranging from Upper Caninian to Lower Dibunophyllidan. The calcareous rocks are underlain by conglomerates and sandstones, 500 feet thick, which rest unconformably on sediments of Old Red Sandstone age, and overlain by a thick series of goniatitic black shales of Bollandian and Namurian age. Compared with the Lower Carboniferous rocks lying north of the Ox Mountains, the Carrick succession is less thick and is of markedly different facies. This is attributed principally to influence of the contemporary barrier of the Ox anticline and possibly of the Curlew anticline. The Carrick syncline is a composite downwarp consisting of a system of pitching folds arranged in echelon and traversed by a number of faults including important wrench-faults. The structures have a caledonoid trend derived from the sub-Carboniferous foundation.

Diagenesis and Paragenesis in Limestones and Dolomites: ABSTRACT

The apparent confusion among geologists about the meaning of probably arises from the fact that there is no universally accepted definition of the word. There seems to be agreement that begins with, or immediately after, deposition of a sediment, but considerable disagreement arises as to when ceases. In this paper, diagenesis refers to the chemical and or physical processes that change the textural and mineralogical characteristics of a sediment. On the basis of a study of some of these characteristics, discernible mostly in thin sections, it is possible to establish reliable sequences of mineral deposition. For this paper, such sequences observed in some limestones and dolomites are shown by paragenetic diagrams, and textural relations on which the paragenetic diagrams are based are shown in photomicrographs. From an established sequence of mineral deposition in a rock it should be possible to determine an orderly progression of the diagenetic reactions that have taken place. A simple illustration would be a crinoidal limestone in which calcite forms not only distinct overgrowths on the crinoid fragments but also a cement between the enlarged fragments. Basically, the paragenesis would be (1) calcite: crinoid and other fossil fragments, (2) calcite: overgrowths, and (3) calcite: cement. If the deposition of the three calcites is definitely successive (that is, if the deposition of one calcite is completed before the other begins), some geologists might consider that lithification was complete after deposition of calcite (2) and that diagenesis ceased at that time. However, if there is any verlap in time of deposition of the three calcites, especially of (2) and (3), it seems logical to assume that diagenesis and lithification both terminated at the end of deposition of calcite (3). End_of_Article - Last_Page 1766------------

Mississippian Bioherms in Northeast Oklahoma

In northeast Oklahoma, the Mississippian Boone formation contains numerous striking bioherms composed of crinoidal debris. Most of the exposed bioherms occur in the St. Joe, or lowest member of the Boone formation. The Reeds Spring, or middle member, contains no bioherms, and the Keokuk, or upper member, contains a few known bioherms. The bioherms occur in widespread, thin-bedded crinoidal limestones and are related in origin to the thin-bedded limestones. Each bioherm consists of a lens-shaped core of massive limestone which is surrounded by and interfingers with thin-bedded limestones. The massive cores are 10-40 feet thick and 50-1,000 feet wide. The thin-bedded limestones dip steeply away from the massive cores. The bioherms are interpreted as having been formed by the accentuated growth and accumulation of crinoids. The bioherms formed mounds which were elevated as much as 50 feet above the floor of a shallow sea. The massive bioherm cores were probably created by the continuous deposition of crinoidal debris; whereas the bedded limestones represent deposition which was interrupted from time to time. The thin-bedded limestones on the flanks of bioherms were deposited on slopes as steep as 45°. A debris-binding agent, such as inorganically precipitated calcium carbonate or calcareous algae, is postulated to have bound the deposits of crinoidal debris together, permitting them to persist on such steep slopes. The bioherms are considered to have been organic reefs because they formed wave- esistant organic structures which formed elevated prominences in a shallow sea.

Stratigraphic Relations in Chiricahua and Dos Cabezas Mountains, Arizona

The Chiricahua and Dos Cabezas mountains are typical of the Basin-and-Range Province in southeast Arizona. Well exposed Paleozoic and Mesozoic marine strata overlie a Precambrian basement complex of schist and granite. The 7,600 feet of Paleozoic strata were deposited on a slowly subsiding shelf, beginning in middle(?) or late Cambrian time. Sedimentation was interrupted by an epeirogenic uplift that caused the widespread unconformity between Lower Ordovician and Upper Devonian strata. The pronounced angular discordance between the Paleozoic strata and the Lower Cretaceous beds was caused by a pre-Cretaceous orogeny. Compressive stresses during the post-Comanche--pre-Pliocene orogeny strongly deformed all the strata. The range was uplifted later in the Cenozoic by high-an le faulting. The oldest formation is the Bolsa quartzite (Middle(?) to Upper Cambrian), which unconformably overlies the basement rocks. It is arkosic to quartzose and ranges from 320 to 600 feet in thickness. The El Paso formation (Upper Cambrian-Lower Ordovician) conformably overlies the Bolsa and ranges in thickness from 340 to 715 feet. It consists of dolomite and limestone. The overlying Portal formation (Upper Devonian) is here named for exposures 2½ miles northwest of the village of Portal. The formation ranges from 200 to 342 feet in thickness and consists of the following four members, in ascending order: (1) alternating limestone and shale, (2) black siliceous shale, (3) alternating limestone and shale, (4) bioclastic limestone. The Escabrosa limestone (Lower Mississippian) consists of cherty crinoidal limestone ranging from 630 to 730 feet in thickness. It is overlain by the Paradise formation (Upper Mississippian) which consists of about 150 feet of alternating thin limestone and clastic strata. The Pennsylvanian and Permian systems are represented by the Naco group, composed of about 5,500 feet of limestone and clastic strata. The Naco group is subdivided into the following five formations, in ascending order: Horquilla, Earp, Colina, Scherrer, and Concha. The Horquilla is dominantly limestone and ranges in age from the Morrow to Missouri series. It is conformably overlain by the Earp formation (Missouri to Wolfcamp age) which consists of alternating limestone, sandstone, and shale. The Colina (Wolfcamp to Leonard age) is aphanitic black limestone. The Scherrer (Leonard age) consists of quartzitic sandstone with a basal red siltstone member. The uppermost formation is the Concha limestone (Leonard to Guadalupe? age) which is very fossiliferous and cherty. The dominantly carb nate Naco sequence of southeast Arizona is equivalent to the thinner, dominantly clastic Pennsylvanian-Permian strata of the Colorado Plateau. The Mesozoic era is represented by the Bisbee group of Lower Cretaceous clastic strata. The Glance conglomerate at the base reaches a maximum thickness in excess of 1,000 feet. The overlying 2,500 feet of the Bisbee group is divided into three lithologic units, but no formal names are proposed. The lowest unit is a red siltstone. This is overlain by the thin middle limestone unit, which is probably equivalent to the Mural limestone at Bisbee. The uppermost unit consists of alternating quartzite and siltstone.