Phosphatic Conglomerate Research Articles

Record of a dense succession of drowning phases in the Alpstein mountains, north-eastern Switzerland: part I—the Lower Cretaceous Tierwis Formation (latest Hauterivian to latest Barremian)

In the Alpstein massif of north-eastern Switzerland, a complete succession of uppermost Hauterivian to uppermost Barremian condensed hemipelagic sediments crops out. This succession is known as Tierwis Formation, comprising in ascending order, the Altmann and Drusberg members. The sedimentary succession bears a number of fossiliferous glauconite- or phosphate-rich beds. A large number of newly discovered ammonites from these key beds and from several poorly explored levels of the Tierwis Formation allows for a new age calibration. The new dating as well as revised sequence stratigraphic interpretations and geochemistry contribute to a better understanding of the lithostratigraphic complexity of the Tierwis Formation and its spatio-temporal relationship with the Schrattenkalk Formation. The new lithostratigraphic observations, backed by ammonites, shows that the Altmann type-section and the Tierwis paratype-section do not cover the same stratigraphic interval because of dynamic sedimentation processes as erosion and sedimentation in submarine channels. We suggest that a phosphatic conglomerate in the Dursberg Member of middle late Barremian age corresponds to the Chopf Bed, which we recognised for the first time in the Alsptein massif. The Drusberg Member strongly thickens toward the southeast and progressively covers an upward extended stratigraphic range. Furthermore, the new dating of the key-surfaces and beds highlight a dense succession of drowning phases which occurred through the latest Hauterivian to late Barremian time interval. The latest Hauterivian onset of the glauconite-rich sedimentation of the Altmann Member is associated with a first major drowning phase, followed by the Faraoni oceanic anoxic event. The change of sedimentation to a rhythmic marl-limestone alternation of the Drusberg Member takes place over a polyzonal phosphatic conglomerate. This conglomerate coincides with a second major drowning phase and the onset of the Mid-Barremian Event, which is calibrated on the Tethyan ammonite biozonation.

Record of latest Barremian-Cenomanian environmental change in tectonically controlled depressions from the Jura-Burgundy threshold (Jura Mountains, eastern France and western Switzerland)

The area of the western Jura Mountains constitutes the former Jura-Burgundy threshold between the Tethys Ocean and the epicontinental Paris Basin Sea. During the Barremian, the area was covered by a shallow-water Urgonian carbonate platform. Tectonic processes influenced the architecture of the Urgonian platform and were notably responsible for the formation of fault-related depressions on top of the Urgonian series, which were subsequently transformed into incised valleys and then to marine depocenters. Their sedimentary infills are mostly represented by the Perte-du-Rhône Formation and record stepwise environmental change on the innermost platform, which was strongly influenced by the nearby landmass. From the latest Barremian onward, sedimentation progressively changed from: i) predominantly oncoidal and detrital charophyte-rich marly limestones; to ii) heterozoan marly limestones and marls; followed by iii) detrital sediments interspersed with numerous phosphatic conglomerates. These changes are linked with the onset of the latest Barremian Taxy Episode and with the following oceanic anoxic episodes (OAE's) 1a to d in the neighbouring basins. The sedimentary changes are associated with sedimentary sequences which formed during 2nd-order transgressions, and which express the effect of sea level fall followed by drowning events. The presence of a rich ammonite-fauna allowed a precise biostratigraphical dating of these events during the latest Barremian to Cenomanian time interval.

The Perte-du-Rhône Formation, a new Cretaceous (Aptian-Cenomanian) lithostratigraphic unit in the Jura mountains (France and Switzerland)

During the late Early Cretaceous, the Jura domain was occupied by a photozoan carbonate platform, summarised in the Vallorbe Formation. From the earliest Aptian to the Late Cenomanian, open marine heterozoan and detrital marl, carbonate, and sandstone (up to 40 m thick) accumulated on top of the former Urgonian platform, followed during the Late Cretaceous by pelagic chalk and limestone. At the end of the Early Cretaceous, sediment accumulation was interrupted by several emersions and consecutive drowning phases identified in the field by angular discontinuities, iron crusts, and phosphatic conglomerates. Such sedimentary succession is particularly well visible in the present Bellegarde-sur-Valserine Basin of the Ain Department (eastern France). Therefore, in agreement with the Swiss Committee of Stratigraphy, we introduced new lithostratigraphic units to cover the time from the Aptian to the Cenomanian in the Jura mountains based on the type section near the historical outcrop of the Perte-du-Rhone in the Bellegarde Basin. The new Perte-du-Rhone Formation is subdivided in a lower marly unit (Fulie Member), followed by a middle green sandstone unit with phosphatic conglomerates (Mussel Member), which is finally topped by a mica-rich sandstone unit (Poncin Member).

Three successive phases of platform demise during the early Aptian and their association with the oceanic anoxic Selli episode (Ardèche, France)

A stratigraphic and depositional model, constrained by biostratigraphy, geochemistry, total phosphorus contents, and bulk-rock mineralogy, is proposed for lower Aptian sediments from the Languedoc platform in Ardèche, SE France. The upper lower Aptian is documented by the Chabert Formation (upper Deshayesites forbesi Zone to upper Dufrenoyia dufrenoyi Subzone), deposited on a discontinuity surface on top of the Urgonian platform, recording a first emersion phase and consecutive drowning event. The Chabert Formation starts with the marly Violette Member, which passes into crinoidal limestone of the Rocherenard Member. The top of this member is associated with a second discontinuity, recording a further drowning phase, which is followed by the deposition of the glauconitic and partly phosphatic Picourel Member (upper Deshayesites grandis to upper Dufrenoyia dufrenoyi Subzone). A third erosive phase is documented by a phosphatic conglomerate (upper Dufrenoyia furcata Zone), which represents a lag deposit derived from underlying sediments. The formation of this conglomerate was associated with a substantial emersion phase. This emersion was followed by a drowning event reworking the phosphatic conglomerate into the base of the upper Aptian black marls (Frayol Formation). The carbon-isotope record shows a negative excursion which coincides with the onset of the early Aptian oceanic anoxic Selli episode (OAE 1a) in the middle/upper part of the Deshayesites forbesi Zone. Emersion phases were an important factor implied in the formation of the sequence boundaries, which were transformed into drowning unconformities during subsequent phases of significant transgressions. These phases were associated with the installation of higher trophic levels, transforming or impeding carbonate production. The first drowning phase preceded the onset of the Selli episode, suggesting that rapid sea-level change and associated environmental change were already an important element of the early Aptian before the major phase of environmental change during the Selli episode.

The Albian–Cenomanian boundary at Eggardon Hill, Dorset (England): an anomaly resolved?

Re-examination of the classic exposures of the Eggardon Grit (topmost Upper Greensand Formation) at Eggardon Hill, Dorset shows that the upper part of this unit has a more complex stratigraphy than has been previously recognised. The Eggardon Grit Member, as described herein, is capped by a hardground and associated conglomerate, and is entirely of Late Albian age. The hardground is probably the lateral equivalent of the Small Cove Hardground, which marks the top of the Upper Greensand succession in southeast Devon. The conglomerate is overlain by a thin sandy limestone containing Early Cenomanian ammonites. This limestone is almost certainly the horizon of the Early Cenomanian ammonite fauna that has previously been attributed to the top of the Eggardon Grit. The limestone is regarded as a thin lateral equivalent of the Beer Head Limestone Formation (formerly Cenomanian Limestone) exposed on the southeast Devon coast. The fauna of the limestone at Eggardon suggests that it is probably the age equivalent to the two lowest subdivisions of the Beer Head Limestone in southeast Devon, with a remanié fauna of the Pounds Pool Sandy Limestone Member combined with indigenous macrofossils of the Hooken Nodular Limestone Member. The next highest subdivision of the Beer Head Limestone in southeast Devon (Little Beach Bioclastic Limestone Member), equates with the ammonite-rich phosphatic conglomerate of the ‘Chalk Basement Bed’, which caps the Beer Head Limestone at Eggardon, and which was previously regarded as the base of the Chalk Group on Eggardon Hill.Petrographic analysis of the Eggardon Grit shows that lithologically it should more correctly be described as a sandy limestone rather than sandstone. The original stratigraphical definition of the unit should probably be modified to exclude the softer, nodular calcareous sandstones that have traditionally been included in the lower part of the member.Without the apparently clear evidence of unbroken sedimentation across the Albian–Cenomanian boundary, suggested by the previous interpretation of the Eggardon succession, it is harder to argue for this being a prevalent feature of Upper Greensand stratigraphy in southwest England. Correlation of the Eggardon succession with successions in Dorset and southeast Devon reveals a widespread regional break in sedimentation at the Albian–Cenomanian boundary. The sand-rich facies above this unconformity represent the true base of the Chalk Group, rather than the ‘Chalk Basement Bed’ of previous interpretations.Selected elements of regionally important Upper Greensand ammonite faunas previously reported from Shapwick Quarry, near Lyme Regis, and Babcombe Copse, near Newton Abbot, are newly figured herein.

Phosphate Taphonomy of Bone and Coprolite Conglomerates: A Case Study from the Eocene of Mali, NW Africa

Repeated sequences of carbonate and shale are punctuated by condensed sections of phosphatic conglomerate in the epeiric deposits of the Trans-Saharan Seaway in northeastern Mali. To characterize the taphonomic and depositional setting of these phosphates, a thick Eocene conglomerate from the area of Tamaguelelt was targeted for quantitative analysis. Systematic grid sampling demonstrates that nearly all of the clasts are derived from vertebrate sources (bones = 27%, coprolites = 20%, probable coprolites = 53%), and invertebrate body fossils are nearly absent. Bony and cartilaginous fish dominate the bone assemblage, which also includes minor reptilian elements from sea turtles, sea snakes, and dyrosaurid crocodilians. Coprolites are of five distinct varieties, including three spiral forms probably produced by separate fish taxa. Repeated episodes of abrasion and minor bioerosion with modest levels of sorting characterize the taphonomy of the phosphate conglomerate and are consistent with a shallow-marine-to-brackish-water depositional environment between fair-weather and storm-wave base. Early phosphogenesis strongly favored the preservation and lithification of phosphate-rich bones and coprolites, probably during periods of marine transgression and sediment starvation. Combined with evidence from sedimentology, these vertebrate-dominated fossil assemblages appear extensively reworked and highly time averaged as a result of amalgamation and concentration by storm activity during periods of marine transgression.

Bivalve Borings in Phosphatic Coprolites and Bone, Cretaceous-Paleogene, Northeastern Mali

Other| December 01, 2004 Bivalve Borings in Phosphatic Coprolites and Bone, Cretaceous–Paleogene, Northeastern Mali LEIF TAPANILA; LEIF TAPANILA 1Department of Geology and Geophysics, University of Utah, 135 South, 1460 East, Room 719, Salt Lake City, Utah, 84112-0111, ltapanila@mines.utah.edu Search for other works by this author on: GSW Google Scholar ERIC M. ROBERTS; ERIC M. ROBERTS 2Department of Geology and Geophysics, University of Utah, 135 South, 1460 East, Room 719, Salt Lake City, Utah, 84112-0111 Search for other works by this author on: GSW Google Scholar MAMADOU L. BOUARÉ; MAMADOU L. BOUARÉ 3École Nationale des Ingénieurs, Bamako, Mali Search for other works by this author on: GSW Google Scholar FAMORY SISSOKO; FAMORY SISSOKO 4Institut des Sciences Humaines, Bamako, Mali Search for other works by this author on: GSW Google Scholar MAUREEN A. O'LEARY MAUREEN A. O'LEARY 5Department of Anatomical Sciences, Health Sciences Center T8 (040), Stony Brook University, Stony Brook, New York, 11794-8081 Search for other works by this author on: GSW Google Scholar Author and Article Information LEIF TAPANILA 1Department of Geology and Geophysics, University of Utah, 135 South, 1460 East, Room 719, Salt Lake City, Utah, 84112-0111, ltapanila@mines.utah.edu ERIC M. ROBERTS 2Department of Geology and Geophysics, University of Utah, 135 South, 1460 East, Room 719, Salt Lake City, Utah, 84112-0111 MAMADOU L. BOUARÉ 3École Nationale des Ingénieurs, Bamako, Mali FAMORY SISSOKO 4Institut des Sciences Humaines, Bamako, Mali MAUREEN A. O'LEARY 5Department of Anatomical Sciences, Health Sciences Center T8 (040), Stony Brook University, Stony Brook, New York, 11794-8081 Publisher: SEPM Society for Sedimentary Geology Accepted: 09 Feb 2004 First Online: 03 Mar 2017 Online ISSN: 1938-5323 Print ISSN: 0883-1351 Society for Sedimentary Geology PALAIOS (2004) 19 (6): 565–573. https://doi.org/10.1669/0883-1351(2004)019<0565:BBIPCA>2.0.CO;2 Article history Accepted: 09 Feb 2004 First Online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation LEIF TAPANILA, ERIC M. ROBERTS, MAMADOU L. BOUARÉ, FAMORY SISSOKO, MAUREEN A. O'LEARY; Bivalve Borings in Phosphatic Coprolites and Bone, Cretaceous–Paleogene, Northeastern Mali. PALAIOS 2004;; 19 (6): 565–573. doi: https://doi.org/10.1669/0883-1351(2004)019<0565:BBIPCA>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyPALAIOS Search Advanced Search Abstract Bivalve borings are described for the first time in coprolites. They occur along with bored bone from Cretaceous through Eocene phosphatic conglomerates in the Taoudeni and Iullemmeden Basins of northeastern Mali. Coprolites are extensively penetrated by flask-shaped borings (Gastrochaenolites): the oldest known occurrence of the ichnospecies G. ornatus preserves mechanical scratch traces at the base of some early Eocene coprolite borings. The alleged tracemaker, a pholad bivalve of the Subfamily Martesiinae, is preserved as an external mold in one of the early Eocene coprolites. It is the first occurrence of rock-borers in this subfamily from the Paleocene of West Africa. Bored coprolites were very firm to fully lithified (paleocoprolites) when bored by the pholads, indicating early-diagenetic phosphogenesis of the clasts. The intense and repeated bioerosion of coprolites and bone by pholads suggests a long residence time in shallow-marine waters for the clasts. Therefore, the vertebrate fossil assemblages in these Mali conglomerates are considered highly time-averaged. Recognition of Gastrochaenolites borings in vertebrate remains provides useful taphonomic and paleoenvironmental information, despite their destructive impact on the fossils. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Facies and diagenesis of some upper proterozoic dolomites of South China

The Dengying Formation exposed at Sansha, Human Province, central South China, comprises unfossiliferous dolomitized platform carbonates and a solution-collapse breccia sequence. Microfacies analysis indicates a cyclic, increasingly restricted lagoonal to oolithic platform marginal facies. Sedimentation was repeatedly interrupted by periods of subaerial exposure as suggested by features of meteoric diagenesis and karstification. During a final regressive phase selective karst-dissolution of strata, interpreted as dolomite-evaporite interbeds, created a solution-residual/collapse breccia sequence. The Dengying Fm. is disconformably overlain by Lower Cambrian transgressive phosphatic conglomerates and fossiliferous black shales. These local facies data add another piece to the puzzle of Proterozoic/Cambrian Yangtze platform sedimentational history and demonstrate the applicability of microfacies analysis to uppermost Proterozoic strata.

Genesis of gray monazites; evidence from the Paleozoic of Belgium

Systematic mineralogical investigations, carried out on panned stream concentrates from the northwestern part of the Cambro-Ordovician Stavelot massif (Belgium), led to the discovery of nodular gray monazites. These were later identified in situ in black slates associated with conglomeratic levels containing phosphatic material. Microscopic investigation shows that gray monazites are authigenic and predate cleavage. To test the hypothesis of a sedimentary origin for rare earth elements, samples of shales, phosphatic conglomerates, and monazite concentrates were submitted to sequential inductively coupled plasma analysis. Results show that the rare earth element pattern in gray monazites is a reflection of the abundance of these elements in the host rock. This suggests a model invoking desorption of the rare earth elements from their clayey support and differential precipitation of rare earth phosphates in the sediment initiated by P mobility during diagenesis. The genetic model for gray monazite mineralization assumes three essential and complementary phenomena: (1) sedimentation of clays in a reducing marine environment, (2) a phosphate anomaly in the same lithological sequence, and (3) evolution to low-grade epizonal metamorphic conditions.These nonrestrictive genetic conditions clearly suggest that gray monazites have been generally overlooked and must be much more widespread than believed. The economic interest of gray monazite lies in potential stream or beach placers. Their low Th content and comparatively high Eu content are interesting features in the scope of ore beneficiation.

Eustatically controlled ooidal ironstone deposition: facies relationships of the Ordovician open-shelf ironstones of Western Europe

Summary Ooidal ironstones are a characteristic feature of the Ordovician successions of south-west Europe and adjacent areas. They occur in shallow marine clastic sequences deposited on a broad low-topography shelf. Three stratigraphic intervals have a particularly widespread development of ironstones: the early Llanvirn, the early Caradoc and the early Ashgill. These intervals, corresponding to periods of major global eustatic sea-level rise, are marked by the development of thin, but persistent, ooidal ironstones even in relatively offshore areas. The ironstones produced by these three events have been studied in the previously contiguous areas of central Portugal and Armorica. The development of the ironstones, and the nature of the successions within which they occur, show remarkable similarities among the three horizons; this recurrence of similar sedimentary facies defines a cyclicity. In each of these ‘first-order’ cycles the upper part is characterised by the occurrence of a second order of cyclicity, represented by asymmetric coarsening— and thickening—upwards sequences of siltstone and sandstone tempestites, commonly capped by phosphatic conglomerates. The latest ‘second-order’ cycle in each case is characterized by a more distal sequence of dark mudstones with graptolites. The chamositic ooidal ironstones lie disconformably above these upper mudstone-dominated cycles, and are interpreted as forming during ‘second-order’ transgressions late in major global transgressive events. Both ‘first-’ and ‘second-order’ sedimentary cyclicity is attributed to eustatic variation. The ‘first-order’ eustatic cycles have a period of 10–15 Ma, and largely correspond to the international stratigraphic series, while the period of the ‘second-order’ cycles is estimated as 0.2–0.5 Ma.

Phosphatic Hardgrounds and Hiatus Concretions in Neogene Marine Sequences of California Coastal Ranges: ABSTRACT

Distinctive phosphatic intervals mark hiatuses of varying durations at several localities along the central California coast. At Capitola in the La Honda basin, a complex series of carbonate-cemented hardgrounds containing authigenic phosphatic nodules and marine vertebrate bones occurs within 50 cm of the upper Purisima Formation (Pliocene). Paleomagnetic measurements and diatom stratigraphy indicate that this 50-cm interval represents a hiatus of about 1 m.y. (4.5-3.5 Ma). At Naples Beach in the Santa Barbara basin, a severely condensed section of about 7 m of interbedded nodular phosphatic hardgrounds, phosphatic marlstones, and dolomites appears to represent as much as 5 m.y. (14.5-9.5 Ma). In the Lompoc Quarry of the Santa Maria basin, a thin phosphatic hardground at the contact between the Monterey and the Sisquoc Formations represents about 0.5-0.7 m.y. (between 7.0 and 6.0 Ma). At Mussel Rock in the Santa Maria basin, a nodular phosphatic conglomerate occurs at the boundary between the Monterey and Sisquoc Formations; the age of the conglomerate determined from diatom biostratigraphy is approximately 6.2 Ma (Messinian), but no time gap can be recognized given the resolution of diatom age dating. The phosphatic hardgrounds of Capitola, Lompoc Quarry, and Naples Beach appear to correspond to high sea levelmore » stands, whereas the conglomerate at Mussel Rock coincides with a sea level lowstand. During high sea level stands, depocenters shifted landward, leading to clastic-sediment starvation possibly accompanied by current-induced erosion and winnowing, which in term led to condensation and early diagenetic carbonate and phosphatic cementation.« less

Albian and Cenomanian ammonites from the island of Bornholm (Denmark)

The ammonite faunas from the Arnager Greensand and its conglomerate on the island of Bomholm (Denmark) are revised. Three time intervals are represented in the phosphatic conglomerate. The ammo­nites from the primary nodules represent two time intervals in the Lower Albian ( Leymeriella tardefurcata and Douvil/eiceras mammilatum Zones). The ammonites from the phosphatized matric of the secondary nodules are assigned to the Lower Cenomanian (Mantelliceras saxbii and/or M. dixoni Zones). The Arnager Greensand sensu stricto is low in the Middle Cenomanian (Turrilites costatus Zone) on the basis of ammonites and plank tic foraminifers. The transgressive history of the Mid-Cretaceous on the island of Bornholm is outlined.

THE DIAGENESIS AND DEPOSITIONAL ENVIRONMENT OF THE PERMIAN RANGER CANYON AND MOWITCH FORMATIONS, ISHBEL GROUP, FROM THE SOUTHERN CANADIAN ROCKY MOUNTAINS

SummaryThe Ranger Canyon and Mowitch Formations are the youngest Permian strata in the Rocky Mountain Front Ranges. They constitute a thin, widespread and clearly defined stratigraphic entity covering approximately 60,000 sq. miles; they occur below Triassic beds and above a major, regional, intra‐Permian unconformity, initiated by a transgression which started in the northwest. Beds of the above entity are therefore diachronous. The erosion surface is overlain by polymict, phosphatic conglomerates containing mature lag gravels and immature breccio‐conglomerates and corrosion breccias.Rock types are either sandstones or silty and sandy cherts, phosphorites and occasionally dolomites. Clastic components are dominantly mature quartz (some pseudomorphs gypsum), phosphate, some feldspar, negligible clay, and stable detrital minerals, necessitating: (1) many cycled, clastic sedimentary rock; (2) local gypsum; and (3) phosphate producing, distributive provinces.Secondary features include primary cementation by quartz, chalcedony, dolomite, sulphate and phosphate, with replacement mainly by chalcedony. Contraction fractures are cemented by quartz (after anhydrite and fluorite), calcite and barite. Accessory authigenic minerals are hematite, sulphates (pseudomorphed by quartz), fluorite, apatite and dolomite. Diagenesis therefore occurred within a chemically active environment.Minerals present indicate: (1) phosphate producing and (2) evaporite producing environments, the components for which may be obtained from sea water by: (a) a biogeochemical phosphate‐fixation cycle; and (b) concentration of brines by interstitial refluxion.A Recent environmental parallel occurs along the coast of Baja, California. Phosphate produced off‐shore contributes to quartz‐rich sediments prograding seawards; evaporitic conditions produce minimum carbonate and maximum sulphate precipitation.The sequence of events envisaged for the Permian rocks started with phosphate corrosion of bed‐rock and cementation of lag gravels; some phosphate was contributed to shoreline, with quartz and gypsum sands, prograding over the basal conglomerate. Sand cementation by carbonate occurred in shoreline and intertidal zones, and by sulphate in the supratidal zones. Concentration of brines by refluxion, and local silica enrichment, facilitated silica precipitation and replacement. Penecontemporaneous silicification of Late Permian sediments was therefore instrumental in their ultimate preservation.

A correlation of the Uppermost Albian and the Cenomanian of South-West England

In south-west England, the uppermost Albian and the Cenomanian show considerable lateral variation when compared with the Gault, Upper Greensand and Lower Chalk succession of south-east England. From Warminster as far south as Melbury Down, west of Salisbury, there is a continuous succession from Albian to Cenomanian, with some Cenomanian Upper Greensand. In the valley of the Stour, Cenomanian greensands have disappeared, and the top of the Upper Greensand is of Upper Albian Stoliczkaia dispar and Mortoniceras (Durnovarites) perinflatum Sub-zone age as far west as Evershot. A few kilometres south of Evershot, a quartzose sandstone, the Eggardon Grit, appears. At some localities, this grit yields Lower Cenomanian ammonites. The Eggardon Grit and the underlying Chert Beds appear just north of the line of the Hooke Valley, and thicken considerably in a south-westerly direction. These relationships can be interpreted as the result of a major north-west to south-east ‘structure’, the Mid-Dorset swell of Drummond ( in Smith & Drummond, 1962). Westward, the top of the Upper Greensand is highly variable, with boulder beds, sandy limestones and phosphatic conglomerates, the latter yielding rich Lower Cenomanian ammonite faunas. Over most of south-west England the base of the Chalk is a phosphatic conglomerate known as the Chalk Basement Bed. This is diachronous. When traced south-westward, it yields phosphatised and unphosphatised faunas indicating progressively higher horizons in the Middle and Upper Cenomanian. Bed C, of Actinocamax plenus Marls, Metoicoceras gourdoni Zone age, the base of the Middle Chalk on the Devon Coast, is the obvious extension of this Basement Bed.

New Lithologic Marker Horizons in Ordovician Rocks, Eastern Ouachitas of Arkansas: GEOLOGICAL NOTES

During recent geologic field mapping in the eastern Ouachitas of Arkansas, several new lithologic marker horizons have been recognized in the Ordovician rocks. The outcropping formations of Ordovician age include the Blakely, Womble, Bigfork, and Polk Creek. Meta-arkose boulders and gray-black chert layers are characteristic of the upper part of the Blakely Sandstone. The Womble Shale has been divided into 3 members. The lower member consists primarily of interbedded shale and calcareous siltstone. Individual layers often exhibit a very delicate banding. The middle member of the Womble is essentially black shale. The upper member is primarily shale but typically has beds of phosphatic conglomerate in its northern outcrops which are correlative with poorly sorted sandstones in the S. The Bigfork Chert is composed of chert and siliceous shale; commonly a black, sooty, punky, carbonaceous clay is found at the base. The Polk Creek Shale is chiefly a shale with some thin chert beds. No now lithologic markers were found in this unit.