Sandstone Interbeds Research Articles

Discussion on vitrinite reflectivity and the structure and burial history of the Old Red Sandstone of the Midland Valley of Scotland

Gerald M. Friedman writes: the paper by Marshall et al. 1994 uses vitrinite reflectivities from Lower Old Red Sandstone (ORS) sediments of the Midland Valley of Scotland to interpret burial and uplift history. Vitrinite reflectivies give values of 1.3% for the Stonehaven Group (Kincardineshire) and 1.3% for the Strathmore Group (Perthshire). They conclude that 'the Stonehaven Group reflectivity values are considerably lower than expected under the 9 km of sediments aggregated for the Lower ORS of Kincardineshire and demonstrate a maximum burial of 3 to 5 km’. The ORS correlates with the Devonian red beds of the Appalachian Basin. Here carbonized plant debris in uppermost Middle Devonian strata within the eastern Catskill Mountains of New York have been converted to anthracite (Friedman & Sanders 1982). The anthracite has a vitrinite reflectance of 2.5%. This implies a level of organic metamorphism (LOM) of 16. A similar degree of thermal activity is implied by the black colour (Staplin kerogen alteration index of 4) of the associated (possibly recycled) carbonized kerogen; a conodont alteration index of 4; and authigenic chlorite and local sericite fillings of the interparticle pores of interbedded sandstones. The specimens are about 350 Ma old. If 200 Ma is taken as the duration of exposure to the maximum geothermal temperature, then the LOM of 16 and other thermal indicators imply a maximum temperature of 190 °C. Using a geothermal gradient of 26°C km −1 , a former depth of burial of 6.5 km is implied. Such a former depth is consistent with

Palaeoecology and sedimentaryenvironments of the Jurassic-Cretaceous transition beds in Sweden

A section of the Vitabäck Clays (Annero Formation) at Eriksdal, SE Scania, Sweden has been investigated petrographically, sedimentologically and biostratigraphically. The section is predominantly argillaceous with occasional sandstone interbeds. The clays are composed mainly of illite, kaolinite, and a mixed layer component of smectiteillite. Fe-oxyhydroxides and siderite constitute important minerals in certain beds. Infill sequences, washover fans and pedogenic horizons together with faunal and floral elements indicate deposition within freshwater-brackishmarine environments such as coastal plains, lakes and lagoons. The fossil content includes molluscs, ostracodes and palynomorphs whose stratigraphical ranges have confirmed and narrowed the age to the Jurassic-Cretaceous transition. The dispersed palynomorphs, mainly pollen grains and spores which were derived from the contemporary parental coastal plants, provide the most complete palaeontological documentation of the transition. From the variations in the vegetation throughout the section it is possible to suggest a climatic change to more humid conditions. An interpretation of the contemporary palaeoecology and sedimentary environment is presented.

Petrography and Geochemistry of Sandstones Interbedded with the Rooiberg Felsite Group (Transvaal Sequence, South Africa): Implications for Provenance and Tectonic Setting

ABSTRACT The 2150 Ma Rooiberg Felsite Group, Transvaal Sequence, contains thin sandstone interbeds within its 3500-5000 m thick volcanic succession. Whereas uppermost feldspathic arenites and wackes are thought to represent reworked felsitic material, quartz arenites and subordinate lithic sandstones, present throughout most of the Rooiberg sequence, comprise mainly reworked sedimentary detritus, probably belonging to the underlying Pretoria Group. Sandy braided-river systems probably transported clastic material into the basin from relatively stable source areas, subject to intense chemical weathering. Geochemistry, and sandstone petrography indicate that mixing of Pretoria Group detritus with sediment derived from erosion of felsitic material occurred within the basin during late Rooiberg ti es. The inferred hiatuses in volcanism represented by the predominant siliceous sedimentary interbeds appear to have been of relatively short duration and occur throughout much of the Rooiberg stratigraphy. Upper arkosic sandstones indicate longer breaks in volcanism as Rooiberg eruptions came to an end. The sandstones provide evidence compatible with an impact origin for the Rooiberg Felsite Group, and for its successor, the Bushveld Complex.

Early Carboniferous Transgression on a Passive Continental Margin: Deposition of the Kekiktuk Conglomerate, Northeastern Brooks Range, Alaska

The base of the Ellesmerian sequence in the northeastern Brooks Range is a widespread terrigenous clastic succession assigned to the Kekiktuk Conglomerate (late Tournaisian-Visean). The Kekiktuk is situated above a regional angular unconformity and is the base of a transgressive succession that is thought to record coastal onlap associated with the formation of a passive continental margin. Six depositional units have been recognized in the Kekiktuk Conglomerate. Unit A consists of clast-supported and rare matrix-supported pebble and cobble conglomerate with minor interbeds of pebbly sandstone. It records deposition at the base of incised paleovalleys from high-sediment-concentration flows and debris flows. Units B, C, and D consist of multistory fining-upward cycles that record deposition within incised paleovalleys from low-sinuosity bed-load-dominated streams, low to moderate-sinuosity bed-load-dominated streams, and high-sinuosity mixed-load streams, respectively. Unit E consists of multistory fining-upward cycles and laminated sandstone with thin conglomerate stringers. It records deposition in the distal regions of incised paleovalleys in a variety of marginal-mari e settings, including fluvial channel, channel-mouth bar, beach ridge, and coastal swamp environments. Unit F consists of horizontally bedded sandstone, interbedded sandstone, and organic-rich mudstone. It records deposition over paleotopographic highs on the sub-Mississippian unconformity from sheetfloods and in associated coastal swamp and mud-flat settings. Relative sea level rise is thought to have initiated fluvial deposition in the northeastern Brooks Range by the latest Tournaisian-earliest Visean. Coal-bearing fluvial channel-fill and flood-basin successions and palynologic data suggest that this part of Arctic Alaska was situated in a humid climatic zone during the Early Carboniferous. The stratigraphic and sedimentologic organization of the Kekiktuk Conglomerate indicates that once base-level rise began to affect the northeastern Brooks Range, local and regional variations in paleotopography along the sub-Mississippian unconformity exerted primary control on accommodation and consequent distribution of Kekiktuk depositional environments. The stratigraphic position of the Kekiktuk Conglomerate above a regional angular unconformity nd below marginal-marine and shallow-marine shales, combined with its limited thickness, widespread distribution, and suite of paleoenvironments, suggests deposition in an upland region situated landward of the tectonic hinge zone along a steadily subsiding passive continental margin. Thermal subsidence combined with eustatic sea level rise were probably responsible for relative sea level rise in Arctic Alaska and the resulting regional transgression. End_Page 679------------------------------

Depositional facies, paleogeography and palaeoclimatology of the Lower Jurassic McCoy Brook Formation, Fundy rift basin, Nova Scotia

The McCoy Brook Formation is a 230+ m sequence of redbeds of Early Jurassic age that accumulated in the subtropical Fundy rift basin. The basin was asymmetrical, bordered on the northern side by the Cobequid fault system and adjacent highlands. The formation is a mosaic of terrestrial facies. (a) Multi- and single-storey fluvial sandstones were deposited by rivers that collected drainage from the norhern highlands, and flowed southwest across the valley floor. (b) Graded-bed sequences of sandstone → mudstone were deposited by flood events on the fluvial floodplains and the margins of playas. (c) Interbedded sandstones and finely laminated to disrupted red mudstones represent deposits of interfingering sandflats at the toes of alluvial fans and playas, respectively. The fabrics of both lithologies are commonly disrupted by desiccation and intrastratal precipitation of calcite and minor gypsum. (d) Plane-bedded conglomerate and pebbly sandstone record deposition in shallow channels that traversed alluvial fans. (e) Non-bedded breccias of basalt boulders up to 8 m in size in a sand to mud matrix formed as talus against palaeocliffs of the Lower Jurassic North Mountain Basalt. The palaeocliffs resulted from synsedimentary faulting within the basin. (f) Basalt conglomerate beds with matrix- to clast-supported fabrics comprise sequences deposited by debris-flows, probably generated by remobilization of talus. (g) A thin sequence of limestone, siltstone, and muddy sandstone containing fish scales and bones formed in a shallow lake on a basalt flow. (h) Sandstones containing m-scale crossbeds and a three-fold hierarchy of erosional surfaces accumulated as barchans and barchanoid transverse ridges that moved southwest in the valley. All of these facies occur within an area of 18 km 2 located south of the fault-bounded highlands. Synsedimentary faulting within the basin during early deposition of the McCoy Brook Formation created palaeocliffs of North Mountain Basalt. Talus and debris-flow deposits accumulated adjacent to these cliffs, interfingering with eolian, fluvial, and lacustrine sediments. During later deposition of the McCoy Brook Formation, the early fault-controlled topography was buried as alluvial fans prograded southward from the highlands. Broad sandflats extended basinward from the toes of the fans, interfingering with playa mudflats on the valley floor. The palaeoclimate was semi-arid as evidenced by gypsum-bearing playa redbeds, eolian sandstones, and caliche palaeosols.

The Saint‐Daniel Melange: Evolution of an accretionary complex in the Dunnage Terrane of the Quebec Appalachians

The Saint‐Daniel Mélange is part of a series of mélanges located along the Baie Verte‐Brompton line in the Northern Appalachians. This line marks the suture between rocks of oceanic affinities and those of the ancient passive margin of North America with which they collided during the Taconian (Middle to Late Ordovician) orogeny. The Saint‐Daniel Mélange contains a wide variety of lithologies including well‐bedded to dismembered sedimentary sequences, pebbly mudstone, olistostromes, and slivers of igneous and metamorphic rocks. Black shales with interbeds of green shale, calcareous siltstone, or sandstone are the dominant units. They exhibit various stages of mélange formation such as those present in shallow parts of an accretionary complex. Units of oceanic origin include sediments derived from the forearc basin and slivers of an ophiolite and of a magmatic arc. Units derived from sediments of the passive margin of North America are also present. The ratio between these various lithologies changes greatly within the mélange on a kilometric scale along strike. The Saint‐Daniel Mélange is a structural complex in which the various units were assigned a sequential order mimicking a stratigraphic order. The Saint‐Daniel Mélange is interpreted as the relict of an accretionary complex because of its actual structural position within the Northern Appalachians and because all its lithologies and their structural fabric can be found in modern accretionary complexes.

Pore pressure variation within the Tuscaloosa trend: Morganza and Moore‐Sams Fields, Louisiana Gulf Coast

Repeat formation tester (RFT, mark of Schlumberger) pore pressure data from the lower Tuscaloosa Formation (Upper Cretaceous) are analyzed across a pressure transition zone in two gas fields of the Tuscaloosa trend, Louisiana, documenting a pressure seal within a sandstone‐dominated interval that maintains a pressure anomaly of ∼20 MPa. Pressure measurements within the formation span a depth interval of 5500–6060 m and are limited to the crests of rollover structures associated with growth faults; the onset of overpressure occurs at a depth between 5620 m and 5690 m in two fault blocks, while the strata are displaced across the fault by 100 to 120 m, suggesting that the pressure seal may be near‐horizontal, crosscutting stratigraphic boundaries. The pressure seal zone is as thin as 38 m and occurs within interbedded sandstones and shales. Overpressures in all wells of the Moore‐Sams Field and in two wells of the Morganza Field follow local hydrostatic gradients, indicating that portions of the overpressured zone are in fluid communication though isolated from shallower, normally pressured fluids. In other wells, overpressures increase with depth in a stair‐step manner to 117 MPa at depths of 5.9 km. The occurrence of the pressure seal in interbedded sandstones and shales where higher permeability is expected suggests that the sandstones of the seal zone are unusually tight. A companion petrographic study of sandstones near the pressure seal suggests that extreme compaction of the sandstones after dissolution of grain‐supporting cements may have contributed to the low permeability of the seal zone.

East‐directed overthrusting in the Melbourne Zone, Lachlan Fold Belt

Inclined northeast‐vergent asymmetric regional F1 folds cut by thrust faults characterize a fold‐to‐fault transition into the Mt Wellington Fault Zone on the eastern margin of the Melbourne Zone of central Victoria. In the most southerly exposed part of the Mt Wellington Fault Zone at Lake Glenmaggie, polydeformed interbedded sandstone and mudstone of Silurian(?) age are unconformably overlain by weakly deformed Late Devonian conglomerate and volcanic rocks. Three deformations pre‐date the Late Devonian succession, which has itself been warped into a series of open folds. Thrusting (D1) along southwest dipping reverse faults and thrusts produced imbrication and juxtaposition of different lithological groups. Fault slices of strongly cleaved, interbedded sandstone and mudstone abut against a weakly deformed mudstone‐siltstone sequence, thereby suggesting significant displacement on some of these faults. Thrust sheets consisting of large panels of overturned homoclinally dipping strata represent the faulted...

Evolution of Lower Jurassic coastal plain and fan delta sediments in the Beryl Embayment, North Sea

Coastal plain and fan delta sediments accumulated throughout the Sinemurian to late Pliensbachian in the Beryl Embayment area, North Sea. These sediments represent a marginal, clastic fringe along the southern extension of the Boreal Ocean into the northern North Sea. Two informal formations are recognized in these predominantly sandy deposits. The lower formation comprises interbedded sandstones, siltstones and coals of minor fluvial channel and associated coastal floodbasin marsh, lake and sheetflood origin. The overlying formation represents a fan delta system, possibly generated as a result of footwall uplift and associated erosion. This formation is dominated by probable channel sandstones, with marine-influenced debris flow deposits developed at a number of levels indicating an active, coastal fan setting. Fan sediments are capped by a marine sandstone, beds of which interfinger with the overlying offshore siltstones. Thickness variations are related to major normal faults, suggesting at least a limited fault control on the deposition and distribution of facies during a time conventionally regarded as tectonically quiescent in the northern North Sea. There is no clear evidence for a eustatic control on sedimentation, although deposition of post-fan delta shallow marine siltstones may have been principally controlled by eustatic sea level rise recording the acme of the early Jurassic expansion of Boreal waters into the Viking Graben.

Sequence Stratigraphy and Facies Architecture of a Middle Eocene Shallow Marine to Basin Transition, Sespe Wilderness Area, California

Several unconformity bounded depositional sequences are present in the Middle Eocene Coldwater, Cozy Dell, and Matilija formations in the Sespe Wilderness Area of California. Five of the sequence-bounding unconformities can be correlated from the canyon walls of Lower Sespe Gorge to the Sespe oil fields 5 km to the east. Three of these unconformities are characterized by a basinward shift of the overlying facies. The Cozy Dell unconformity is a flooded sequence boundary which formed as a surface of bypass and erosion during base-level fall but which was subsequently flooded during subsidence-induced deepening of the basin. The middle Coldwater unconformity is incised 58 m into a depositional sequence recognized on well logs, and alluvial valleys incised into the same unconformity in outcrop contain 58 m of estuarine fill. The unconformity on the paleotopographic highs is a combined flooding surface/sequence boundary with deeper water facies above the unconformity than below. In contrast, the unconformity in the paleotopographic lows is a sequence boundary with shallower water facies above the unconformity than below. The Cozy Dell Formation in outcrop is equivalent to a highstand systems tract, whereas the interval identified by lithology as Cozy Dell in well logs is equivalent to the condensedmore » section at the base of the same systems tract. The systems tract is dominantly shale and contains interbedded lenticular turbidite sandstones on the downthrown, basinward side of an Eocene growth fault. Continuing landward, the turbidites are eventually replaced by shoaling upward parasequences.« less

PART I: INTRODUCTION AND THE NORTHWESTERN AND CENTRAL FAUNAS

The continental Cenozoic of this country is very extensive, holds important natural resources, and sustains much of the environmental impact; yet it remains geologically poorly known. Scarcity of time markers and very limited detailed mapping have hampered the studies on this subject. In an effort to improve the knowledge on the Tertiary, the available geologic information on the Cenozoic sequence of nine representative areas belonging to six states is critically reviewed in this paper, their vertebrate faunas described, and the results are integrated to assess their geologic and paleontologic significance.The areas and faunas treated in this part of the study are: From Northwestern Mexico: (1) La Misión, B. C. N; (2) La Purísima, B. C. S.; (3) Tubutama, Son.; (4) Yécora, Son.; and from Central Mexico: (5) Zoyatal, Ags. In La Misión, the Tertiary includes the Rosarito Beach Formation (basalt flows interbedded by marine and continental litharenitic sandstones), that bears the La Mision Faunule of Hemingfordian (medial Miocene age); it is overlain by tuffs, undifferentiated volcanics and arkosic strata, which in turn are unconformably covered by Quaternary basaltic flows and sediments. The faunule consists of Desmostylus sp. (a marine mammal), and a Camelidae Gen et sp. indet.In La Purísima, the Tertiary includes the Oligocene San Gregorio Formation (phosphatic and tuffaceous sandstones) unconformably overlain by the intertongued Isidro (litharenitic sandstones and volcanics) and Comondú (volcaniclastic deposits) Formations bearing the La Purísima faunule of late Hemingfordian-early Barstovian (medial Miocene) age; they are overlain by late Tertiary basaltic flows -forming two units- in turn unconformably overlain by Quaternary deposits. The faunule includes Desmostylus hesperus and Eupiocyon cf. E. praedator, the earliest record of Miocene carnivores in México.In Tubutama, the Tertiary includes five, largely lacustrine informal units (subarkosic sandstones) of medial Miocene age unconformably overlain by the late Tertiary ?Baucarit Formation (tuffaceous and litharenitic sandstones), which in turn is unconformably covered by Quaternary deposits. In the third informal unit, the camel Stenomylus tubutamensis sp. nov. occurs.In Yecora, the Tertiary includes Oligocene undifferentiated volcanics (chiefly andesitic flows and pyroclastics), unconformably overlain by fluviolacustrine tuffaceous sandstone strata that bear the late Hemingfordian (medial Miocene) leporid Hypolagus sonoranus; these strata are in turn unconformably overlain by late Tertiary volcanics and Quaternary deposits.In Zoyatal, the Tertiary includes the ?Oligocene Ojo Caliente Rhyolite, unconformably overlain by the Zoyatal Tuff (mostly crystal-vitric, rhyolitic tuffs with tuffaceous sandstone interbeds) that bears the homonymous local fauna of medial Hemingfordian age (medial Miocene); it is unconformably overlain by the Aguascalientes Tuff and other Quaternery deposits. The Zoyatal local fauna consists of the peccary Dyseohyrus ef. D. stirtoni, the rhinocerotid cf Menoceras sp., the oreodon Merychyus elegans and the camelid Aguascalientia wilsoni.Summing up, the Precenozoic basement-Cretaceous marine bodies- crops out only in areas (1) and (3); the Tertiary sequence is formally differentiated in areas (1), (2) and (5); the fossiliferous units are in all areas tuffaceous-arikosic fluviolacustrine sandstones; marine-continental intertonging accurs in areas (1) and (2), hence there the possibility of direct correlation of marine and continental strata and faunas could be investigated; radioisotopic dating affording some measure of age-calibration is available in areas (2) and (3); the mammal record is meager but highly significant, includes exclusively genera and species of strict North American biogeographic affinity; the faunas and single occurrences discussed, closely correlate to well known Hemingfordian and to a lesser extent early Barstovian- faunas in North America, being in fact partial samples of such assemblages.

Kinematic analysis of shear zones in sandstone and mudstone of the Shimanto belt, Shikoku, SW Japan

I describe kinematic criteria for sense-of-shear in unmetamorphosed interbedded sandstone and mudstone of the Tertiary Shimanto accretionary complex at Ariigawa, Kǒchi Prefecture. In certain zones parallel to bedding and up to several meters wide, sandstone layers with internal bedding-parallel turbidite laminations are broken into blocky rectangular fragments or inclusions. Many inclusions rotated counterclockwise and so indicate a sinistral sense-of-shear with respect to the present orientation of bedding. Asymmetric folds corroborate this interpretation. Amounts of rotation are variable and are inversely proportional to the axial ratios of fragments. Fragments with axial ratios greater than about 4–6 did not rotate, while those with ratios between 4 and 1 record progressively greater maximum rotations. These observations accord with theoretical and experimental analyses describing the behavior of rigid inclusions in a matrix subjected to simultaneous simple shear and pure shear. The data from the Shimanto suggest that: (1) the rotation-inducing deformation included a coaxial component and departed from strictly non-coaxial simple shear; and (2) total shear strains were of the order of 3–5.

Stratigraphy and depositional history of the Upper Palaeozoic and Triassic sediments in the Wandel Sea Basin, central and eastern North Greenland

A lithostratigraphic scheme is erected for the Lower Carboniferous to Triassic sediments of the Wandel Sea Basin, from Lockwood Ø in the west to Holm Land in the east. The scheme is based on the subdivision into the Upper Carboniferous - Lower Permian Mallemuk Mountain Group and the Upper Permian - Triassic Trolle Land Group. In addition the Upper Carboniferous Sortebakker Formation and the Upper Permian Kap Kraka Formation are defined. Three formations and four members are included in the Mallemuk Mountain Group. Lithostratigraphic units include: Kap Jungersen Formation (new) composed of interbedded limestones, sandstones and shales with minor gypsum - early Moscovian; Foldedal Formation composed of interbedded limestones and sandstones -late Moseovian to late Gzhelian; Kim Fjelde Formation composed of well bedded Iimestones - late Gzhelian to Kungurian. The Trolle Land Group includes three formations: Midnatfjeld Formation composed of dark shales, sandstones and limestones - Late Permian; Parish Bjerg Formation composed of a basal conglomeratic sandstone overlain by shales and sandstones - ?Early Triassic (Scythian); Dunken Formation composed of dark shales and sandstones - Triassic (Scythian-Anisian). The Sortebakker Formation (new) is composed of interbedded sandstones, shales and minor coal of floodplain origin. The age is Early Carboniferous. The Kap Kraka Formation (new) includes poorly known hematitic sandstones, conglomerates and shales of Late Permian age.

New Interpretations of Pennsylvanian and Permian Stratigraphy, San Juan Basin and Southeast Paradox Basin: ABSTRACT

The Honaker Trail, Paradox, and Pinkerton Trail Formations of the Hermosa Group are recognized throughout most of the San Juan basin. The Paradox Formation is extended southeastward beyond the limits of its evaporite facies into the basin, where it consists of thick shelf-carbonate rocks and thin black shale, sandstone, and siltstone interbeds. Where the Hermosa Group thins onto the marginal uplifts, the Paradox loses the thick carbonate rocks and becomes indistinguishable from the rest of the Hermosa. The Hermosa is correlated in the subsurface with the Madera and Sandia Formations to the southeast. The transitional Rico Formation, between the marine Hermosa Group and the continental Cutler Formation, is identified throughout the subsurface of the San Juan basin and is correlated with similar deposits out-cropping along the northern and eastern margins. The Cutler Formation includes the Organ Rock, Cedar Mesa, and Halgaito members throughout most of the basin. In the vicinity of the Hogback monocline, the Cedar Mesa Sandstone Member undergoes a gradational eastward facies change from cyclic evaporite and sandstone to thick-bedded sandstone. The subsurface Cedar Mesa is correlated in part with similar rocks in the outcropping Abo and Supai Formations.

Submarine fan sedimentation at a convergent margin: the cretaceous mangapokia formation, New Zealand

The middle Cretaceous Mangapokia Formation (Pahaoa Group) near Te Awaiti, southeast North Island, New Zealand, consists of indurated, poorly fossiliferous, alternating sandstone and argillite, minor conglomerate, grit, pebbly-sandstone, and pebbly-mudstone (terrigenous sedimentary assemblage), and minor basalt, coloured argillite, chert, and micritic limestone (ocean-floor assemblage). Seven lithofacies are distinguished in the sedimentary assemblage on the basis of lithology, bed thickness and geometry, sand/mud ratio, grain size and internal sedimentary features. Facies 1 (10–15% of total exposure), which includes all sediments coarser than sand grade, comprises seven subfacies as follows: lenticular and erosive beds of coarse-grained (Subfacies 1Ai), medium-grained (Subfacies 1Aii) and fine-grained (Subfacies 1Aiii) predominantly clast-supported conglomerate, grit (Subfacies 1Aiv) and pebbly-sandstone (Subfacies 1Av) displaying numerous types of graded bedding and sedimentary structures, were all deposited predominantly from high-concentration turbidity currents or bed-load inertia flows. Minor chaotic sand or mud matrix-supported conglomerate lenses (Subfacies 1Bi), and beds which show clear evidence of post-depositional remobilisation (Subfacies 1Bii), represent debris flow deposits. Thick lenses of sandstone and minor argillite interbeds (Facies 2) were deposited from large-volume inertia flows, possibly grainflows. Facies 3, the most common lithofacies, consists of laterally more extensive, medium thickness, graded beds of alternating sandstone and argillite with rare Bouma sequences. These deposits are proximal turbidites which accumulated in environments more distal than Facies 1 and 2. Thin-bedded (Facies 4) and very thin-bedded (Facies 5) alternating sandstone and argillite, and argillite-dominated sequences with minor interbedded sandstone (Facies 6) were deposited in interchannel depressions, on channel levees, or in areas distant from high-concentration flows. Rare discontinuous graded bedded sandstones containing common flame structures (Facies 7) interbedded with conglomerate facies represent within-channel turbidites. A lower inner-submarine fan channel-overbank assemblage (Facies Association A), an inner-middle submarine fan channel-lobe assemblage (Facies Association B), and a mid-fan assemblage (Facies Association C) are distinguished. Two large-scale progradational suites of facies reflecting large-scale migration of the inner-fan channel complex are superimposed upon numerous small-scale depositional cycles resulting from lateral migration of numerous braided channels within the inner and mid-fan environments. The volumetrically minor ocean-floor assemblage is restricted to zones of melange. Chemically the basalts are tholeiitic, with trace element concentrations similar to modern ocean-floor basalts. Radiolarian-bearing cherts and coloured argillites are highly oxidised, and chemically similar to some modern oceanic pelagic sediments. The juxtaposition of relatively coherent, strongly folded and tectonically imbricated trench-fill submarine fan sediments and minor ocean-floor material is consistent with contemporaneous sedimentation and accretion at the base of the i inner trench slope in a convergent plate setting. The Mangapokia Formation is interpreted to represent the youngest part of the Torlesse terrane, and hence in southern North Island, Torlesse deposition and concurrent subduction tectonism were probably continuous at least until the middle Cretaceous.

Structural analysis of the Shimanto Accretionary Complex, Kyushu, Japan, based on foraminiferal biostratigraphy

The Paleogene Hyuga Group in the Shimanto Supergroup, southern Kyushu, Japan, is lithologically divided into four formations: the Aradani, Tashiro, Unama, and Ouchibaru. These formations are characterized by imbricate structure, and each formation is repeatedly sliced and distributed by thrusts dipping to the north (landward). The mudstone unit of the Tashiro Formation is composed mainly of massive mudstones with thin sandstone interbeds. It occurs in seven belts, of which the Shiitani, Nagano, and Unama belts have larger widths than others. Based on a biostratigraphic study of abundant planktonic foraminiferal fossils from the Tashiro Formation, five planktonic foraminiferal zones were recognized in the strata of these three belts. They are Morozovella spinulosa Zone (I), Globigerinatheka index Zone (II), Globorotalia cerroazulensis cocoaensis Zone (III), Pseudohastigerina barbadoensis Zone (IV), and Chiloguembelina cubensis Zone (V) in ascending order. They are firmly correlated with the well‐calibrated zones known from low‐latitude regions and are assigned to latest Middle Eocene, earliest Late Eocene, Latest Eocene, Earliest Oligocene, and Early Oligocene age, respectively. On the basis of the distribution of the fossil zones in the three belts the stratigraphy and geologic structure of mudstone unit of the Tashiro Formation are summarized as follows: 1. The strata of each belt are cut into many sheets mainly by thrusts dipping to the north (landward), forming an imbricate structure, and the same strata appear repeatedly many times with a disordered array within a single belt: 2. Many of the sheets show a normal sequence, but a few sheets are overturned: 3. The thickness of each sheet is less than 200 m: 4. In each belt, the older planktonic foraminiferal zones occur widely in the southern part and the younger ones in the northern part. Therefore, the strata of each belt show a tendency of northward (landward or inward) younging: 5. If the distribution of the foraminiferal zones of the three sections is viewed collectively, the upper zones (IV–V) are more widely distributed in the Shiitani belt in the southernmost part of the investigated area than the Nagano belt that is in the central part: 6. To date, the youngest zone (V) has been found only in the Shiitani belt. Therefore it is concluded that each belt shows a roughly northward (landward) younging internally, but the arrangement of the overall foraminiferal zones among the three belts analyzed shows a southward (oceanward) younging. The stratigraphic arrangement and structure of the three mudstone belts of the Tashiro Formation may be characteristic of accretionary prisms.

Model of Fluvial Deposition for Control of Oil Migration and Entrapment in Upper Eocene to Oligocene Sespe Formation, West Montalvo Field, Ventura County, California: ABSTRACT

The Sespe Formation consists of continental red beds deposited during the tectonism that resulted as the Pacific-Farallon spreading ridge approached the North American plate. The Sespe at West Montalvo field is over 7000 ft thick and consists predominantly of fine to medium-grained sandstones interbedded with siltstone and mudstone deposited in the central part of the Oligocene basin. Oil production was established in 1951 from the upper 2000 ft, known as the Colonia zone. The Colonia zone has been subdivided into six sandstone packages 350-600 ft thick, based on a model of laterally migrating fluvial systems that created local intraformational unconformities. These systems had unique depositional characteristics that can be inferred from well-log analysis and related to facies described in outcrops surrounding the Ventura basin. These characteristics include sandstone to shale ratios, relative bed thicknesses, lateral continuity of sandstone and shale interbeds, and whether the sandstone beds exhibit normal or reverse grading, or have sharp bases and tops. The fluvial environments include small braided distributary streams, larger trunk streams, and broad shallow braided streams. All of the sandstone packages contain oil-bearing beds, but the package in which the sediments were apparently deposited in a system of broad, shallow braided streams ismore » the most oil-prone. These sandstones are relatively thin, have sharp bases and tops, and are laterally continuous across parts of the field. The rapid sedimentation and the cut-and-fill processes of braided streams may have created interconnecting fluid pathways that allowed the migration and updip accumulation of oil.« less

Submarine Canyon, Slope, and Shelf Sedimentation in an Upper Eocene-Oligocene Progradational System (Limnos Island, North Aegean Sea, Greece): ABSTRACT

The only well-exposed outcrops of a post-Alpine late Eocene-Oligocene basin in the north Aegean Sea are at Limnos Island. These mostly consist of typical slope deposits overlain by remnants of shallow marine shelf and continental (braided-river) deposits. Three main slope lithofacies are distinguished. Canyon deposits consist of thick-bedded, massive, and pebbly sandstones (facies B), conglomerates (facies A), pebbly mudstone where the matrix shows flow (facies F), rock falls (nummulitic limestones), and zones of slump folds in sandstones (facies F). Channelized facies of massive sandstones, classical turbidites, and thin interbeds of sandstone and mudstone (facies E - probably overbank or levee deposits) are associated with canyon deposits. Pelagic and hemipelagic slope deposits consist of mudstone (facies G) and thin-bedded sandstone (facies D) with occasional small to medium-scale slump folds. These mud-rich slope deposits are incised by several channels filled with conglomerates, thick massive sandstones with well-developed dish structures, and turbidites interbedded with thin layers of mudstone (facies C). This unit composes a typical thinning and fining-upward sequence.

Trace Fossils Indicating Estuarine Deposystems for the Devonian-Mississippian Cloyd Conglomerate Member, Price Formation, Central Appalachians

The use of ichnologic data is crucial to the understanding of some paleoenvironmental regimes. The Cloyd Conglomerate Member of the Price Formation has been interpreted as entirely fluvial, partly fluvial and partly marine bar, and entirely marine bar. Trace fossils provide data to resolve these divergent opinions. The Cloyd preserves a thin sedimentary wedge composed of a lower, regressive nonmarine phase and an upper, transgressive marine phase bearing trace fossils and a restricted marine fauna. Massive, quartz-pebble conglomerates of the lower Cloyd were deposited in fluvio-distributaty systems during a regression near the Devonian-Mississippian boundaty. Regressive channel systems became estuaries during a subsequent transgression. Trace fossils in the upper Cloyd constitute a mixed Skolithos-Cruziana Ichnofacies of suspension-feeding sand dwellers and depositfeeding mud dwellers. Though not abundant, trace fossils occur most commonly in the uppermost sandstone beds of fining-upward, estuary pointbar deposits. A point-bar assemblage includes large, deep Skolithos and Cylindrichnus suspension feeders that inhabited bar slip faces. Deep burrowing behavior suggests that point-bar trace makers sought to moderate environmental extremes, e.g., salinity and temperature fluctuation and possible subaerial exposure of bar surfaces. The Teichichnus trace maker inhabited lower-energy, submerged sand flats on point-bar surfaces, possibly following channel abandonment. Tidal-flat facies composed of organic-rich silty shales with interbedded siltstones and sandstones overlie point-bar sandstones. Well-circulated tidal-flat facies are dominated by shallow-burrowing suspension feeders in sandstone interbeds. The trace assemblage includes Arenicolites, Diplocraterion, Monocraterion, Planolites, Rusophycus?, and shallow Skolithos. A backwater assemblage includes Chondrites and Planolites deposit feeders that inhabited organic-rich, black

Paleogeography of Late Cretaceous Clastic Shelf Deposits, Northeastern Sacramento Valley, California: ABSTRACT

The Chico Formation of the northeastern Sacramento Valley, California, is a thick sand-dominated sequence of late Coniacian to early Campanian age that deepens upward from fluvial and deltaic or shelf conditions, reflecting Late Cretaceous transgression and subsidence. During maximum marine conditions in the late Santonian and early Campanian, the shoreline was located near the southern end of the Chico outcrop area, and sedimentary environments deepened to the northwest. The southernmost exposures of the Chico Formation, represented by the Pentz Road member, consist of cross-stratified sandstone with minor shale and lenses of metamorphic pebbles. These strata are of estuarine origin, and they contain a brackish to marine fauna that includes species transported from the rocky intertidal zone. Inner shelf conditions are represented to the north at Butte Creek and Big Chico Creek, where the Ten Mile member consists of erosively based beds of fine-grained, hummocky cross-stratified sandstone. Bioturbation is common in the Ten Mile Member, and basal shell lags of storm-surge origin contain a diverse marine fauna. Farther north, at Mill Creek and Antelope Creek, an outer shelf environment is represented by the Kingsley Cave member, which consists of parallel-laminated to highly bioturbated mudstone and siltstone with an inoceramid-dominated fauna. Similarmore » mudstones at Tuscan Springs, at the northern end of the Chico outcrop area, include slope-related interbeds of parallel-laminated sandstone and matrix-supported, intrabasinal conglomerate. The outer shelf area between Mill Creek and Tuscan Springs extended an unknown distance to the north-northeast, where Upper Cretaceous deposits are completely covered by Tertiary volcanics.« less