Fault-controlled Basin Research Articles

Geology of the Deer Butte Formation, Malheur county, Oregon: faulting, sedimentation and volcanism in a post-caldera setting

The Deer Butte Formation accumulated during the middle Miocene in fault-controlled basins in an extensional setting. The basins developed as regional faults asserted influence after eruption of ash-flow sheets and collapse of calderas of the Lake Owyhee volcanic field. The sequences of Hurley Flat, Dry Creek, and Oxbow Basin contain a lower basalt tephradominated unit formed by basalt hydrovolcanism overlain by fine-grained fluvial and lacustrine volcaniclastic sedimentary units. The sequence of Freezeout Creek was deposited in an erosional valley that was incised into older units and cut across the concurrently active Wall Rock Ridge fault zone. The sequence of Hurley Flat and Dry Creek contain alkaline tholeiitic basalt flows and tephra deposits, whereas the sequences of Freezeout Creek and Oxbow Basin contain subalkaline calcalkaline basaltic andesite. The compositional change occurred after local uplift due to faulting along the Wall Rock Ridge fault zone. The youngest unit, well-sorted, medium-grained, muscovite-bearing arkose of the arkose of Dry Creek Buttes, was deposited in a large river that drained westward from source areas in western Idaho. The Deer Butte Formation was deposited between approximately 15 and 12.6 Ma, while basin and range-type faulting dominated regional structural patterns.

Arenig sedimentation and basin tectonics in the Harlech Dome area (Dolgellau Basin), North Wales

AbstractArenig (Ordovician) clastic sediments crop out in the Harlech Dome region (North Wales), and are placed in a single stratigraphic unit: the Allt Lwyd Formation. This unit records a marine transgression onto an erosion surface produced during late Tremadoc arc volcanicity. Four discrete petrofacies are denned, and reflect differing proportions of detritus derived from Tremadoc-type basic-intermediate igneous rocks, and the local sedimentary basement. Initial shallow marine siliciclastic sandstones and conglomerates are overlain by extensive deep water mud-rich units. These generally shallow up into a complex arc-apron deposit, with sediments derived from the eroding Tremadoc arc, as well as from similar, synchronous volcanics. Predominantly epiclastic sandstones and conglomerates were deposited in deltaic and tidal environments in an arc-apron complex, and capped by condensed mudstones and an ironstone, deposited as sea level rose across these systems. Sediments were ponded in north–south orientated troughs and derived from uplifted blocks. Facies and petrofacies distribution were controlled by syn-sedimentary north-south and northeast–southwest faults. The Allt Lwyd Formation was ponded in a fault-controlled basin (the Dolgellau Basin), one of a series of interconnected sub-basins flooded by the Arenig transgression. The sediments preserved reflect deposition during the transgression of a volcanic arc, prior to the extrusion of marginal basin-type volcanics.

An exceptionally thick upper Proterozoic (Sturtian) glacial succession in the Mount Painter area, South Australia

The Yudnamutana Subgroup is a thick succession of upper Proterozoic (Sturtian) glacigenic rocks preserved in the Yudnamutana trough in the northeastern part of the Adelaide geosyncline. In ascending order, the subgroup comprises the Fitton, Bolla Bollana, and Lyndhurst Formations. The name Creek Member is proposed for a locally developed conglomeratic facies at the base of the Fitton Formation. These rocks are interpreted as the products of two glacial advances. The first is evidenced by the Hamilton Creek Member, which includes sediment gravity flows derived from subaqueous glacial outwash. The Fitton Formation is composed mainly of basinal siltstones and mudstones with associated coarser-grained rocks that are largely sediment gravity flows. Diamictites in the upper part of the formation signify the onset of a second glacial advance. Thick diamictites of the Bolla Bollana Formation are interpreted as rainout deposits from floating ice derived from ice streams debouching from a steep and high hinterland into a fault-controlled basin. The Lyndhurst Formation is mainly fine-grained rocks. Dropstones and sparse diamictites attest to the presence of some floating ice, but some of the diamictites are probably mass-flow deposits. Paleocurrent data from the Yudnamutana Subgroup indicate transport to the west-northwest. The Sturtian glacigenic rocks are overlain by a transgressive shale. Stratigraphic similarities exist between the Yudnamutana Subgroup and thinner successions to the west. These stratigraphic similarities probably reflect paleoclimatic changes. The twice-repeated sequence of diamictite followed by mudstone is attributed to two glacial advance-retreat cycles. Local variations in thickness and stratigraphic succession are probably related to contemporaneous faulting.

Tidally influenced coal-bearing sediments in the Tertiary Bering River Coalfield, south-central Alaska

Tertiary coal-bearing sediments of the Tokun Formation in the structurally complex Bering River Coalfield, south-central Alaska, comprise stacked, glauconitic, fining- and rare coarsening-upward sequences up to 7 m thick. Fining-upward sequences consist of a scoured base, overlain by shale pellet conglomerate, coarse- to medium-grained, well-sorted, planar and trough cross-bedded sandstone, containing skeletal remnants of crabs and bivalves, and carbonaceous shale and coal beds. Coarsening-upward sequences are the inverse of the fining-upward sequences; stacking of these sequences produces megasequences up to 50 m thick. The fining- and coarsening-upward sequences are interpreted to have been deposited in tidal inlet and back barrier lagoon, and marsh environments that formed part of a mesotidal barrier island-estuarine complex. Seaward migration of inlet channels and flood tidal deltas produced fining-upward sequences capped by back barrier lagoon and marsh deposits. Vertical stacking of sequences is attributed to a series of transgressive-regressive events controlled by differential rates of sediment supply and subsidence. Consideration of the plate tectonic setting of the Bering River Coalfield and the established tectonic control on Tertiary sedimentation in Alaska, suggests that megasequence deposition was most probably related to longer term regional tectonic events, such as fault-controlled basin subsidence.

Structure of Cuyama Valley, Caliente Range, and Carrizo Plain, and Its Significance to Structural Style of Southern Coast Ranges and Western Transverse Ranges: ABSTRACT

A concerted exploration program in the Cuyama area by ARCO during the last 10 years, employing surface mapping, deep drilling, high-quality seismic profiles, biostratigraphic analysis, and balanced cross sections, has revealed several significant structural characteristics. (1) Thrust faults, such as the Morales and White Rock, flatten under the Caliente Range and merge into a single system with up to 14 km of slip. This system intersects the San Andreas fault at a high angle and may detach the San Andreas fault at depth from 7 to 8 km. (2) Integration of surface and subsurface mapping shows that these thrust faults have little or no component of strike-slip and are younger than 3 Ma. (3) Major surface folds, such as the Caliente Mountain anticline, are due to thrust faults ramping upsection and producing fault-bend and fault-propagation folds. (4) The young thrust system has cut across a late Oligocene to early Miocene extensional fault-controlled basin, and both tectonically transported and overthrust portions of this older basin. These structural characteristics strongly suggest that the structural style of the Cuyama area and possibly adjacent areas in the southern Coast Ranges and western Transverse Ranges is that of a fold-and-thrust belt and is not relatedmore » to wrench tectonics.« less

Modern carbonate-siliciclastic transitions: Humid and arid tropical examples

Climate and tectonic setting largely control characteristics of transitions between carbonate and siliciclastic sediments in depositional systems. Three examples, selected from both humid and arid tropical settings, are discussed order to illustrate details of these relationships. The eastern shelf area of Nicaragua is an area where siliciclastic and carbonate sediments actively interfinger under humid tropical conditions. Extremely high rainfall (4.5 m yr −1 in the southern part of coastal plain), coupled with a correspondently high sediment flux to the nearshore shelf, results in an inner shelf that is dominated by siliciclastics. Density gradients and persistent momentum provided by the trade winds force turbid coastal waters to the south and confine siliciclastic sediments to a coast-parallel zone approximately 20 km wide. The remainder of the broad shelf is characterized by carbonate sediments and reefs. The mid-shelf area has a coarse carbonate sediment cover composed largely of the calcareous green alga Halimeda. The outer shelf displays rugged topography associated with reef development. Skeletal carbonates containing abundant coral and coralline algal grains are characteristic of the seaward margin of the shelf. Significant amounts of siliciclastic sediment are confined to the nearshore part of the shelf, and the transition from siliclastics to carbonates occurs over a distance of 20 km or less. A second example from the humid tropics focuses on the living reefs of Indonesia that are surrounded by fine-grained siliciclastics. A warm, rainy climate results in rapid weathering of volcanic rocks from the Indonesian archipelago and abundant sediment input to the shallow (40–70 m deep) Sunda Shelf. In the northern hemisphere winter, monsoonally driven marine processes rrive water and sediment to the east down the axis of the Java Sea. During summer the direction of transport is reversed. This bidirectional process regime not only leaves its imprint in reef and carbonate platform morphology but also results in widespread distribution of siliciclastics from adjacent source areas. Isolated reefs as well as small and generally elongate reef-rimmed carbonate platforms (0.5–10 km long) rise 40–50 m above a sea floor composed largely of siliciclastic sediments. In these cases, facies transitions are very abrupt. Until environmental conditions curtail the carbonate-producing communities of these platforms, carbonate buildups and associated skeletal sediments will coexist with wider spread and more abundant siliciclastics. A third example from a low-latitude arid setting shows that facies transitions from carbonates to siliciclastics typically take place over short distances. The lack of significant rainfall, coupled with high relief along fault-controlled basin margins, promotes aperiodic but powerful episodes of siliciclastic that build directly into the marine environment. High-sloping and narrow coastal plains composed of overlapping fans result. The steep seaward ends of these fans are maintained and protected from marine erosion by coral reefs and associated carbonates. Regionally, siliciclastic to carbonate facies changes in these arid settings take place over very shor distances, a few kilometers at most. However, on a local scale, carbonates may be replaced by siliciclastics over distances as small as a few tens of meters.

Wet-sediment deformation in the Upper Ordovician Point Leamington Formation: an active thrust-imbricate system during sedimentation, Notre Dame Bay, north-central Newfoundland

Summary The Upper Ordovician (late Caradoc-Ashgill) Point Leamington Formation is essentially composed of fine-grained, thin-bedded turbidites that accumulated in a deep marine environment below wave-base. Immediately underlying, and apparently conformable with the formation, there are about 120 m of red, red-green and grey bioturbated cherts overlain by late Llandeilo-early Caradoc black shales—these argillaceous, ‘pelagic’, sediments conformably overlie about 800 m of mafic pillow lavas and flows of the Lawrence Head Volcanics (middle Exploits Group). The Point Leamington Formation, up to 2200 m thick, together with the mainly fine-grained, thin-bedded turbidities, contain locally abundant wet-sediment deformation in horizons up to tens of metres thick, although most of the thickest horizons appear to represent multiple events. Conglomerate-filled gullies, channels and canyons occur within the finer grained Point Leamington Formation facies-associations. The formation is overlain, gradationally, by the deep-water conglomeratic Goldson Formation, interpreted as submarine canyon deposits; with the Ordovician-Silurian boundary occurring towards the top of the Point Leamington Formation or towards the base of the Goldson Formation, based on the presence of lower Llandovery corals in limestone boulders within an olistostrome near the gradational boundary between these formations. Wet-sediment deformation occurs as: (i) coherent folded layers; (ii) semi-coherent folded layers; (iii) chaotic balled or brecciated sediments; (iv) boudinaged layers; (v) faulted layers, involving normal, reverse and thrust faults, and best seen on a micro- to meso-scopic scale. Clastic dykes, other liquefaction and fluidization structures, together with convolute lamination, also occur either isolated from, or in association with, the above listed wet-sediment deformation styles. The wet-sediment deformation generally appears to be related to gravity-controlled slope failure, in surface to near-surface mass failure or at unspecified depth of burial, on a margin with a regional south to south-eastward downslope dip, although some folds may be interpreted as thrust-related (tectonic) deformation in wet sediments. The overall stratigraphy, sedimentology and structure suggests Upper Ordovician to Lower Silurian deep marine sedimentation in small fault-bounded and fault-controlled basins, some of which contained relatively coarse-grained, small-diameter, submarine fans. Regional considerations are consistent with the succession having accumulated in a thrust imbricate system, active during sedimentation, with new evidence to indicate Ashgill-Llandovery volcanism and magmatism that can be related to initial subduction followed by possible crustal extension. Sometime during the Ashgill, subduction appears to have ceased as all the intervening oceanic crust was consumed and volcanic arc activity shut down. During late Ashgill-Llandovery time, thin continental crust of the ‘Gander’ terrane to the east then underplated the ‘Dunnage Zone’, probably leading to foreland basin development, analogous to the plate tectonic processes in the Banda Arc today. Thus, there was a reversal of subduction polarity from the eastwards subduction of pre-Caradoc times, such that the remnant backarc basin became, for a while, a forearc as the wide backarc or marginal basin floored, at least in part by oceanic crust, telescoped. Contemporaneous sinistral shear appears to have been important in the late Ordovician-Silurian, during which time unspecified ‘suspect’ terranes may have slipped in and out of the Dunnage Zone. The contemporaneous Ashgill-Llandovery bimodal igneous activity can be explained by phases of crustal transtension as re-entrants in opposing plates slid past each other, followed by phases of transpression to re-activate the thrust imbricate system.

The petroleum geology of the Unst Basin, North Sea

The Unst Basin is situated in the northern North Sea between the East Shetland Basin and the Shetland Isles. The basin is essentially a three-armed, Permo-Triassic fault-controlled basin containing up to 3600 m of red-beds. This is overlain by a westerly thickening Jurassic and early Cretaceous sequence, the stratigraphy of which is very similar to that of the East Shetland Basin. In particular, the Brent Group (140 m), Humber Group (685 m) and Cromer Knoll Group (300 m) are well represented. As a result of Laramide uplift of the area, the thick Upper Cretaceous and Palaeocene strata of the East Shetland Basin are absent from the Unst Basin. This uplift resulted in substantial erosion within the Unst Basin providing the major source for Palaeocene sands in the Viking Graben and the Faeroes Basin. Late Palaeocene and younger Tertiary strata transgress westwards across this erosion surface. Petroleum exploration within the basin culminated in the drilling of two exploration wells. These wells encountered potential reservoir and source rocks in the Jurassic section. However, geochemical analyses indicate these source rocks are immature for hydrocarbon generation within the Unst Basin. It is concluded that the Unst Basin has a low petroleum potential.

Geological Setting of North Slope Oil Fields, Alaska: ABSTRACT

The North Slope is a prolific hydrocarbon province in which discoveries to date amount to some 60 billion bbl of oil in place and 50 tcf of gas in place. Reservoirs and prolific source rocks occur throughout the stratigraphic column, which consists of a lower (or Ellesmerian) megasequence of Carboniferous to Jurassic age and an upper (or Brookian) megasequence of Early Cretaceous to Recent age. Discovered oil is almost equally divided between Ellesmerian and Brookian reservoirs. Patterns of hydrocarbon generation and migration have been controlled by deposition of clastic sedimentary wedges derived from the Brooks Range orogen. In the Late Jurassic to Early Cretaceous, the main oil kitchen was located in the Western Colville trough. Clastic depocenters and associated kitchen areas migrated progressively eastward with time and are now located in the East Beaufort offshore. Important source rocks include the Jurassic Kingak and Late Triassic Shublik Formations of the Ellesmerian megasequence, and the Aptian-Cenomanian HRZ and Turonian-Paleocene Shale Wall formations of the Brookian megasequence. In the Ellesmerian megasequence, productive reservoirs are known at several stratigraphic levels, with best reservoir properties associated with secondary porosity development in subcrop beneath a mid-Hauterivian unconformity. The Kekiktuk Formation (Mississippian), oil- and gas-bearing in the Endicott field (ca. 1 billion STBOIP), is a fluvially dominated unit, locally deposited in fault-controlled basins. The Lisburne Group (Mississippian to Early Permian) contains oil in the Lisburne pool of the Prudhoe area (?2-3 billion STBOIP). The reservoir is primarily early diagenetic dolomites within a thick platform carbonate sequence. The major reservoir on the North Slope is the Early Triassic Ivishak Formation, reservoir to the Prudhoe field (22 STBOIP), the Seal Island discovery (?1 billion STBOIP), and target in the recent Mukluk well. Sands and conglomerates were deposited by a series of alluvial fan deltas shed from a nearby northern landmass. Arctic Ocean rift events culminated in mid-Hauterivian continental breakup, which generated the subcrop unconformity of the rift margin uplift. Post-unconformity sands associated with local erosion of Ellesmerian End_Page 663------------------------------ and basement strata occur in widespread sheets. These sands can be important reservoirs,such as the Kuparuk and Point Thomson formations, of late Hauterivian-Barremian age (?ca. 11 STBOIP total). Finally, within the Brookian megasequence, large volumes (?ca. 20 billion STBOIP) of relatively heavy oils are trapped in the Late Cretaceous to early Tertiary West Sak and Ugnu formations. These sands are of marine-shelf to fluvial/deltaic depositional environments, topset strata of a Laramide prograding clastic wedge. End_of_Article - Last_Page 664------------

Walker Lake, Nevada: Sedimentation in an Active, Strike-Slip Related Basin: ABSTRACT

Walker Lake, Nevada, is in an active fault-controlled basin related to the right-lateral, northwest-trending Walker Lane Shear Zone on the western side of the Basin and Range province. The lake occurs in a half graben bounded on its west side by a high-angle normal fault zone along the Wassuk Range front. This fault zone may merge to the north into the Walker Lane fault system, which forms the northeast boundary of the basin. To the south of Walker Lake, the Wassuk front fault merges with an east-northeast trending left-lateral fault. The Walker Lake basin is interpreted to be a pull-apart basin formed within the triangular zone bounded by the Wassuk front, the Walker Lane, and left-lateral faults. The Walker River drainage basin occupies about 10,000 km2 (3,800 mi2) in western Nevada and parts of California and is essentially a closed hydrologic system that drains from the crest of the Sierra Nevada in California and terminates in Walker Lake. Walker Lake trends north-northwest and is 27.4 km (17 mi) long and 8 km (5 mi) wide with water depths exceeding 30 m (100 ft). Lake Lahontan (Wisconsinian) shorelines ring Walker Lake and suggest water depths of 150 m (500 ft) above the present lake level. The lake is situated in an asymmetric basin with steep alluvial fans flanking the western shoreline (Wassuk Range) and gentle, areally more extensive fans flanking the eastern shoreline (Gillis Range). The Walker River delta enters the lake from the north and is a ajor sediment point source for the basin. Older dissected shoreline, alluvial fan, Gilbert delta, and beach ridge deposits were built largely of coarse-grained, locally derived materials. Stromatolites, oncolites, and tufas formed along the shorelines, whereas mud and organic sediments accumulated in the lake on the west side of the basin. Extensive submerged sand flats and local sand dunes occur on the east side of the basin. End_of_Article - Last_Page 501------------

Sedimentology of Upper Ordovician – Silurian sequences on New World Island, Newfoundland: separate fault-controlled basins?: Discussion

Amott's interpretation of New World Island (NWI) stratigraphy suggests that Upper Ordovician and Silurian sedimentation took place in three distinct sedimentary basins that were bounded by synsedimentary faults. The stratigraphic and sedimentologic data offer a significant contribution to NWI geology but, in view of the geologic complexity of this area of the northern Appalachians, the paper would have benefitted by a presentation of paleocurrent data in rose diagrams or spherical projections rather than as azimuths, by a discussion of how paleocurrent data were rotated to inferred pre-deformational positions, and by an addition of error bars showing uncertainties in age determinations of fossils.

Sedimentology of Upper Ordovician – Silurian sequences on New World Island, Newfoundland: separate fault-controlled basins?

Remapping of northeast New World Island, Newfoundland demonstrates that two major faults separate three distinct sedimentary sequences, Paleontology and sedimentology indicate that these sequences are partly equivalent in age but were deposited in separate basins of deposition that were adjacent to each other. Active Silurian faults, the Boyds Island and Byrne Cove Faults (new names), bounded the margins of these basins and directly influenced sedimentation by uplifting Ordovician volcanics, limestone, and black shale, which are found both in situ and as blocks within Silurian sediments. Silurian sediments deposited adjacent to these faults are dominated by pebbly mudstones and chaotic bedding interpreted as debris flow deposits and slumped horizons. Away from the fault scarps sedimentation was predominantly axial; it comprises resedimented conglomerates and thick- and thin-bedded sandstone turbidites.West of New World Island, similar synsedimentary faults are confined to a narrow belt south of the Lukes Arm – Sops Head Fault. Two stages of Acadian deformation overprint all structures associated with the Silurian faulting.

Origin of the lithospheric tension causing basin formation

Most fault-controlled basin formation within plate interiors occurs by normal faulting in response to horizontal deviatoric tension in the continental crust. It is suggested that the tension originates either from the plate boundary forces acting at trenches or as a result of isostatically compensated uplifted regions such as East Africa. The tension produced by both mechanisms is greatest in high heat-flow regions where the upper elastic part of the lithosphere is thinned and weakened. Particularly widespread tension in the continental lithosphere occurs when subduction takes place on opposite sides of a large continental mass, such as Pangaea in the early Mesozoic, where it led to widespread graben formation and, in association with hot spot activity, to continental splitting.

Oligocene unconformities and nodular phosphate — hardground horizons in western Southland and northern West Coast

Abstract Oligocene strata generally occur as paraconformable or condensed sequences in the New Zealand region. Descriptions are given of well-exposed sections through Oligocene unconformities in the Waiau River (western Southland) and at Whitecliffs, Buller River (northern West Coast). In both cases the unconformities are preserved in sediment sequences of inferred basin-margin origin with respect to contemporaneous, Oligo-Miocene, fault-controlled basins. At Whitecliffs, an early Oligocene angular unconformity separates the regionally transgressive Maruia Group below from the more locally transgressive, mid-late Oligocene Cobden Group (Whitecliffs Formation) above; abundant fresh detritus from the Maruia Group occurs in breccias associated with the Cobden Group, which is separated from the overlying early Miocene Blue Bottom Group (Inangahua Formation) by a further unconformity, marked by a condensed sequence shellbed with rolled, phosphatized and glauconitized macrofossils and abundant pelagic microfaun...

Southwest Pacific Island Arcs: Sedimentary Basins and Petroleum Prospects in New Hebrides and Solomons: ABSTRACT

Several thousand meters of Miocene-Pliocene sediments are predominantly fine to coarse-grained volcaniclastics deposited in shelf to deep marine environments, commonly as turbidites; coralgal reef limestones and fore-reef calcarenites formed locally. Original basins in both the Solomons and New Hebrides measured 375 to 435 mi (600 to 700 km) by 62 to 125 mi (100 to 200 km) but parts of their margins are strongly deformed, uplifted, and eroded. Cross-faulting and Holocene volcanism caused segmentation and further reduction of basinal areas. In both island arcs, the sediments are little deformed along a median structural basin which was downfaulted in Pleistocene to Recent time in the Solomons, and from Pliocene onward in the central New Hebrides; no downfaulting occurred i the northern New Hebrides. Little is known of hydrocarbon source potential and degree of maturation. Back-reef or rapidly buried fore-reef environments may be the principal areas for source rock formation. Reefal limestones are the main potential reservoir rocks; they have, however, lost porosity locally because of recrystallization. Turbidite sandstones may form additional reservoirs, but volcanic derivation keeps permeability generally low. The main structural traps are fault-controlled near basin margins, but limited folding also occurs. The pre-Pliocene unconformity in the northern New Hebrides could generate stratigraphic traps. Water depth in the main prospective areas is several hundred to 5,000 ft (1,500 m) in the Solomons, and up to 10,000 ft (3,000 m) in the New Hebrides. Nearshore and onland prospect are extremely limited in both island arcs. End_of_Article - Last_Page 972------------

Sedimentary succession and palaeoenvironments within a fault-controlled basin: The ‘wealden’ of the Santander area, northern Spain

The so-called Wealden sediments are no less than 2000 m thick in Santander and north Burgos (northern Spain). The succession rests on an extensive scoured and channeled surface over marine carbonates of Early Callovian age, and ends with the incoming of the Urgonian marine strata and equivalents (Lower Aptian). Two major sedimentary cycles (‘megacylothems’) and the beginning of a third are recognized. Following a widespread break in the succession, each megacyclothem opens with a coarse-grained clastic interval of fluviatile origin and then grades up into finer-grained clastics and carbonates recording fluviatile, lacustrine and shallow-marine conditions. The succession accumulated in an elongated fault-bounded basin trending E—W. Detritus came from source areas lying to the south and west of the depositional area, which was at times connected with the sea to the east and possibly to the north. The development of the basin was controlled by the reactivation of Late Hercynian faults acting contemporaneously with sedimentation. The frequency of faulting varied considerably, periods of relative calm alternating with periods of strong activity. These determined the characteristics of the successive megacyclothems.

Controls on Diatomaceous Lithofacies in Obliquely Rifted Marginal Basin: Gulf of California: ABSTRACT

DSDP Leg 64 dissected Quaternary sedimentation patterns in the Guaymas Basin which confirm many similarities but underline some differences with other Neogene circum-Pacific diatomite basins. Tectonic setting in this morphologically complex basin includes broad hemipelagic slopes, fault-controlled outer slope basins and highs, and relatively small transform-bounded, obliquely rifted deeper basins with complex ocean crust. Frequent mass flows are triggered from either muddy delta foreslopes or hemipelagic diatom ooze drape. These accumulate as mud turbidites in the narrow rift zones at rates exceeding 2,000 m/m.y. Interaction of climatic and oceanographic parameters control the intensity of biogenic productivity (ergo, the oxygen budget) producing alternating sequences of aminated and homogenous diatomaceous ooze, generally confined to slope regions (400 m/m.y.). Laminated diatom-ooze also accumulated in deeper basins which were deprived of turbidity flows during limited periods. Sediments in slope areas contain a uniform 4% carbon but CaCO3 (mostly foraminifera) ranges episodically from 2 to 25%. Phosphate occurs as fish-debris-rich laminae or rare, soft, centimeter-size pellets. Diagenetic dissolution of silica is recorded at Site 479 on the slope where finely laminated hard muds occurring below an unconformity at 380 m subbottom are now devoid of frustules, except those cemented in dolomite beds. Paradoxically, porcellanites were not encountered, although traces of clinoptilolite suggest that some silica reactions are presently active. Chert only occurs in proximity to basaltic intrusions. Dolomite precipitation occurs at shallow subbottom depths in zones of high alkalinity and methanogenesis, gradually forming decimeter-thick hard layers by slow vertical accretion. These layers commonly preserve primary fabrics. Petrologic and heavy carbon isotope evidence suggest that ions for dolomite precipitation are mainly derived from interstitial waters. End_of_Article - Last_Page 945------------

Sedimentation and tectonic controls in the Early Jurassic Central High Atlas trough, Morocco

Stratigraphical and sedimentological studies of Lower Jurassic deposits at the western end of the Central High Atlas Mountains, Morocco, has led to a reappraisal of the tectonic significance of this linear feature during the critical period of the opening of the Atlantic Ocean. Lower Jurassic deposits south and east of Marrakech represent an extremely shallow water and supratidal marine carbonate accumulation that formed in fault-controlled basins at the western end of the trough. The basins were aligned east-northeast and the character and distribution of the deposits strongly suggests that no connection with the basins of the Western High Atlas existed. To the east, sediments indicate more open conditions, but no evidence of bathyal or abyssal conditions was seen. Sedimentation was probably structurally controlled by vertical tensional and compressional movements as the result of reorientation of stress between major strike-slip faults. Volcanic flows and shallow intrusions, which occur at the base of the Jurassic sequence, and associated carbonate and clastic sediments show no evidence of being part of ophiolite sequences; they more closely resemble extrusive basalt sequences associated with rifting as seen in the Triassic of the Eastern United States and the Tertiary of Northwest Europe. Evidence from this area suggests that the High Atlas seaway represented only an arm of the Tethyan Ocean, which extended along a long-standing zone of crustal weakness.

Upper Jurassic coal-bearing shoreline deposits, Hochstetter Forland, east Greenland

Upper Jurassic coal-bearing yellow sandstones occur at several localities on Hochstetter Forland, northern east Greenland. The sandstones have been divided into eight sedimentary facies, each facies characterized by an assemblage of sedimentary structures, a specific lithology and, occasionally by the content of fossils, notably oysters. The interpretations of the established facies range from high subtidal oyster banks over intertidal beach sand to lagoonal and coastal swamp deposits. The sedimentary facies occur in a vertical sequence where two slightly different facies associations could be discerned. It is suggested that both facies associations have been deposited in a barrier—lagoon coastal area characterized by minor shoreline oscillations. The palaeogeographic setting of the investigated sequence in the fault-controlled basins along the east Greenland continental margin is discussed.

Anatomy of an Orogen

Mapping a strip of 15′ quadrangles across the northern Coast Ranges of California by student colleagues and myself has resulted in the development of a model for the origin and emplacement of the Franciscan complex and its relation to the coeval Great Valley sequence that differs from earlier models. Rocks of both Franciscan and Great Valley are predominantly graywacke, silty mudstone, and local conglomerate. Most of the Great Valley rocks are graded, and bottom markings are common; these features are present but not characteristic of the Franciscan rocks. Franciscan graywacke and silty mudstone tend to be broken and sheared, or chaotically deformed. The chaotic zones typically contain exotic blocks, mostly of ophiolitic rocks, but also blue-schist, mica schist, metagraywacke, and blocks similar to the enclosing sheared matrix. These chaotic units are best characterized as melanges. Within the Franciscan complex, mappable units have been defined, based on lithology of matrix and content of exotic blocks. The units dip and face easterly and are oldest to the east (Tithonian or older for the South Fork Mountain Schist). Progressively younger rocks occur down section to the west; the youngest are the Coastal Facies largely of Eocene age. The probable origin involves deposition of Franciscan sediments in a trench, followed by thrusting of the sediments against and under the landward side of the trench, while the axis of the trench migrated progressively westward. The process began in Tithonian time, slowed during Late Cretaceous, and ceased in the Eocene. Models are suggested for the formation of melanges during sedimentation and underthrusting. Each melange unit contains a unique assemblage of exotic blocks. It is suggested that these assemblages in part reflect the lithology of the basement rocks on which the sediments were deposited. If true, the composition of the basement varied from dominantly ophiolite-suite rocks to dominantly blueschist. A melange of ophiolite blocks also formed, the basement beneath a part of the Great Valley sediments. Samples of radiolarian chert from the ophiolite suite throughout the Franciscan were determined to be of Tithonian age by Emile Pessagno. If the ophiolite represents ocean floor, formed at a spreading ridge, the ridge must have been located adjacent to the trench in Tithonian time; after the Tithonian, no oceanic rocks were added to the Franciscan pile. A diapiric origin for the Tithonian ophiolite is suggested. Northwest-trending isolated slabs of Great Valley—type rocks and of shallow-water to continental coal-bearing clastic rocks of Cretaceous to Miocene age are scattered within the Franciscan complex. Evidence is presented that these were deposited in separate, probably fault-controlled basins, developed on the surface of the tectonically active Franciscan complex during and following underthrusting. They do not appear to represent a klippen of a Coast Range thrust fault, as has been suggested by others.