John J. Miller

Seismic stratigraphy and tectonic development of Virgin River depression and associated basins, southeastern Nevada and northwestern Arizona

Virgin River depression is a Neogene basin, with a surface area that exceeds 1,500 km 2 in the Basin and Range structural province of south-eastern Nevada and northwestern Arizona. The depression formed within the foreland of the Sevier orogenic zone, a region that was characterized in Paleogene time by a flat-lying section of Cambrian to Cretaceous platform strata about 5 km thick. Well data from Mobil Virgin 1A on Mormon Mesa reveal 2,000+ m of Neogene basin fill that consists mostly of the Muddy Creek Formation (4-10 Ma), the red sandstone unit (10-12 Ma) of Bohannon (1984), and the Lovell Wash Member of the Horse Spring Formation (12-13 Ma). Seismic reflection data from six primacord and two vibroseis lines show that the Muddy Creek Formation uniformly fills Virgin River depression to a depth of 1-2 km. Two older and less-extensive basins, the Mormon and Mesquite, lie beneath the Muddy Creek and are separated from one another by a complex buried ridge. The basins are mostly filled with rocks of the red sandstone unit and Lovell Wash Member to depths locally exceeding 6 km. Two older members of the Horse Spring Formation, the Rainbow Gardens and Thumb, also occur in Mormon basin (the western one), where they rest disconformably on the pre-Tertiary strata. The basins are east-tilted half grabens that are bounded on the east and southeast by large listric normal fault systems. The faults that bound Mormon basin are buried by the Muddy Creek Formation, but the Piedmont fault, on the east side of Mesquite basin, cuts Quaternary alluvium. The Virgin River depression formed in three stages. The period from 24-13 Ma is characterized by slow subsidence in Mormon basin and little noticeable deformation of the basin substrate. The Mormon and Mesquite basins became fully differentiated during the period from 13-10 Ma. This stage is associated with large displacements on the normal faults bounding both basins and the buried ridge. Proterozoic crystalline rocks were exposed locally, providing a source for part of the red sandstone unit deposited in the basins. Tectonic denudation during the 13-10 Ma stage locally removed large amounts of the pre-basin section. By 10 Ma, most of the fault activity had ceased, the ridge between the basins was overlapped, and Virgin River depression began to subside uniformly over a wide area. This stage lasted until the commencement of the modern period of dissection associated with the Colorado River. Our structural analysis suggests that upper crustal extension within the basin, mostly during the 13-10 Ma stage, might have exceeded 60%. The basin subsidence was partly due to extension in the upper crust and partly due to viscous flow in the deeper crust beneath the basin. It is not clear to us what caused the uplifts that flank the depression, but isostatic rebound due to tectonic denudation remains aviable possibility.

Subducting plate geology in three great earthquake ruptures of the western Alaska margin, Kodiak to Unimak

Three destructive earthquakes along the Alaska subduction zone sourced transoceanic tsunamis during the past 70 years. Since it is reasoned that past rupture areas might again source tsunamis in the future, we studied potential asperities and barriers in the subduction zone by examining Quaternary Gulf of Alaska plate history, geophysical data, and morphology. We relate the aftershock areas to subducting lower plate relief and dissimilar materials in the seismogenic zone in the 1964 Kodiak and adjacent 1938 Semidi Islands earthquake segments. In the 1946 Unimak earthquake segment, the exposed lower plate seafloor lacks major relief that might organize great earthquake rupture. However, the upper plate contains a deep transverse-trending basin and basement ridges associated with the Eocene continental Alaska convergent margin transition to the Aleutian island arc. These upper plate features are sufficiently large to have affected rupture propagation. In addition, massive slope failure in the Unimak area may explain the local 42-m-high 1946 tsunami runup. Although Quaternary geologic and tectonic processes included accretion to form a frontal prism, the study of seismic images, samples, and continental slope physiography shows a previous history of tectonic erosion. Implied asperities and barriers in the seismogenic zone could organize future great earthquake rupture.

The destructive 1946 Unimak near‐field tsunami: New evidence for a submarine slide source from reprocessed marine geophysical data

The Mw 8.6 earthquake in 1946 off the Pacific shore of Unimak Island at the end of the Alaska Peninsula generated a far-field tsunami that crossed the Pacific to Antarctica. Its tsunami magnitude, 9.3, is comparable to the 9.1 magnitude of the 2011 Tohoku tsunami. On Unimak Islands Pacific shore, a runup of 42u2009m destroyed the lighthouse at Scotch Cap. Elsewhere, localized tsunamis with such high runups have been interpreted as caused by large submarine landslides. However, previous to this study, no landslide large enough to generate this runup was found in the area that is limited by the time interval between earthquake shaking and tsunami inundation at Scotch Cap. Reworking of a seismic reflection transect and colocated multibeam bathymetric surveys reveal a landslide block that may explain the 1946 high runup. It is seaward of Scotch Cap on the midslope terrace and within the time-limited area.

Seismic reflection and gravity study of proposed Taconic suture under the New Jersey Coastal Plain: Implications for continental growth

A vibroseis seismic reflection line near Buena, New Jersey, crosses the linear positive Salisbury Bouguer gravity and magnetic anomaly that follows the proposed Taconic suture. Two reflections at 1.4 and 1.7 sec under the positive gravity anomaly and beneath the Coastal Plain basement reflections at 1.1 sec are from interpreted Taconic suture rocks with ophiolitic crustal fragments (7.1-7.9 km/sec). A gently southeast-dipping reflection (2.0-2.7 sec) is interpreted as the detachment thrust below the allochthonous suture rocks. A stratified sequence (5.4-6.3 km/sec) is interpreted as remnants of parautochthonous lower Paleozoic drift-stage sedimentary rocks beneath the detachment thrust. Below 3.0 sec (8 km depth), the reflections are subhorizontal or gently dipping (11°) all the way to the Moho reflection at 11.0 sec. On the northwest end of the vibroseis line, a northwest-tilted Mesozoic rift-basin basement reflection is seen from 1.1 to 1.5 sec. These Mesozoic rocks (4.5-4.8 km/sec) are at least 1 km thick. The denser suture rocks with ophiolitic fragments and the less dense Mesozoic rocks explain almost all of the Salisbury Bouguer anomaly. No significant lateral density variations are needed below 8 km depth. These results are compatible with the Taconic suture under the New Jersey Coastal Plain being allochthonous and underlain by parautochthonous Grenville-age continental crust. A deep-seated Taconic suture at the edge of the Grenville North American basement may be southeast of the New Jersey Coastal Plain. This implies a greater amount of Taconic overthrusting and lesser amounts of accretion of the North American crust than previously proposed.

Multiple phases of basin formation along the Stateline fault system in the Pahrump and Mesquite Valleys, Nevada and California

Two phases of deformation are needed to describe the Cenozoic tectonic evolution of the Pahrump and Mesquite basins in the southern Great Basin and eastern Mojave Desert, United States. By interpreting seismic reflection and gravity observations along with bedrock and surficial mapping, we infer an extensional phase of basin formation followed by a transtensional phase, in this area straddling the border of southern Nevada and southeastern California. We reprocessed ∼220 line km of industry seismic reflection data from the Pahrump and Mesquite Valleys to emphasize reflections in the basin fill, and combined these results with analysis of gravity data. The seismic lines portray the complex geometry of the Stateline fault system, a major Neogene dextral strike-slip system that passes through these valleys, and provide evidence for multiple ages of faulting along structures that bound the Pahrump basin. Locally thick sequences of preextensional Tertiary sedimentary rocks are cut by large-offset, relatively high-angle normal faults that record a phase of extensional basin formation that preceded transtension. The existence of preextensional basins beneath the Pahrump and Mesquite Valleys bears on tectonic reconstruction of the region and suggests that tilted ranges blocks to the west of these valleys need not restore to positions immediately adjacent to the Spring Mountains to the east. Subsequent dextral offset on the Stateline fault system resulted in the formation of steep-sided basins, local arching and tectonic inversion, and the burial of earlier-formed normal faults with coarse clastic detritus. Gravity models that are constrained to match the basin architecture observed in the seismic lines require lateral variations in basin-fill and bedrock density, and they confirm that the Paleozoic outcrop of Black Butte, a topographic high separating the Pahrump and Mesquite Valleys, is unrooted to underlying bedrock.

Integrated Geologic and Geophysical Assessment of the Eileen Gas Hydrate Accumulation, North Slope, Alaska

Using detailed analysis and interpretation of 2-D and 3-D seismic data, along with modeling and correlation of specially processed log data, a viable methodology has been developed for identifying sub-permafrost gas hydrate prospects within the Gas Hydrate Stability Zone (HSZ) and associated sub-hydrate free gas prospects in the Milne Point area of northern Alaska (Figure 1). The seismic data, in conjunction with modeling results from a related study, was used to characterize the conditions under which gas hydrate prospects can be delineated using conventional seismic data, and to analyze reservoir fluid properties. Monte Carlo style gas hydrate volumetric estimates using Crystal Ball{trademark} software to estimate expected in-place reserves shows that the identified prospects have considerable potential as gas resources. Future exploratory drilling in the Milne Point area should provide answers about the producibility of these shallow gas hydrates.

Interpreted Reflection Seismic Events Near the North Central Corporation Well, Newark Basin, Bucks County, Pennsylvania

dril led a 10,500-ft well in Bucks County, Pennsylvania, to test for oil and gas in TriassicJurassic strata of the Newark Group in the Newark basin. The hole was located about 5 mi southeast of the border fault on the crest of the Revere anticline, a transverse structure that plunges northwestward to-ward the border fault on the crest of the Revere anticline, a transverse structure that plunges northwestward toward the border fault. This drill hole, No. 1 Cabot-KBI, penetrated in descending order 2900 ft of lacustrine red and gray shale (Passaic Formation of Jurassic and Triassic age), 3600 ft of Lacustrine gray and black shale (Lockatong Formation of Triassic age), and 4000 ft of fluvial sandstone and red shale (Stockton Formation of Triassic age). Although the drill hole was abandoned, it revealed excellent gas shows throughout the Lockatong Formation and parts of the Stockton Formation. Acoustic and density logs from the well were converted to a synthetic seismogram that ties stratigraphic intervals in the borehole with seismic events on the nearby, 33-mi-long Seitel ND-l seismic line. Formation contacts, probable intraformational unconformities and lithologic units such as 50-ft-thick fluvial sandstones in the Stockton Formation are identified and traced for tens of miles away from the well. Moreover, beneath the drill site in the Stockton Formation, the 48-fold seismic line shows a large anticline that is subparallel to the border fault. This anticline has no surface expression; Interpreted reflection seismic events near the North Central Corporation Well, Newark Basin, Bucks County, Pennsylvania

Modeling Seismic Reflection Data in the Vicinity of the Békés-2 Well

In the vicinity of the Bekes-2 well, located near the center of the basin, seismic reflection data show a complex geologic structure in the Mesozoic-age basement rocks below a depth of 3400 m. To assist in the interpretation of the geometry of the structure, three nearby seismic profiles were reprocessed, vertical seismic profiles (VSPs) were recorded in the well, and a new surface seismic line was recorded concurrently with one of the VSPs. Interpretations of the surface seismic data were integrated with those of the VSP data and a structural model of the Mesozoic-age rocks was developed. Two-dimensional seismic modeling aided in modifying the interpretation and the synthetic seismic response of the final interpretation gave a reasonable match to the observed data. This final interpretation is a good approximation of the geometric relationships between seismic reflection boundaries in the Mesozoic-age rocks in the area of the Bekes-2 well site.