Limestone Member Research Articles

Rifting of a Mississippian continental arc system: Little Salmon formation, Yukon–Tanana terrane, northern Canadian Cordillera

The Yukon–Tanana terrane in the northern Canadian Cordillera records the development of a series of mid- to late Paleozoic arc systems, punctuated by intra-arc deformation, uplift, and episodic rifting coeval with back-arc extension, built upon a metasedimentary basement of northwestern Laurentian affinity. In central Yukon, the Little Kalzas formation records the development of one of these Mississippian continental arcs, whereas the Little Salmon formation records the development of an intra-arc rift basin within a continental arc. The Little Salmon formation lower succession comprises mainly volcaniclastic rocks derived from erosion of Early Mississippian and older units, including rocks of the Little Kalzas continental arc. Above a medial limestone member, the upper succession of the Little Salmon formation includes alkali basalt, breccia, and crystal and ash tuffs in the north and predominantly epiclastic rocks interbedded with crystal and ash tuffs in the south. The alkali basalts have the geochemical characteristics of ocean-island basalts and their positive ε Nd 340 (+7.3) and low 87Sr/86Sr values (0.705) suggest a primitive magma source with little or no involvement of continental crust. The transition between the northern and southern facies of the upper succession of the Little Salmon formation coincides with a northeast-trending synvolcanic fault inferred to have controlled alkali basalt eruptions and deposition of Mn-bearing exhalite in the north and basin plain sedimentation in the south. The environment of deposition of the Little Salmon formation resembles that of the modern Sumisu rift in the Izu–Bonin–Mariana arc system or the early stages of development of the Japan island-arc system.

The Middle Pleistocene Al Mahrúqah Formation in the Murzuq Basin, northern Sahara, Libya evidence for orbitally-forced humid episodes during the last 500,000 years

Geological and geomorphological work in the endorheic Murzuq Basin (Fezzan) in the northern Sahara Desert, SW Libya, has revealed that the Middle Pleistocene Al Mahrúqah Formation consists of at least four limestone beds at four distinct elevations. The corresponding limestone beds are named (from oldest to youngest) the Antalkhatá Member, the Brak Member, the Bi'r az Zallaf Member, and the Aqar Member. The four members are interpreted as lacustrine deposits, and they are well distinguished by their spatial extent and lithology. Twenty-five radiometric and thermal ionisation mass spectrometric (TIMS) 230Th/U ages ranging from about 490–130 ka have been determined on these lacustrine limestones. According to these radiometric 230Th/U ages, the accumulation of the four limestone members of the Al Mahrúqah Formation followed the orbitally-forced (Milankovitch) eccentricity cycle of 100 ka during at least the last 400 ka. The four members formed in humid episodes of several tens of thousands of years' duration, when large lakes might have existed. The δ 13C and δ 18O values from samples of the four members of the Al Mahrúqah Formation provide independent genetic evidence on the lacustrine origin of the deposits in the Murzuq Basin. The spatial extent of the limestone deposits, and thus the intensity of the humid episodes, decreases at least from the Brak Member to the Aqar Member from about 52,000 km 2 to 1400 km 2, respectively. In spite of the limited precision and reliability of the new 230Th/U ages, it seems possible that short humid episodes interrupted the intervening arid episodes. Other long-term palaeohydrological records from northern and southern Africa as well as the Mediterranean region support the palaeoclimatic results from the Murzuq Basin.

Pennsylvanian (Late Carboniferous) chondrichthyans from the LaSalle Limestone Member (Bond Formation) of Illinois, USA

Pennsylvanian (Late Carboniferous) chondrichthyans from the LaSalle Limestone Member (Bond Formation) of Illinois, USA

Quantification of deep-time orbital forcing by average spectral misfit

Quantification of Milankovitch orbital cyclicity within ancient strata has become a principal tool for refinement of the geologic time scale. However, accurate characterization of the orbital signal in deep time paleoclimate records is commonly challenged by inadequate radiometric time constraints for calibration of the spatial rhythms to temporal periods. This problem can potentially introduce large errors into derivative orbital timescales. In this study we develop a new method for the identification and calibration of orbital cyclicity in cyclostratigraphic records. The method (average spectral misfit, or ASM) yields an objective estimate of the optimal sedimentation rate for a stratigraphic interval that preserves a record of orbital forcing. The technique also provides a formal statistical test for rejecting the null hypothesis (no orbital signal). Application of the method to assess orbital cyclicity in the upper Bridge Creek Limestone Member (Turonian) of the Western Interior Basin highlights the utility of this new cyclostratigraphic tool, and provides a means to independently evaluate conflicting interpretations of the lithologic cycles. Importantly, ASM offers a new consistent standard by which orbital timescales may be compared. Hence, the quality of an orbital timescale can be formally qualified by reporting its average spectral misfit and null hypothesis significance level. This technique will permit improvement of Mesozoic/Cenozoic orbital timescales and extension of orbital time scale development into the Paleozoic, as the method is not dependent upon well-constrained radiometric age data.

Historical analysis and reinterpretation of the lower part of the Canyon Group (Missourian, Upper Pennsylvanian), Wise County, Texas: A fine can of worms

Late Pennsylvanian (Early Missourian) age limestone strata exposed in several isolated outcrops in the Trinity River Valley in southwest Wise County, Texas were last described by Bose (1917), and Scott and Armstrong (1932). These strata can be correlated with the Upper Pennsylvanian Palo Pinto and Posideon formations of Laury (1962) in the lower part of the Canyon Group in the Brazos River Valley. The two reports were stratigraphically inconsistent concerning the seven limestone units of the Palo Pinto Formation as named by Scott and Armstrong (1932), and both included many poorly located type sections. The Bose (1917) report included no map and many key geographic references were to 1914 ranch names. Wise County records have helped to pinpoint these locations and allowed the reconciliation of the Bose (1917) Palo Pinto limestone localities to those of Scott and Armstrong (1932). Examination of exposures of these strata, and their associated fusulinid and conodont faunas has demonstrated that the following correlations to the Canyon Group in the type area in the Brazos River Valley in Palo Pinto County can be made (in descending order): (i) a conodont-rich black mudstone (indicating maximum transgression and a deep basin environment) at the base of the Wolf Mountain Shale and just above the Wiles Limestone correlates to a similar interval just above the Willow Point Limestone, well exposed in the area around the southwest shore of Lake Bridgeport; (ii) the Willow Point Limestone (= Bridgeport Limestone of Bose) correlates to the Wiles Limestone (top of the Posideon Formation); (iii) strata in the Martin Lake area just south of Bridgeport correlate to equivalent age conodont-rich black mudstones present in the middle part of the Posideon Formation; (iv) the Martin Lake Limestone is a fusulinid/algal grainstone containing a distinctive nankinellid fusulinacean at all outcrops and indicates very shallow marine deposition. It correlates with the top part (= Wynn Limestone) of the Palo Pinto Formation; (v) the Hudson Bridge Limestone correlates with the lower part of the Palo Pinto Formation. A black shale interval a few centimeters thick found in a core taken in an abandoned quarry in the Palo Pinto Formation in southwestern Wise County is correlated to a crinoid - conodont-rich 10-25cm unit that marks the base of the Palo Pinto Formation on top of the Keechi Creek Shale in much of Palo Pinto County. The new correlation of the Willow Point Limestone, the uppermost member that Scott and Armstrong (1932) reported as being in the Palo Pinto Formation, and its placement into the overlying Posideon Formation, leaves six limestone members in the Palo Pinto Formation identified in Wise County. Its top and bottom members, the Martin Lake Limestone and the Hudson Bridge Limestone, respectively, now are firmly correlated to the top and bottom, respectively, of the Palo Pinto Formation in the type area. A 1937 revision of the geologic map of Wise County raised the lower boundary of the Palo Pinto Formation, placing its lowest two members into the older Mineral Wells Formation of the Strawn Group (Armstrong 1937).

PLACUNOPSIS BIOHERMS: THE FIRST METAZOAN BUILDUPS FOLLOWING THE END-PERMIAN MASS EXTINCTION

Abstract Outcrops of the Lower Triassic (Spathian) Virgin Limestone Member (Moenkopi Formation) in the southwestern United States contain the oldest known metazoan bioherms formed in the aftermath of the end-Permian mass extinction. These small buildups, up to 1.0 m in diameter and 0.2 m high, were constructed by cementing bivalves. The bivalve bioherms accreted in a shallow, subtidal environment above storm wave base atop an oolitic hardground on a carbonate ramp. Similarities in shell microstructure and bioherm morphology between the Lower Triassic buildups reported here and previously described Middle Triassic occurrences suggest that the Lower Triassic bioherms were likely built by a bivalve assignable to Placunopsis. Although taxonomic assignment of the bivalve remains uncertain, the presence of cemented bivalve bioherms in Lower Triassic sections of the southwestern United States demonstrates that cementing bivalves were geographically widespread, even early in their Mesozoic evolutionary history. D...

The Takena Formation of the Lhasa terrane, southern Tibet: The record of a Late Cretaceous retroarc foreland basin

Our understanding of the processes involved in the Indo-Asian collision and the construction of the Tibetan Plateau are, in part, predicated on our understanding of the tectonic setting and crustal conditions of southern Tibet in the time period immediately preceding the Indo-Asian collision (Late Cretaceous). Several hypotheses have been proposed that describe the middle to Late Cretaceous tectonic and paleogeographic evolution of southern Tibet, each with different implications for when and how the Tibetan Plateau was uplifted. We examined the mid-upper Cretaceous Takena Formation of the Lhasa terrane in southern Tibet in order to reconstruct the middle to Late Cretaceous tectono-sedimentary history of this area and test the competing tectonic hypotheses. The Takena Formation consists of >2 km of sedimentary strata that include a lower marine limestone member (Penbo Member) and an upper member of clastic red beds (Lhunzhub Member). The Aptian-Albian Penbo Member consists of ~250 m of orbitolinid-limestone beds that were deposited in a shallow-marine seaway during a time when there was little regional subsidence. The overlying Lhunzhub Member (>1500 m) consists primarily of fl strata that were deposited during a period of increased subsidence. Overall, the Lhunzhub Member coarsens upward from meandering and anastomosing stream deposits interbedded with numerous paleosols near the base, to multistory-multilateral braided stream deposits near the top. The fl uvial sandstone units are lithic-rich and contain abundant plagioclase and volcanic grains, and paleocurrent data record northwest-directed transport; these data indicate that the sediment was derived from the Gangdese volcanic arc that developed along the southern margin of the Lhasa terrane. Following deposition, but prior to ca. 70 Ma, the beds of the Takena Formation were folded and partially eroded. The sedimentary and stratigraphic characteristics of the Takena Formation are most consistent with deposition in a retroarc foreland basin setting. The limestone beds of the Penbo Member were deposited in a shallowmarine seaway that was eventually infi lled by clastic sediment derived principally from the Gangdese volcanic arc. The low subsidence rate recorded in the lowermost strata of the Takena Formation, including the Penbo Member and the paleosol-rich interval of the Lhunzhub Member, is interpreted to be associated with the passage of a fl exural forebulge. The overlying, upward-coarsening fl uvial strata were deposited in progressively more proximal locations within the foredeep of a foreland basin. The Late Cretaceous folding of the Takena Formation indicates that the foreland basin strata were eventually incorporated into a fold-and-thrust belt. The upper Cretaceous sedimentary strata of the Lhasa terrane of southern Tibet predict that a north-verging fold-and-thrust belt existed along the southern margin of the Lhasa terrane during Late Cretaceous time, prior to the Indo-Asian collision. Collectively, the evidence indicates that the southern margin of the Lhasa terrane had thickened crust and was likely to have been at high elevations immediately before the Indo-Asian collision.

Stevns Klint, Denmark: Uppermost Maastrichtian chalk, Cretaceous–Tertiary boundary, and lower Danian bryozoan mound complex

Stevns Klint is a 14.5 km long coastal cliff south of Copenhagen, Denmark. It is a classical Cretaceous–Tertiary boundary locality and constitutes the type locality of the Danian Stage together with the nearby Faxe quarry. The irregular coastal topography, the presence of numerous old quarries in the upper, Danian part of the cliff section, and the inaccessibility of parts of the coast makes it difficult to construct an exact geological profile of the whole length of the cliff. Until now the only profile is a fine engraved pen drawing from a boat by Puggaard in 1853. We present a new profile of the cliff constructed on the basis of multi-model photogrammetric mapping of a long series of overlapping photographs taken from a small airplane. This was supplemented by photography and observations from a boat and from the beach. The exposed succession is about 45 m thick and is subdivided into a series of new lithostratigraphic units. It shows the stratigraphic evolution from the uppermost Maastrichtian, across the Cretaceous–Tertiary boundary and into the lower Danian. The position of the boundary varies from about 5 m below to about 30 m above present day sea level. The irregular relief has been interpreted as due to Paleogene folding but it actually represents a depositional sea-floor topography. The lower part of the succession comprises about 25 m of upper Maastrichtian benthos-rich chalk of the newSigerslev Member showing irregular low-amplitude mounded bedding outlined by thin layers of nodular flint, passing upward into benthos-poor, gently wavy to almost horizontally bedded chalk poor in flint. The member is capped by a double incipient hardground and a prominent marker band of nodular flint, occurring 30–50 cm beneath the upper hardground can be followed along the length of the cliff. The upper hardground is overlain by uppermost Maastrichtian bryozoan chalk wackestone deposited in low asymmetrical mounds of the new Højerup Member. It is 4–5 m thick in the southern part of the cliff, thins gradually to the north, and has wedged out almost completely at the northern end of the cliff. The new Sigerslev and Højerup Members form the top part of the Maastrichtian Tor Formation in the Stevns Klint area. It is overlain by the new lowermost Danian Fiskeler Member (P0 foraminifer Zone) which drapes the troughs between the mound crests and wedges out gradually towards the margins of the troughs. It is generally about 5–10 cm thick and passes gradually upwards into the strongly burrowed new Cerithium Limestone Member (Pα–P1a Zones), which is up to 60 cm thick. The Fiskeler and Cerithium Limestone Members form the new Rødvig Formation. An erosional hardground surface truncates the top of the Cerithium Limestone Member and the intervening crests of the uppermost Maastrichtian mounds of the Højerup Member. The surface is overlain by lower Danian bryozoan packstone–rudstone mound complexes (foraminifer zones P1b–P1c , local coccolith zones 2–3, Nannoplankton Zones D2–D3). The internal architecture of the mounds is outlined by thick black flint layers and the mounded bryozoan limestone belongs to the new Korsnæb Member of the new Stevns Klint Formation. Up to 20 m of bryozoan limestone are preserved beneath the Quaternary deposits, forming the top of the cliff succession. The new photogrammetric profile may serve as an excursion guide, as a basis for detailed sedimentological study, and for biostratigraphical, geochemical and palaeomagnetic sampling. Finally, it illustrates the geometry, dimensions and architecture of one of the finest carbonate mound complexes known.

The cardioceratid ammonite fauna of the Upware Limestone and Dimmock’s Cote Marl (Upper Jurassic) in Cambridgeshire, England

SUMMARY The West Walton Formation (Dimmock’s Cote Marl and Upware Limestone members) of Upware, Cambridgeshire, has yielded an abundant cardioceratid fauna. Three assemblages of Cardioceras, comprising eight nominal species, are described and figured. Faunas such as these from Maltonense–Tenuiserratum Subzone boundary beds are rare in the UK, and the Upware ammonites add significantly to our understanding of the manner in which Miticardioceras, a subgenus of Cardioceras important for correlation of Middle Oxfordian strata across northern Europe, appears in the UK.

The Sacta Limestone Member (early Wenlock): Cool-water, temperate carbonate deposition at the distal foreland of Gondwana's active margin, Bolivia

The Sacta Limestone Member is located towards the base of the Silurian Kirusillas Formation in central Bolivia, and forms part of the sedimentary record of the Peru–Bolivia Palaeozoic Basin. The early Wenlock (Sheinwoodian) age of this thin carbonate unit has been established by conodont and foraminifer biostratigraphy. During the Silurian, a thick siliciclastic wedge was deposited in the foredeep of the basin, related to tectonism along the active margin of Gondwana. Strong subsidence and downslope resedimentation (slides, slumps, debris flows, turbidites) dominated towards the west of the basin during the Llandovery and early Wenlock. In contrast, a much thinner record was deposited at the distal margin of the foreland basin, pinching out towards the northeast and onlapping the stable Precambrian basement. The Sacta Member represents cool-water, temperate carbonate deposition at mid-to-high latitude on the distal stable margin of this foreland basin. Its sedimentation took place towards the end of a phase of important tectonic instability, and the carbonate bed was itself also partially resedimented. Late Ashgill?–Llandovery glaciation of nearby reliefs along the active margin of Gondwana was followed by brief and local cool-water, temperate carbonate deposition in the early Wenlock, and subsequently by the development of a storm- and wave-dominated shallow clastic shelf during the rest of the Silurian. The Sacta Member records carbonate deposition in western Gondwana during a brief and singular event with important palaeogeographic and palaeoclimatic implications.

Carboniferous–Permian Boundary in Kansas, Midcontinent, U.S.A.

The placement of the Carboniferous (Pennsylvanian)-Permian boundary in Kansas has been debated since the rocks of this age were first described and named. With the ratification of the Global Stratotype Section and Point (GSSP) for the base of the Permian System in the southern Ural Mountains, the Carboniferous-Permian boundary in Kansas can now be confidently defined. Based on the identification of the first occurrence of the conodont Streptognathodus isolatus that definitively correlates the Kansas rock section to the basal Permian GSSP, the Carboniferous-Permian boundary in Kansas can be placed at the base of the Bennett Shale Member of the Red Eagle Limestone.
 The Kansas Geological Survey proposes that the Tuttle Creek Lake Spillway section, located in northeast Kansas, be considered for the Carboniferous-Permian boundary stratotype in Kansas. It is further suggested that the stratigraphic position of the Carboniferous-Permian boundary in the Tuttle Creek Lake Spillway section be considered as a potential North American stratotype.
 In addition to being a significant biostratigraphic boundary, the Carboniferous-Permian boundary and enclosing strata also have significance because they reflect important geologic events and changes that occurred on a regional and global scale.

Sedimentology and geochemistry of extensive very coarse deepwater submarine fan sediments in the Middle Jurassic of Oman, emplaced by giant tsunami triggered by submarine mass flows

Unusual fining upwards coarse conglomerates overlain by sandstones, thin cherts and green shales occur at the top of the deep-water submarine fan deposits of the Oolitic Limestone Member of the Jurassic Guwayza Formation of Oman. They separate the dominantly submarine fan deposits of the Guwayza Formation from the pelagic shales, fine-grained limestones and cherts of the overlying Sidr Formation. The cross-bedded and graded framework conglomerates occur in extensive, tabular units and are dominated by earlier Mesozoic carbonate clasts with sandy oolitic and peloidal grains derived from fault escarpments and shelf sediments far to the southwest. Subordinate inverse grading, very thick beds, very large floating clasts (up to 100 m long in places) indicate deposition from catastrophic debris flows. Though most palaeocurrents indicate flow from off the platform to the southwest, hummocky cross-bedding shows divergent palaeocurrents suggesting movement in part by deep-water waves. The beds are too coarse for antidune formation and the conglomerate to sand hummocks indicate decelerating flow. There are no nearby large objects to deflect turbidity currents to form divergent flows. We consider that the hummocky cross-stratification, like that in shallow water, was formed by interfering waves. That such coarse, tabular conglomerates affected by wave action occur over extensive areas across deep submarine fan environments, suggests deposition by high-velocity seaward-moving debris and grain flows followed by reworking by waves large enough to redistribute coarse sediment in deep water. The only waves large enough are those of giant tsunami. Petrology and geochemistry show no impact or explosive volcanic constituents in the finer units and the waves involved are too large for generation directly by submarine fault displacements. We suggest that the top Guwayza conglomerates were deposited by very large submarine slides which were then reworked by the tsunami generated by them. Such contemporary massive slope failure deposits are present on the adjacent slope and shelf margin.

The irregular echinoid Nucleopygus from the Upper Cretaceous of western Jamaica

Late Cretaceous echinoids of Jamaica are moderately diverse, but specimens are rare. The upper Santonian Clifton Limestone Member (Mount Peace Formation) of the Lucea Inlier (western Jamaica) includes common tests of Nucleopygus cubensis Weisbord preserved in oosparites. This species was previously reported from Cuba. The test is ovate, rounded anteriorly and rounded to more truncate posteriorly, moderately high domed, highest posterior to the apical system; the posterior sulcus is deep, narrow and short; and the ambulacral petals are open, but closing. The local accumulation of this echinoid is interpreted as a habitat preference for this echinoid, which is unknown from other lithofacies. It is commonly preserved as a complete test, suggesting it was particularly robust by analogy with material from other echinoid “conglomerates”.

Stratigraphic Implications of Lower Ordovician Conodonts from the Munising and Au Train Formations at Pictured Rocks National Lakeshore, Upper Peninsula of Michigan

Early Ordovician conodonts have been recovered from the upper Munising Formation and the lower part of the Au Train Formation at Pictured Rock National Lakeshore in the Upper Peninsula of Michigan. Two trilobites from the lower Au Train probably represent the Symphysurina Zone ( S. woosteri Subzone) and confirm the Early Ordovician age of the Au Train. The presence of middle Late Cambrian (Franconian) trilobites in the underlying Munising Formation led to traditional assumption of a major hiatus between the Munising and the Au Train. An unconformity between these formations has been difficult to recognize in outcrops because upper Munising sandstones grade upward into weathered, reworked beach sands deposited during Ordovician transgression. The reworked sands traditionally have been assigned to the Munising, with the base of the Au Train placed at the lowest occurrence of carbonate rocks in the sequence. Conodonts from the uppermost Munising and lower Au Train are characteristic of the Rossodus manitouensis Zone (upper Skullrockian Stage, Millardan Series, Lower Ordovician). The conodont and trilobite data permit correlation of the rocks of this interval with sequence stratigraphic units in the Burnout Canyon Member of the House Limestone in the Ibex area of Utah and with equivalent strata in other parts of Laurentia.

Correlation of Mississippian (Upper Viséan) foraminiferan, conodont, miospore and ammonoid zonal schemes, and correlation with the Asbian–Brigantian boundary in northwest Ireland

AbstractThe microbiota of the upper Viséan (Asbian–Brigantian) rocks in the Lough Allen Basin in northwest Ireland is analysed. The Middle Mississippian sequence studied extends from the upper part of the Dartry Limestone/Bricklieve Limestone formations of the Tyrone Group to the Carraun Shale Formation of the Leitrim Group. The rocks have been traditionally dated by ammonoid faunas representing the B2a to P2c subzones. The Meenymore Formation (base of the Leitrim Group) also contains conodont faunas of the informal partial‐range Mestognathus bipluti zone. The upper Brigantian Lochriea nodosa Conodont Zone was recognized by previous authors in the middle of the Carraun Shale Formation (Ardvarney Limestone Member), where it coincides with upper Brigantian ammonoids of the Lusitanoceras granosus Subzone (P2a). Foraminifera and algae in the top of the Dartry Limestone Formation are assigned to the upper Cf6γ Foraminifera Subzone (highest Asbian), whereas those in the Meenymore Formation belong to the lower Cf6δ Foraminifera Subzone (lower Brigantian). The Dartry Limestone Formation–Meenymore Formation boundary is thus correlated with the Asbian–Brigantian boundary in northwest Ireland. For the first time, based on new data, a correlation between the ammonoid, miospore, foraminiferan and conodont zonal schemes is demonstrated. The foraminiferans and algae, conodonts and ammonoids are compared with those from other basins in Ireland, northern England, and the German Rhenish Massif. Historically, the Asbian–Brigantian boundary has been correlated with several levels within the P1a Ammonoid Subzone. However, the new integrated biostratigraphical data indicate that the Asbian–Brigantian boundary in northwest Ireland is probably located within the B2a Ammonoid Subzone and the NM Miospore Zone, but the scarcity of ammonoids in the Tyrone Group precludes an accurate placement of that boundary within this subzone. Copyright © 2006 John Wiley & Sons, Ltd.

FOSSIL MARINE VERTEBRATES FROM THE LOWERMOST GREENHORN LIMESTONE (UPPER CRETACEOUS: MIDDLE CENOMANIAN) IN SOUTHEASTERN COLORADO

Fossiliferous deposits of the Greenhorn Limestone (Upper Cretaceous) are found on the Comanche National Grassland in southeastern Colorado. The lowest portion of the Greenhorn Limestone, the base of the Lincoln Limestone Member, contains calcarenite beds rich in disarticulated remains of marine vertebrates. We examined the composition of the vertebrate paleofauna from one particular location (Tobe locality) through surface collecting and acid solution of rocks. The paleofauna is taxonomically diverse, consisting of 22 chondrichthyans, at least 15 osteichthyan fishes, and six aquatic reptiles. Molluscan taxa indicate that the basal Lincoln Limestone in southeastern Colorado was deposited sometime between middle Middle Cenomanian (ca. 95 Ma) and late Middle Cenomanian (ca. 94.7 Ma), and the composition of the vertebrate fauna (primarily chondrichthyan taxa) agrees with this interpretation. The Tobe locality was situated far from the shorelines of the Western Interior Seaway, with relatively high-energy waves impinging upon the seafloor, concentrating biogenic remains. Fossil remains within the calcarenite appear to have undergone minor time-averaging, based upon erosional rounding and breakage present in numerous specimens. Nevertheless, the deposit contains one of the best representative Middle Cenomanian vertebrate communities so far described from the Western Interior Seaway of North America. Considering that the paleofauna consists of taxa diverse in size, morphology, and inferred ecology, the trophic structure of the paleocommunity was probably complex.

Integrated biostratigraphy of foraminifers, radiolarians and conodonts in shallow and deep water Middle Permian (Capitanian) deposits of the "Rader slide", Guadalupe Mountains, West Texas

A diverse assemblage of microfossils is present in a 6m thick sequence of three debris flow deposits interbedded with thin turbidite limestone beds and fine grained siliciclastics exposed above the megaconglomerate in a section (known as the “Rader Slide" in numerous guidebook stops) of the Rader Limestone Member of the Bell Canyon Formation of Capitanian age (Middle Permian) in the Guadalupe Mountains of West Texas. Each debris flow, derived from nearby Capitan Reef shelf-margin and slope deposits, contains a distinct microfossil assemblage. Small foraminifers and fusulinaceans, conodonts, radiolarians, sponge spicules, fish dermal plates and teeth, and other fragmental fossils are present in this sequence. Conodonts are relatively scarce in the first (or lowest) debris flow, except in its upper part, but they are common to abundant in the other two debris flows, and very abundant in several of the thin turbidite limestone beds. All of the conodonts present appear to be morphotypes of one population of the species Jinogondolella postserrata, except for one new conodont species, and the Jinogondolella postserrata Zone is clearly documented in this sequence. The debris flows contain the fusulinaceans Rauserella, rare Codonofusiella, Polydiexodina, Leella? and various species of the small foraminifers Globivalvulina, Hemigordius, Baisalina, Abadehella, Deckerella, Neoendothyranella, Vachardella, Geinitzina, and Polarisella. Some of the thin turbidite limestone beds contain a foraminiferal assemblage similar to that found in the debris flows, but with lower diversity. Many small foraminiferal species appear to be endemic, although a few are closely related to species known in Permian age strata in Italy, Greenland, the Russian Far East, northeastern part of Russia (Omolon massif), and the Zechstein of Germany and the Baltic area. Two thin limestone beds above the second debris flow contain primarily radiolarian species known from the Follicucullus japonicus Zone of Japan. Nine new species of small foraminifers (Bisphaera? improvisa, Vissariotaxis? nativus, Multidiscus raderensis, Baisalina miscella, Agathammina minuscula, Polarisella globosa, Geinitzina jucunda, Robustopachyphloia texana and Spireitlina capitanensis) and one new conodont species Jinogondolella gladirobusta are described.

Slimhole Technology Maximizes Middle East Horizontal-Drilling Programs

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 92376, “New Slimhole Technology Maximizes Productivity in Middle East Horizontal-Drilling Programs,” by Tariq Al-Wahedi, Mahmoud Abdel Hay, and Tamer Waheed, Abu Dhabi Co. for Onshore Oil Operations, and Bobby Grimes, SPE, and Sarvesh Tyagi, SPE, Hughes Christensen, prepared for the 2005 SPE/IADC Drilling Conference, Amsterdam, 23–25 February. Many operators use advanced slimhole drilling technologies throughout the Middle East. These programs involve new and re-entry wells with long horizontal sections, typically 6 or 6⅛ in. in diameter. A metal-sealed slimhole roller-cone bit was developed for use in these slim wellbores. The longevity of the new bearing has enabled operators to gain considerable confidence, which has translated into more on-bottom drilling time and longer drilled sections. Introduction Historically, tungsten carbide insert (TCI) bits with elastomer bearing seals were used to drill the slimhole sections, 6 in. in diameter in Abu Dhabi. In other Middle East countries, 6⅛ in. is the most common bit size. Typically in Abu Dhabi, the 6-in.-diameter hole section is drilled horizontally through the limestone reservoir (Shuaiba or Thammama group), sometimes crossing from zone to zone through a dense limestone. A tight tolerance requires geosteering to keep the well on target. The reservoir section is a porous and fractured limestone, with unconfined compressive strength (UCS) less than 5,000 psi. The UCS of the dense limestone caprock ranges between 12,000 and 15,000 psi. The section length typically varies from 3,000 to 4,000 ft, depending on the well, and sometimes extends to 6,000 ft. Two elastomer-sealed Intl. Assn. Drilling Contractors (IADC) 4-3-7-type bits were required to drill the 4,000-ft sections because of bit-use time limit. The instantaneous rate of penetration (ROP) is greater than 150 ft/hr, at times limited by data-capture speed of the measurement-while-drilling system. The additional trips for bit change-out cost rig time and increase the time that the reservoir is exposed to drilling fluid, which can result in greater formation damage. Horizontal sections through limestone reservoirs are drilled in several other countries in the Middle East (e.g., Saudi Arabia, Oman, and Kuwait). The section length varies depending on country, operator, and well type, but in general, the length of bit run is limited by the risk of lost cones after undetected seal failures, not by the cutting-structure wear. General Well Plan/Profile There are two general Abu Dhabi well plans and profiles. In Case 1, the 12¼-in. upper vertical section is drilled, and 9⅝-in. casing is set at the top of a massive shale formation. The 8½-in. section is drilled through the shale and limestone reservoir section while building inclination from vertical to horizontal. The massive shale formation is open in this type of well. In Case 2, the 12¼-in. upper vertical hole is drilled to the top of the reservoir section, where the 9⅝-in. casing is set in a limestone member or below to cover the shale formation.

FIRST RECORD OF A HETERODONT BIVALVE (MOLLUSCA) FROM THE EARLY TRIASSIC: PALAEOECOLOGICAL SIGNIFICANCE AND IMPLICATIONS FOR THE ‘LAZARUS PROBLEM’

: The heterodont bivalve Sinbadiella pygmaea gen. et sp. nov. is described from the Sinbad Limestone Member (Olenekian, Smithian) of the Lower Triassic Moenkopi Formation of Utah (USA). The new taxon is tentatively assigned to the Lucinidae on the grounds of its external morphology, larval shell morphology and hinge characters. It is the only definite representative of the Superorder Heterodonta in the Olenekian, removing this major clade of bivalves from the long list of Early Triassic Lazarus taxa. Although at least locally very abundant, the new taxon has been overlooked by earlier workers, probably due to its minute size. Dwarfism is a widespread phenomenon in Early Triassic faunas, which has been attributed to ongoing reduction of primary production. If S. pygmaea was a lucinid, a chemosymbiotic feeding mode can be inferred, which might explain why it was able to thrive in conditions that suppressed filter feeding heterodonts.

Earliest record of the genus Tylosaurus (Squamata; Mosasauridae) from the Fort Hays Limestone (Lower Coniacian) of western Kansas

Abstract Fossil vertebrates are extremely rare in the Fort Hays Limestone member of the Niobrara Chalk in comparison to the rich variety of well-preserved fish, turtles, mosasaurs, pteranodons, and toothed birds collected from the overlying Smoky Hill Chalk, and to a lesser extent, the underlying Carlile Shale. Although mosasaurs are well documented from the Smoky Hill Chalk (Upper Coniacian – Lower Campanian), and occur rarely in the Carlile Shale (Middle Turonian), they had not been previously reported from the intervening Fort Hays Limestone (Lower Coniacian). Here we report the remains of a mosasaur (FHSM VP-2297) preserving 14 articulated vertebrae, the left coracoid, ribs, and fragments of sternal cartilage collected by M.V. Walker in 1967 from Ellis County, Kansas. Serrated bite marks on several of the ribs are attributable to the anacoracid shark, Squalicorax falcatus, and suggest postmortem scavenging of the remains. The specimen is identified as Tylosaurus sp. on the basis of the characteristic ...