Kristian Soegaard

Detailed internal architecture of a fluvial channel sandstone determined from outcrop, cores, and 3-D ground-penetrating radar: Example from the middle Cretaceous Ferron Sandstone, east-central Utah

Ideally, characterization of hydrocarbon reservoirs requires information about heterogeneity at a submeter scale in three dimensions. Detailed geologic information and permeability data from surface and cliff face outcrops and boreholes in the alluvial part of the Ferron Sandstone are integrated here with three-dimensional (3-D) ground-penetrating radar (GPR) data for analysis of a near-surface sandstone reservoir analog in fluvial channel deposits. The GPR survey covers a volume with a surface area of 40 x 16.5 m and a depth of 12 m. Five architectural elements are identified and described in outcrop and well cores, using a sixfold hierarchy of bounding surfaces. Internally, the lower four units consist of fine-grained, parallel-laminated sandstone, and the upper unit consists of medium-grained, trough cross-bedded sandstone. The same sedimentary architectural elements and associated bounding surfaces are distinguished in the GPR data by making use of principles developed in seismic stratigraphic analysis. To facilitate comparison of geologic features in the depth domain and radar reflectors in the time domain, the radar data are depth migrated. The GPR interpretation is carried out mainly on migrated 100 MHz data with a vertical resolution of about 0.5 m. Measures of the spatial continuity and variation of the first- and second-order bounding surfaces are obtained by computing 3-D experimental variograms for each architectural element (each radar (Begin page 1584) facies). The maximum correlation length of the dominant internal features ranges between 4 and 6 m, and the anisotropy factor ranges between 0.6 and 0.95.

Evidence of Tide, Storm, and Wave Interaction on a Precambrian Siliciclastic Shelf: The 1,700 M.Y. Ortega Group, New Mexico

ABSTRACT The 1,700 m.y. Ortega Group in northern New Mexico is a sequence of quartz arenites and subordinate mudstones in excess of 1,000 m. Sedimentation took place in diverse shallow-shelf environments under the influence of tidal, wave, and storm processes. Time-velocity asymmetry of tidal flow on the inner shelf produced large-scale trough cross beds and tabular-planar cross beds with multiple reactivation surfaces. In contrast, tabular cosets structured internally by small-scale trough cross beds and herringbone cross beds resulted from symmetrical tidal flow on the inner shelf. Fair-weather waves reworked the upper surface of tidal sand bodies on the inner shelf during quiet periods within the tidal rhythm. During storms, sand was entrained into the water column on the proximal inner she f leaving winnowed pebble lags at the top of tidal sand bodies. Sand transported by storm-induced gradient currents was deposited on the outer shelf as discrete lobes. Progradation of sand lobes produced 1-11-m-thick genetic packages which display an upward thickening of sandstone beds and are capped by cosets of tabular, planar-laminated sandstones. Storm sands were reworked by wave processes on the proximal outer shelf, whereas storm-deposited sands on the distal outer shelf remained unmodified. No evidence of tidal processes exists on the outer shelf. Distribution of facies and paleocurrent patterns indicates that the shelf sloped to the south and experienced an overall transgression which culminated in the drowning of the outer shelf with onlap of black, basinal muds. Absence of shallow-water turbidites below storm wave base reflects low gradients on the shelf. Vertical transition from tide- to storm-dominated sedimentation in the Ortega Group does not represent a change in depositional style with time on the shelf. Rather, all facies coexisted across the shelf, with tidal processes prevailing on the shallow inner shelf and storm processes on the outer shelf. The German Bight in the North Sea and parts of the Bering Sea off Alaska are Holocene counterparts for the depositional framework envisaged for the Ortega Group; in both, pro imal, tide-dominated environments pass distally into deeper-water, storm-dominated environments.

Late Middle Proterozoic Hazel Formation near Van Horn, Trans-Pecos Texas: Evidence for transpressive deformation in Grenvillian basement

The latest Middle Proterozoic Hazel Formation near Van Horn in the Trans-Pecos region of west Texas is an orogenic elastic wedge that provides insight into paleotectonic evolution of the southern margin of the Laurentian craton. The Hazel Formation is >2.5 km thick adjacent to the contemporaneous Streeruwitz thrust fault and consists exclusively of terrestrial sedimentary rocks. Boulder-sized clasts in conglomerate were deposited within alluvial fans north of the Streeruwitz thrust sheet, 0along the southern margin of a basin of unknown geometry and extent. The alluvial-fan conglomerate interfingers with eolian sedimentary rocks in the northern part of the basin. Alluvial-fan conglomerate megasequences indicate that at least two episodes of faulting accompanied Hazel sedimentation. Alluvial megasequences are overall aggradational within the Hazel succession as revealed by clast size variations within the conglomerate. Systematic variation of pebble composition up through the Hazel conglomerate provides an erosional history within the source terrain. Carbonate and subordinate mafic volcanic rocks, derived from the pre orogenic Allamoore and Tumbledown Formations, were the principal source off the Hazel Formation. Metavolcanic and metasedimentary rocks from the Carrizo Mountain Group in the present tectonic hinterland are notably absent in the Hazel conglomerate. Moreover, a rhyolite terrain, presently not exposed in the Van Horn area, was a significant component of the Hazel source. Juxtaposition between source terrain and sedimentary basin, coupled with aggradation of alluvial-fan megasequences along the southern fault-bounded basin margin, supports a transpressive regime for this orogenic belt rather than a purely convergent margin. Deformation associated with the Hazel orogenic sequence post-dated a 1,126- to 1,070-m.y.-old regional felsic igneous event; Grenville deformation in the Llano uplift of central Texas predated the same igneous event. This implies that the Hazel tectonic episode was a second, later Grenvillian deformation. The terms Llano orogeny and Hazel orogeny are introduced here for the Grenville province in Texas and may be comparable with the Elzevirian and Ottawan orogenies within the Grenville province of Canada.

Fan-delta and braid-delta systems in Pennsylvanian Sandia Formation, Taos Trough, northern New Mexico: Depositional and tectonic implications

Fan-delta and braid-delta deposits are distinguished in the Pennsylvanian Sandia Formation, Taos Trough, New Mexico, according to guidelines recently defined by McPherson and others. Subaqueous sedimentation styles and depositional sequences also are used for distinguishing fan-delta and braid-delta deposits in the Taos Trough. Fan deltas were deposited in close proximity to the active western boundary fault of the Taos Trough and occur as coarse-grained aggradational sequences. Alluvial processes in fan-delta complexes were characterized by unrestricted sheetflood and ephemeral streamflow deposition. Subaqueously deposited, fan-delta conglomerates and sandstones arranged in lenticular units encased by basinal mudstones were emplaced by a variety of high-density and dilute gravity-flow processes. Braid-delta complexes were characterized by channelized alluvial deposition in which discharge was variable, but perennial. Perennial discharge in braid deltas was in response to expansion of the drainage area in the Taos Trough hinterland through time. This resulted in increased sediment discharge into the trough and enabled basinward extension of the alluvial plain by braid-delta progradation. This study concludes that fan deltas are associated with steep basin margins in which overall subsidence rates exceed sedimentation rates. Braid deltas are related to low-gradient basin margins along which sedimentation rates are greater than subsidence rates. Distribution of fan-delta and braid-delta complexes in the Sandia Formation is dictated by tectonic load-induced subsidence in the Taos Trough and may be used as a guide for resolving evolutionary history of other types of active sedimentary basins.

Origin of thick, first-cycle quartz arenite successions: Evidence from the 1.7 Ga Ortega Group, northern New Mexico

Abstract The 1700-Ma-old Ortega Group in northern New Mexico represents a > 1000-m-thick succession of metamorphosed quartz arenites and subordinate pelites. Published ages of detrital zircon grains from the Ortega Group indicate that the sediments were derived mainly from 1760-1700-Ma-old volcano-plutonic basement rocks and not from the Archean Wyoming Province to the north. Evidence for recycling within the Ortega Group also is lacking. Consequently, the Ortega Group quartz arenites are considered to be first-cycle sediments. The compositional and textural maturity of these sandstones is attributed to intense chemical and mechanical weathering. Abnormally high Al contents of pelites within the Ortega Group imply intense chemical weathering in the provenance similar to present-day low-relief tropical regions. Quartz arenites in the Ortega Group were deposited exclusively within a shallow-marine shelf environment influenced by tide, storm and wave processes. Prolonged sediment transport, especially on the tide-dominated inner shelf, in conjunction with inferred gradual subsidence optimized mechanical destruction of labile grains and rounding of quartz grains. Although the maturity of sediments in the Ortega Group can be accounted for, the mode of origin of > 850 m of quartz arenites remains enigmatic. Thick quartz arenite sequences of global distribution in the Phanerozoic coincide with first-order eustatic sea-level rises. World-wide occurrences of 1700-Ma-old, thick quartz arenite shelf successions suggest that the Ortega Group may have accumulated in response to a similar major eustatic sea-level rise.

Transition from Arc Volcanism to Stable-Shelf and Subsequent Convergent-Margin Sedimentation in Northern New Mexico from 1.76 Ga

The Precambrian of northern New Mexico is part of an extensive 1,800 to 1,500 m.y.-old terrane extending from Colorado through northern New Mexico into central Arizona. Three lithostratigraphic sequences are present in New Mexico. The oldest consists of 1,760 to 1,720 m.y.-old metamorphosed bimodal volcanic and volcaniclastic rocks to which no basement has been recognized. This juvenile crust developed as a magmatic arc complex and represents an early period of crustal instability. Between 1,740 and 1,700 m.y., the volcanogenic sequence was intruded by voluminous, coeval granodiorites and tonalites which progressively stabilized the early crust. Unconformably overlying the volcano-plutonic terrane is a thick sequence of metamorphosed quartz arenites and subordinate argillites, the Ortega Group, which accumulated on a sTable continental shelf. The inner shelf was dominated by tidal processes with subordinate reworking by storm and fair-weather waves. Storm processes were responsible for sand deposition on the outer shelf. The Ortega shelf sloped gently to the southeast and experienced an overall transgression which culminated in drowning of the outer shelf with onlap of black basinal muds from the south. STable-shelf sedimentation resulted from prolonged thermal subsidence following cratonization of the juvenile volcanogenic crust by intrusion of granitoid plutons. The third lithostratigraphic sequence, the Marquenas Quartzite, consists of polymictic metaconglomerates and texturally-immature metasandstones deposited in a braided-alluvial environment. These terrigenous sediments were supplied from a general southerly direction, and pebble compositions indicate derivation from the underlying volcanogenic and shelf sequences. The Marquenas Quartzite signifies cannibalization of the underlying magmatic arc and shelf succession in response to deformation of the craton margin to the south. This margin defined the southern limit of the proto-North American craton, which extended from central Arizona to southern Wisconsin at ca. 1,700 m.y. ago and was destroyed between 1,660 and 1,630 m.y. ago.

Anatomy of the Grenville Orogen in West Texas

Exposures of Precambrian basement rocks near Van Horn, Texas provide an important transect across the Grenville age deformational front. To the south, metamorphic rocks of the Carizzo Mountain Group (CMG) are interpreted to be an allocthonous package of ∼1350 Ma rift-related rocks. The CMG has been subdivided into three structural domains. In the northwest portion of the mountains a large east-west trending, reclined synform characterizes the hanging wall of the Streeruwitz thrust. The southern limb of this synform has been truncated by the Mineral Creek fault zone (MCfz), a 350 meter thick imbricate package characterized by mylonitized rhyolite. The central domain is in the hangingwall of MCfz and contains large (5 km wavelength) tight, inclined folds with subhorizontal axes. These folds are cut by several northerly directed thrust faults. In the southeast domain, southerly trending structures suggest a zone of refolding plunging beneath the Paleozoic cover. The CMG is translated towards the NNW on the Streeruwitz thrust. In the foreland, sedimentary rocks of the AUamoore Fm. record shallow marine carbonate deposition associated with (andesitic) volcanism of 1250 Ma. Initial deformation is documented in the Tumbledown Fm, a volcaniclastic package containing megabreccias of the Allamoore Fm. This initial deformation is interpreted to reflect a transtensional setting between 1250-1100 Ma. Deformation within the Tumbledown Fm. includes penetrative cleavage and complex conical folds. Post-dating these events is the development of a large alluvial fan comprised of coarse conglomerate and siltstone, the Hazel Fm. The source of these sediments are the Allamoore and Tumbledown Fm. and a granitic/rhyolite basement of ∼1100 Ma, not the CMG. This observation suggests continued strike slip deformation and basement uplift in front of the advancing Streeruwitz thrust. The Hazel alluvial fan is deformed into large kink folds which are subsequently overridden by complex thrust sheets. The thrust sheets are folded and klippe of CMG are emplaced during this final stage of deformation. Recurrent motion has occurred along many of the basement faults in the foreland area. Isotopic data suggests two distinct stages of deformation, an early stage ∼1250 Ma and a younger stage between 1100-1000 Ma. Between these two events is a regional igneous event of ∼1100 Ma, possibly coincident with rifting in the midcontinent.

Architectural Elements of Fan-Delta Complex in Pennsylvanian Taos Trough, New Mexico: ABSTRACT

Identification of architectural elements within alluvial-fan and subaqueous fan-delta gravel units is fundamental to resolving depositional processes within fan-delta complexes of the Pennsylvanian Taos trough, New Mexico. Subaqueous fan-delta deposits consist of lenticular gravel-body complexes encased by black, basinal shales. Gravel-body complexes are composed of a series of stacked gravel lenses, each of which is enveloped by fifth-order bounding surfaces. The central portion of individual gravel lenses contains a channel complex. Channels are outlined by third- and fourth-order bounding surfaces and are infilled by high-density gravity flow deposits. The fringe of submarine gravel lenses consists of stacked, laterally continuous Bouma sequences separated by second-order bounding surfaces. Bouma sequences were deposited by dilute turbidity flows during evacuation of submarine channels. Subaqueous channel complexes within gravel lenses represent midfan channels, whereas the fringe of lenticular gravel lenses represent outer-fan lobes. Recognition of depositional processes and architectural elements of fan deltas in the Sandia Formation enables distinction between these and other types of coarse-grained deltas in the Taos trough. This, in turn, has implications for resolving evolution of the trough.

Storm-Deposited Outer Shelf Facies from Precambrian Ortega Group, New Mexico: ABSTRACT

The 1,700 million year old Ortega Group in northern New Mexico accumulated in diverse shallow shelf environments under the influence of tidal, wave, and storm processes. Tidal and fair-weather wave reworking dominated the inner shelf but a significant storm overprint is indicated by offshore-directed trough cross-stratification, and winnowed lags and scour channels at the top of tabular units. Storm-surge currents supplied sand to the outer, mud dominated shelf where deposition occurred predominantly under flat-bed conditions. Amalgamated, upward-thickening depositional units of horizontally stratified sandstone comprise 1 to 7 m (3 to 23 ft) thick genetic packages. Based on their position in the progradational shelf sequence, these sandstones are inferred to have accumul ted in proximal reaches of the outer shelf. The upper parts of individual 2 to 25 cm (.78 to 9.8 in.) thick depositional units are commonly defined by inter-laminated siltstone and mudstone, and the thinner basal sandstones frequently have wave-rippled tops. Scour channels are often present at the top of the sandstone packages. The sandstone:mudstone ratio decreases outward on the shelf with discrete, 2 to 5 cm (.78 to 1.9 in.) thick, horizontally-stratified sandstone beds and rare hummocky cross-stratified beds passing distally into mm-thick horizontally stratified sandstones. Associated lenticular sandstones are exclusively wave rippled. The preponderance of horizontal stratification in outer shelf sandstones coupled with the resemblance of individual depositional units to b-d turbidit beds suggests suspension fallout under conditions of high but waning bed shear. Such conditions may have been related to unidirectional storm surge currents or oscillatory storm waves; the paucity of hummocky cross-stratification may favor the former process. Wave-rippled sandstones developed through fair-weather reworking of the storm-deposited sandstones. In the absence of bioturbation, Precambrian shelf sequences provide an excellent opportunity for studying outer shelf depositional facies and processes. End_of_Article - Last_Page 456------------