Middle Ordovician Sediments Research Articles

Early­­­­–Middle Ordovician sedimentation and bimodal volcanism at the margin of Iapetus: the Trollhøtta–Kinna Basin of the central Norwegian Caledonides

Abstract The late Neoproterozoic–Paleozoic Iapetus Ocean developed between Laurentia, Baltica, Siberia and Gondwana. Its Paleozoic closure history is recorded by volcano-sedimentary successions within the Caledonian orogen of Scandinavia, the British Isles and Newfoundland. We present new lithological, geochemical and geochronological data relevant for the Iapetan closure history from the hitherto poorly known Trollhøtta–Kinna Basin (central Norwegian Caledonides). This basin consists of alternating siliciclastic rocks, mid-ocean ridge basalts (MORBs), and felsic volcanic rocks highly enriched in, for example, Th, U and light REEs. Rhyolites from the stratigraphically upper part are dated by zircon U–Pb thermal ionization mass spectrometry to 473.3 ± 1.0 and 472.4 ± 0.7 Ma. Detrital zircon spectra indicate deposition after c. 480 Ma, with sediments derived from composite Cambro-Ordovician and Archean–Neoproterozoic landmass(es), possibly the Laurentian margin or a related microcontinent. The peculiar bimodal volcanic association is interpreted as an intermittent phase of marginal basin rifting, derived from a heterogeneous mantle source previously metasomatized by continental material. The tectonic mechanisms behind rifting could be slab retreat and/or break-off, or far-field tectonic forces within the Iapetan realm. Comparison of this basin with other Iapetus-related, similarly-aged volcano-sedimentary successions along the Caledonian–Appalachian orogen indicate that the bimodal MORBs and highly enriched rocks reflect a palaeotectonic setting hitherto unknown in the orogen.

Zircon provenance of SW Caledonian phyllites reveals a distant Timanian sediment source

Cambrian to middle Ordovician schists and phyllites in southwestern Baltica, now exposed in the (par-)autochthon to Lower Allochthon nappes of the Scandinavian Caledonides in southern Norway, contain previously unrecognized far-travelled detrital zircons with ages in the intervals 0.47–0.8 and 1.85–3.2 Ga and ε Hf in the range −27 to +18. These ages are assigned to Timanian and Fennoscandian Shield sources respectively and contrast with the locally derived detritus with zircon ages of c . 0.9–1.8 Ga and ε Hf values c . −13 to +10. The far-travelled zircons provide evidence that a steady, long-haul, source-to-sink drainage system existed from the northeastern fringe of Baltica to the SW passive margin across the whole palaeocontinent ( c . 2000 km) since c . 521 Ma, and that the Timanian orogen shed detritus across large distances towards both its foreland (Baltica) and hinterland (Arctica). There are several arguments against an Avalonian source for these zircons. Recycling of the detrital zircon from the Cambrian to middle Ordovician sediments is probably responsible for the presence of Cryogenian to Middle Ordovician zircon ages in younger sedimentary sequences of southwestern Baltica. The development of an ophiolitic mélange associated with Ordovician phyllites underlying the Jotun Nappe Complex seems to mark the change to an active continental margin in the middle Ordovician, heralding the Caledonian orogeny. The study demonstrates that detrital zircon-poor fine-grained (siltstone to mudstone) sediments are an extremely valuable indicator for distal sources and favourably complement zircon-rich coarser sandstone in provenance analysis. Supplementary material: Results of LA-ICP-MS and SIMS U–Pb–Hf analyses of zircons with details of analytical methods and the collection of CL images of the youngest (<500 Ma) detrital zircon grains are available at http://www.geolsoc.org.uk/SUP18827 .

Five buried crater structures imaged on reflection seismic data in Saudi Arabia

ABSTRACT Reflection seismic data acquired for hydrocarbon exploration in Saudi Arabia have revealed five buried crater structures ranging in diameter from 5 km to 34 km. These structures have little or no present-day surface expression and span a range of ages from Ordovician to Cenozoic. The Saqqar structure (29°35′N, 38°42″E) is 34 km in diameter and is formed in Devonian siliciclastics. The structure is partially eroded and is unconformably overlain by Upper Cretaceous and Paleogene strata up to 400 m thick. The Jalamid structure (31°27′N, 39°35″E) is 19 km in diameter at Lower Ordovician level and is infilled by Middle Ordovician sediments, at a present-day depth of 4,500 m. The Banat Baqar structure (29°07′N, 37°36″E) is 12 km in diameter at Middle Ordovician level and infilled by Upper Ordovician sediments, at a depth of 2,000 m. The Hamidan structure (20°36′N, 54°44″E) is 16 km in diameter at Paleogene level and is overlain by a thin veneer of Recent sediment. The Zaynan structure (20°23′N, 50°08″E) is 5 km in diameter and affects Triassic sediments at depth of 3,800 m, and is infilled by Jurassic strata. In addition to reflection seismic imaging, various amounts of gravity and magnetic data and drilled wells are available in or near these structures. Various models including impact cratering are discussed here for each structure. One structure (Saqqar) has yielded quartz grains with possible shock metamorphic features that, contingent on future work, may support a meteorite impact crater interpretation.

The Carbon Isotope Organic Geochemistry of Early Ordovician Rocks from the Annascaul Formation, County Kerry

Ireland is well known to geologists as containing some of the thickest successions of Early Ordovician (485! 470 Ma; Walker et al. 2012) sedimentary rocks in the world. The carbon stable isotope compositions (d 13 C value) of similarly aged rocks have been reported for only very few places in the world (i.e., Argentina, southern China, and southern France), and no such analyses have been performed on the Early Ordovician, organic-rich rocks of Ireland. Here we report the d 13 C values of bulk organic material and organic isolates recovered from the Annascaul Formation of Southwestern Ireland. Members of the Annascaul Formation spanning the Early Ordovician were sampled at multiple sites within five localities on the Dingle Peninsula, County Kerry. Mean bulk organic d 13 C values for the organic matter within rocks of the Farranacarriga, Tinal, Illaunglass, Bealacoon, and Killelton Members ranged from 28.7! (Farranacarriga) to 22.1! (Illaunglass); the average d 13 C values of organics isolated from the Farranacarriga and Bealacoon Members were 29.0 and 28.4! , respectively. No statistical difference was observed between the d 13 C value of isolates and the d 13 C value of bulk sediment from which organics had been isolated (P# 0.85, Farranacarriga, n# 6; P# 0.81, Bealacoon, n# 5; paired t-test). The d 13 C values we present here agree well with the previously published d 13 C values for Early Ordovician organic carbon (average # 28.4, 25.4, 25.5! ) from Argentina, southern China, and southern France (respectively). Our new data from the Annascaul Formation are ! 3.3! higher than the average value reported for d 13 C values of organic matter of marine origin for the same period, raising the possibility that terrestrial bryophytes (or other terrestrial photosynthesizers) contributed to these Early Ordovician sediments. Further equivocal evidence is provided by the abundant organic macerals found within the Farranacarriga Member with possible vitrinite origin. The d 13 C values of palynomorphsized organic isolates from the Annascaul Formation reflect a marine origin for these organisms, consistent with their previous identification as acritarchs. Given the recognition of the earliest thalloid macrofossils and land-plant cryptospores in the Middle Ordovician sediments of the Appalachian basin (USA) and Argentina, respectively, our results highlight the Early! Middle Ordovician boundary as a potentially crucial time of terrestrial ecosystem expansion and development.

THE CARBON ISOTOPE ORGANIC GEOCHEMISTRY OF EARLY ORDOVICIAN ROCKS FROM THE ANNASCAUL FORMATION, COUNTY KERRY

Ireland is well known to geologists as containing some of the thickest successions of Early Ordovician (485—470 Ma; Walker et al 2012) sedimentary rocks in the world. The carbon stable isotope compositions (δ¹³C value) of similarly aged rocks have been reported for only very few places in the world (i.e., Argentina, southern China, and southern France), and no such analyses have been performed on the Early Ordovician, organic-rich rocks of Ireland. Here we report the δ¹³C values of bulk organic material and organic isolates recovered from the Annascaul Formation of Southwestern Ireland. Members of the Annascaul Formation spanning the Early Ordovician were sampled at multiple sites within five localities on the Dingle Peninsula, County Kerry. Mean bulk organic δ¹³C values for the organic matter within rocks of the Farranacarriga, Tinal, Illaunglass, Bealacoon, and Killelton Members ranged from — 28.7‰, (Farranacarriga) to — 22.1‰ (Illaunglass); the average δ¹³C values of organics isolated from the Farranacarriga and Bealacoon Members were — 29.0 and — 28.4‰, respectively. No statistical difference was observed between the δ¹³C value of isolates and the δ¹³C value of bulk sediment from which organics had been isolated (P= 0.85, Farranacarriga, N = 6; p = 0.81, Bealacoon, n = 5; paired Mest). The δ¹³C values we present here agree well with the previously published 5 l3 C values for Early Ordovician organic carbon (average = –28.4, –25.4, –25.5‰) from Argentina, southern China, and southern France (respectively). Our new data from the Annascaul Formation are ~3.3‰ higher than the average value reported for δ¹³C values of organic matter of marine origin for the same period, raising the possibility that terrestrial bryophytes (or other terrestrial photosynthesizers) contributed to these Early Ordovician sediments. Further equivocal evidence is provided by the abundant organic macerais found within the Farranacarriga Member with possible vitrinite origin. The δ¹³C values of palynomorphsized organic isolates from the Annascaul Formation reflect a marine origin for these organisms, consistent with their previous identification as acritarchs. Given the recognition of the earliest thalloid macrofossils and land-plant cryptospores in the Middle Ordovician sediments of the Appalachian basin (USA) and Argentina, respectively, our results highlight the Early-Middle Ordovician boundary as a potentially crucial time of terrestrial ecosystem expansion and development.

Evolution of Stromatoporoidea in the Ordovician–Silurian epicontinental basin of the Siberian Platform and Taimyr

The paper discusses the evolution of Stromatoporoidea in the epicontinental sedimentary basin of the Siberian Platform and Taimyr during the Ordovician and Silurian. Specimens of the oldest genus, Priscastroma, were found in the middle of Middle Ordovician sediments. This genus is represented by the species P. gemina Khrom., which has two forms, A and B. Tracing the emergence of new genera over time, we identified two distinct branches in stromatoporoid evolution. The ancestor of the first branch is P. gemina f. A, which gave rise to the genus Cystostroma. The latter is the ancestor of two subbranches with predominant horizontal skeletal elements. The subbranches differ only in tissue microstructure. The genera Stromatocerium, Dermatostroma, and Aulacera display dense fibrous microstructure, whereas the genus Rosenella and its descendants display dense microstructure. The genus Lophiostroma, with a lamellar–fibrous tissue, may be a dead branch of evolution. The ancestor of the second branch is P. gemina f. B, which gave rise to the genus Labechia and its descendants. This branch has a dense tissue, with predominant vertical skeletal elements. Ordovician stromatoporoids from Siberia were compared with those from other basins of the world. Comparison shows that all the Ordovician genera from the epicontinental basin of the Siberian Platform and Taimyr originated here. Thus, this basin was one of the centers of stromatoporoid origin.

The geochemistry of Lower Palaeozoic sediments deposited on the margins of Baltica

A regional survey of the geochemical composition of Lower Palaeozoic shales deposited on Baltica indicates that Llanvirn (Lower/Middle Ordovician) to Lower Silurian shales have higher concentrations of Na, Mg, Cr, Ni and Fe and lower concentrations of K, Rb and Ti compared to Arenig shales. This geochemical signature can be traced from Scania to the Oslo Region, i.e. in areas approximately 500 km apart, but is not present in Middle Ordovician sediments from Avalonia. The geochemical signature matches island arc tholeiites such as those in the Fundsjø Group within the Upper Allochthon of the Norwegian-Swedish Caledonides. Hence, these sediments were probably predominantly derived from island arcs formed during the end phase of closure of the Iapetus Ocean. Simple two component mixing calculations between oceanic and continental sediment sources suggest that the oceanic component diminishes towards the south where modifications related to longer sediment transport distances can be recognised. The introduction of sediment derived from island arcs coincides with increases in subsidence rates in the Oslo Region and may reflect an early stage in foreland basin development. The presence of the geochemical signature in Scania implies that island arcs systems were geographically widespread. The combined evidence indicates that the Arenig/Llanvirn boundary marks an important change in the continuing closure of the Iapetus Ocean. The data suggest that island arcs were obducted onto the outer margins of Baltica presumably during the Arenig. Continued obduction of island arcs in the Mid Ordovician and younger intervals is likely.

Mississippi Valley-type fluorite-barite-sphalerite mineralization in the Sweetwater district, Tennessee

Recent exploration has established that the Sweetwater district, well known for its residual barite, contains significant Mississippi Valley-type (MVT) fluorite-barite-sphalerite mineralization at depth. These minerals are hosted in the upper Knox Group (Lower Ordovician) carbonate sequence, which is composed of limestone, medium- to coarse-grained dolomitized limestone, and fine-grained dolostone. The potential orebodies of the district appear to be of two forms: “breakthrough” ore that cuts across the stratigraphic units, and “bedded” ore that represents lateral extensions of mineralization along individual limestone units. The mineralization is dominantly of the cavity-filling type and is associated mainly with coarse rock-matrix breccias. Correlation of marker horizons in cross-sections constructed from underground borehole data suggests that the distribution of coarse rock-matrix breccias was controlled by foundering of overlying blocks due to partial dissolution of underlying limestone units by hydrothermal fluids. The zones of coarse rock-matrix breccia were permeable enough for the passage of later ore-forming fluids and consequent mineralization. The mineralization appears to have been essentially completed before the deposition of the Middle Ordovician sediments, which unconformably overlie the Knox Group and contain only occasional occurrences of fracture-filling sphalerite.

Sedimentation and tectonism in the Middle Ordovician of the Girvan district, SW Scotland

ABSTRACTThe Kirkland Conglomerate and the Benan Conglomerate and associated sediments of the Barr Group (Ordovician—Llanvirn-Llandeilo) of the Girvan district, SW Scotland record the development of two fan-delta systems situated on the northern margin of Iapetus. The intervening Stinchar Limestone represents a shallow marine fan-delta abandonment facies. Subaerial fan-delta deposits are seen at the lowest exposed horizons within the Kirkland Conglomerate. Transgression and eventual abandonment of the fan-delta system is recorded by (1) matrix-rich gravels forming the topmost horizons of the unit. (2) subaqueous distributary channel sands of the overlying transitional sandstone and associated shallow-marine carbonates of the Auchensoul Limestone and (3) shallow-marine sandstones of the Confinis Flags. Following a period of shallow-water sedimentation (Stinchar Limestone), a phase of rapid subsidence occurred during the upper Llandeilo, broadly synchronous with the Nemagraptus gracilis Zone transgression. Lowermost horizons of the succeeding Benan Conglomerate comprise re-sedimented gravels and laterally equivalent ‘outer shelf’ deposits (Superstes Mudstone). Renewal of coarse clastic sedimentation resulted from source area uplift related to granite plutonism, responsible also for the rapid subsidence of basinal areas. Fan-delta progradation is recorded by the occurrence of braided-fluvial deposits and shallow-marine carbonates at higher stratigraphical levels within the unit. Progradation of the fan-delta complex resulted from a gradual reduction in subsidence rates along basin-margin faults.Fan-deltas of the Barr Group prograded southwards, from a SW—NE-trending faultdelineated basin margin, across a narrow shelf area. To the S of the shelf area defined by Barr Group outcrop, sediments of the Tappins complex accumulated in outer shelf, ?slope and base of slope settings.The thickness of individual conglomerate units (>150 m—Kirkland Conglomerate, and up to 700 m—Benan Conglomerate), the associated high sedimentation rates, narrowness of the shelf area, and distribution and style of basin-margin faults indicate that oblique-slip motion along the northern margin of Iapetus may have provided a major control over Middle Ordovician sedimentation in the Girvan district.

A review of Middle Ordovician sedimentation in the St. Lawrence Lowland, eastern Canada

AbstractMiddle Ordovician sediments of the St. Lawrence Lowland, eastern Canada, and its northeastward extension to St‐Siméon, are subdivided into the numerous formations of the Chazy, Black River and Trenton Groups. Details of each formation and interpretation of environments of deposition are presented and a coherent model for the development of the upper Middle Ordovician Trenton Group throughout the region is presented. In the southwest, around Montreal, a complete and continuous Middle Ordovician sequence is present and Trenton Group sediments overlie well‐developed tidal flat and lagoonal (Black River Group) and mixed shallow subtidal (Chazy Group) sediments. This sequence was deposited on a slowly subsiding, essentially flat, broad shelf environment. Northeastward from Montreal, toward the Montmorency Promontory of the Quebec City area, basal Middle Ordovician sediments become younger and the extent of the shelf area narrowed significantly. The latter resulted in skeletal shoal sediments (lower Trenton Group) developing closer to shore and concomitant less well‐developed clastic‐rich lagoonal sediments (Black River Group and basal Trenton Group). At Montmorency Promontory the shoal sediments (basal Trenton Group) accumulated along an irregular and rugged coastline. Northeast of the Promontory a steep onshore to offshore profile and rapidly deposited basal inshore clastics (Black River Group) precluded the deposition of skeletal shoals and rapid submergence promoted the early development of deeper shelf (middle and upper Trenton Group) and slope and basin (top Trenton Group, Saint Irénée Formation) sediments. In contrast, corresponding offshore sediments (middle and upper Trenton Group) in the southwest reflect a lower depositional gradient and more gradual subsidence. These patterns of deposition were determined by the interaction of the changing nature of the Ordovician coastline southwest, at, and northeast of the Montmorency Promontory and the variable subsidence rates influenced by the eastward evolving Taconic Orogen.

Tideless"Middle Ordovician Sediments: ABSTRACT"

Tideless"Middle Ordovician Sediments: ABSTRACT"

The Unmetamorphosed Sedimentary Fill of the Brent Meteorite Crater, Southeastern Ontario

A 10 000-ft (~3050-m) diameter circular structure that exhibits rimmed crater form, shock-metamorphic features, and underlying unbrecciated basement rocks occurs near Brent, Ontario, close to the southern margin of the Canadian Shield. In its center 863 ft (263 m) of Middle Ordovician sediments are preserved. Shortly after impact a nearly level crater floor was established and the subsequent sequence appears to have been deposited close to mean sea level. Repeated sediment laminae probably reflected wind-tidal marine incursions onto the low relief margin of the contemporary Ordovician epicontinental sea. Throughout much of its period of sediment accumulation, the crater floor appears to have been nearly flat. A 380-ft (115.8 m) sequence of dolostones, arkosic siltstones, and evaporite layers and veins formed within a breccia-rimmed depression. Initially sea water carrying fine silt invaded the crater and refluxed through the porous and permeable crater rim. Subsequently some 100 ft (30.5 m) of silty arkose blanketed the area, probably resulting from further transgression of the Middle Ordovician seas and breaching of the crater walls. An upper 380 ft (115.8 m) of predominantly thin-bedded lagoonal and shallow shelf sea limestones are divided into upper and lower sequences by a middle regressive set of red beds.The implied near-flat crater floor, coupled with preservation of over 800 ft (>244 m) of crater sediments, suggests continued slow subsidence. Earlier on, this subsidence affected only the crater area, but later episodes of subsidence were regional, involving Ordovician, Silurian, and Devonian sedimentation. The superincumbent load further compacted the total crater sequence. Preservation of the rocks described here is due to final depression of the sequence into a position below the general level of the surrounding Precambrian terrain. If Brent can be considered to be a typical peri-marine meteoritic impact crater, all such craters should have in common an inward-dipping succession of open-circulation sediments overlying a crater-rim restricted sedimentary sequence, which in turn overlies a shock-metamorphosed series of breccias.Compaction of the impact-generated breccias and subsequent unmetamorphosed crater-filling sediments influenced both sediment accumulation and the ultimate crater structure.

The Gander Lake and Davidsville Groups of Northeastern Newfoundland: New Data and Geotectonic Implications

Structural studies of rocks in the Gander region of Newfoundland and on the coast to the north have indicated that rocks that were previously assigned to the Middle Ordovician Gander Lake Group actually consist of a composite sequence that contains pre-Middle Ordovician polyphase deformed metasedimentary rocks and even older gneissic rocks as well as Middle Ordovician sediments and mafic volcanics. The possible implications of these results are discussed and map units in the region are redefined.

The Ramapo Fault System in New York and Adjacent Northern New Jersey: A Case of Tectonic Heredity

The Ramapo fault system forms the northwestern boundary of the Newark Triassic basin in New York and adjacent northern New Jersey, and is commonly attributed to Mesozoic crustal fracturing. However, the detailed geology in the vicinity of this fault indicates an earlier, complex tectonic ancestry, perhaps dating from late Precambrian time. Intense cataclastic effects found in Precambrian to Middle Ordovician rocks on both sides of the Ramapo fault are absent from the bordering Triassic rocks of the footwall block. Significant post-Middle Ordovician right-lateral-transcurrent faulting is recorded by actual offset of distinctive units and by the megascopic fabric in mylonite zones. The total cumulative offset since the late Precambrian on these faults is unknown but could be large. Right-lateral displacement of 4 km is indicated for the Canopus Valley area. Intrusive relationships of probable late Precambrian to Late Ordovician diorite plutons and dikes, as well as pegmatites of probable Devonian age, help date the strike-slip movements as Paleozoic and older. Block faulting along the Ramapo fault in the Lower Ordovician affected Middle Ordovician sedimentation and resulted in unconformable relationships between a Middle Ordovician gneiss-boulder conglomerate (Bucher, 1957) and Precambrian gneisses on the upthrown block west of the fault north of Peekskill, New York. Repeated movements of the Highlands block during Late Triassic time produced the coarse clastic fanglomerates of the Hammer Creek Formation adjacent to the Ramapo fault. This syndepositional faulting was followed by post-Brunswick downdropping along the Ramapo fault south of the Hudson River. Recent seismic activity along this fault system reported by Page and others (1968) suggests that the fault may still be active. These observations suggest a possible tectonic longevity of some 700 m.y. for the “Triassic” border fault system here. This fracture system had deep crustal connections along which basic magma rose toward the surface in Late Precambrian, Late Ordovician (Cortlandt), and Late Triassic time (Palisades sill and Triassic flows). The Ramapo fault also marks the boundary between tectonic blocks having different Paleozoic deformational histories. These observations, along with the remarkable longevity, suggest that this fault zone may be part of a fundamental crustal fracture system that was operative during formation of the Appalachian orogen. The ideas presented here require a degree of tectonic permanence not commonly reported in orogenic belts. Activity along this fracture system in pre-Middle Ordovician time as documented here suggests strongly that the northern end of the Reading Prong is not allochthonous in the sense proposed by Isachsen (1964).

Lower Silurian and Cambro-Ordovician Sedimentation of Northern Appalachian Basin: ABSTRACT

Subsurface studies in the Northern Appalachian basin reveal several different histories of sedimentation (Cambro-Ordovician and Lower Silurian). Too few wells have been drilled to the Precambrian to reveal anything but very regional data on the Cambrian sediments. The Cambrian thickness varies from 13,000 feet on the outcrop in south-central Pennsylvania to 900 feet in northeastern Ohio and 0 feet in southwestern Ontario. The section is predominantly dolomite and sandstone. The Upper Cambrian isopach map shows a probable regional high in northeastern Ohio. This high seems to trend north-south. Two regional unconformities have been detected within this section. Middle-Ordovician sedimentation marked a time of emergence with the Adirondack-Tazwell axis in the center of the basin forming a structural and facies barrier separating for the first time the Appalachian basin into two distinct basins of deposition. On the northwest flank of the Allegheny synclinorium, the Crawford arch came into existence along with the Olean embayment and Erie trough (Chatham Sag extension). Unlike the more pronounced Adirondack-Tazwell feature, the Crawford arch did not act as a facies barrier. The Middle Ordovician sediments encountered so far are argillaceous limestones. The exception to this being the dolomitized productive areas on the Findlay arch. Upper Ordovician sedimentation shows regional east to west thinning with occasional interruptions across local highs. These sediments are predominantly shales with occasional layers of siltstones or sands. The subsequent Lower Silurian deposits also show regional thinning eastward with local variations. Lower Silurian production appears to be confined to within the 200-400-foot isopach interval. Variations within this isopach interval are critical as to the quality of production to be found. Detailed study reveals the productive sand bodies are deltaic rather than offshore bars or shoe-string sands. End_of_Article - Last_Page 1252------------

Trenton and Sub-Trenton of Outcrop Areas in New York, Pennsylvania, and Maryland

Trenton and sub-Trenton rocks of the outcrop regions in New York, Pennsylvania, and Maryland are described, and their conditions of origin and paleogeography are discussed. The basement rocks are considered with respect to regional characters, possible causes of magnetic and isostatic anomalies, and relation to deformation of the Paleozoic sediments. New stratigraphic charts, and new isopachous and paleogeographic maps are presented for the Glenarm, the Lower Cambrian, the Middle and Upper Cambrian, the Lower Ordovician, and the Middle Ordovician sediments of the region, with accompanying discussions of the stratigraphy, regional relations, and sedimentation of the deposits. Brief consideration is given to relations of the Trenton and sub-Trenton at the outcrop in New Yor , Pennsylvania, and Maryland, with respect to search for oil and gas in the more westerly subsurface areas of these sediments.

Sand Hills Area, Crane County, Texas

The Sand Hills area of west-central Crane County includes the Tubb and McKnight pools, in which production is obtained from Permian dolomite. In a third small area on the northwest side of the Tubb pool, five wells have encountered flush production of high-gravity oil near the top of the Lower Ordovician dolomite, and two small wells are producing from a sandstone member of the Middle Ordovician. Lower Permian dolomite lies unconformably on the eroded surface of a seemingly complex structural system, which involves Lower and Middle Ordovician sediments. Intermediate beds of probably Upper Ordovician, Silurian, and Devonian ages, respectively, appear in a test which was drilled approximately 8 miles southeast of the Ordovician producing area.

Sand Hills Area, Western Crane County, Texas: ABSTRACT

The Sand Hills area of western Crane County includes two pools, the Tubb and McKnight, in which production is obtained from Permian dolomite. In a third small area on the northwest side of the Tubb pool, three wells have encountered flush production of high-gravity oil in the upper portion of the Lower Ordovician dolomite, and two small wells produce from a sandstone member of the Simpson, having failed in the Lower Ordovician. Lower Permian dolomite lies unconformably on the eroded surface of a seemingly complex structural system, which involves Lower and Middle Ordovician sediments. Intermediate beds of probably Upper Ordovician, Silurian, and Devonian ages, respectively, appear in a test which was drilled approximately eight miles southeast of the Ordovician producing area. End_of_Article - Last_Page 1704------------