A 5-km gap in ground rupture and a decrease in aftershock epicenter density associated with the 1992 M 7.3 Landers earthquake occurs at the intersection of the Johnson Valley and Pinto Mountain faults, near the town of Yucca Valley, California. Field mapping reveals a family of curvilinear faults occupies this region, undisturbed by the Landers seismic event. Wholly contained within Township Section 25, this family of faults strike N20°W at the northern border of the Section and rotates eastward, striking N70oW where last seen at the eastern boundary, with dips ranging between 60° to 80° NE. Where relative motion on the Johnson Valley fault is predominately dextral strike-slip, field evidence supports the curvilinear faulting to contain a component of high angle dip-slip. This study concludes the curvilinear nature of the faulting is the product of warping within the regional rock stresses due to the normal intersection of the Pinto Mountain and Johnson Valley faults. Each fault influences the other, creating a distortion in their respective stress fields subsequently followed by the curvilinear faults.
Field findings suggest the lack of ground rupture and seismic activity within this sector of the Lander earthquake resulted, at least in part, from the presences of the curvilinear and the Pinto Mountain faults. Seismic inactivity to a depth of 3 km beneath the surface trace of the curvilinear faults may be attributed to the oblique orientation and opposing dips of the curvilinear fault planes relative to the north-south aligned stress field of the Landers earthquake. As a result, the curvilinear faults resisted slip that occurred elsewhere during the Landers event. At depths greater than 3 km, seismic activity truncates in the subsurface against a plane plunging 77° N that originates at the surface trace of the Pinto Mountain fault. Field mapping also discovered high-angle reverse thrust faulting (named the Sawtooth fault) along the central axis of the Sawtooth Range to the west of the Section 25. These data are consistent with the Pinto Mountain fault under-thrusting the Mojave Block and uplifting this eastern extension of the San Bernardino Mountains. Field relationships also suggest this occur coincident with the activation of the San Andreas fault and rotation of the East Transverse Range province, thus suggesting a late Neogene to Holocene age of faulting.
Due to the under-thrusting of the Sawtooth range by the Pinto Mountain fault, dip-slip took local precedent over strike-slip separation that was otherwise associated with the Lander event. Consequently, the ground rupture gap represents a localized tectonically locked region. Because of this, the area between the Sawtooth Range and the Johnson Valley fault may be experiencing extension and pull apart. This is supported by a northeast-southwest strike in the fault trace orientation exhibited by the southern-most surface expressions in the Johnson Valley fault.
Presentation Date: Wednesday, October 17, 2018
Start Time: 8:30:00 AM
Location: 211A (Anaheim Convention Center)
Presentation Type: Oral
In this research, high-resolution seismic surveys were carried out by the Allied Geophysical Laboratories at University of Houston to study the near-surface geology and geological evolution of Galveston Bay estuary system. Chirp and Boomer seismic sources were used to image the shallow sediments of Galveston Bay.
Presentation Date: Wednesday, September 27, 2017
Start Time: 10:10 AM
Presentation Type: ORAL
Large offshore Windfarm developments require significant investment decisions at early stages (pre-funding investment decision, so called pre-FID stage), when in many cases geotechnical information at the majority of the proposed turbine positions is not available or has been performed to a limited depth. The generation of a ground model integrating desk study (geology, seabed mobility, depositional environment, formation variability, etc.), geophysical and advanced borehole data is therefore key in the evaluation of future survey strategies, foundation concepts, construction risks and cost estimates of foundation works for final investment decisions.
Current geotechnical design practice is generally based on probabilistic assessments, factors of safety and derivation of design geotechnical profiles from borehole data at the location of the structure. Little guidance exists with regards to the preparation of an advanced geotechnical investigation report that also evaluates the anticipated ground profiles and geotechnical risks at locations where partial, or no, borehole data exists.
This paper puts forward a recommended process, based on a thorough understanding of the geology and the integration of geological models, geotechnical, geophysical data and application of geostatistical methods to report the anticipated ground profiles and uncertainty at positions where only partial borehole information exists. The process is then illustrated with several examples of windfarm cases.
The world of offshore wind power is under significant pressure to reduce costs, while competing with other energy sources such as fossil fuel extraction. Developers of offshore wind are under increasing pressure to define the returns on their investments in order to secure funding pre-FID. This process requires the careful planning and implementation of early ground investigation surveys to determine the ground conditions of the future turbine and substation foundations and cables.
Li, Yuting (Louisiana State University) | Clift, Peter (Louisiana State University) | Boening, Philipp (University of Oldenburg) | Guilderson, Tom (Lawrence Livermore National Laboratory) | Giosan, Liviu (Woods Hole Oceanographic Institution)
Classic sequence stratigraphic models argue that submarine canyons and their associated deep-sea fans should become inactive during periods of rising and high sealevel as accommodation space is generated on the continental shelf. Initial data from the upper Indus Submarine Fan had suggested that this system largely follows this model, as turbidite sedimentation ceased around 11 ka. New cores from the canyon now show that the situation is more complex, with sediment propagating deep through the shelf canyon during the entire Holocene. Sediment accumulation is known to be very rapid in recent times at the head of the canyon, but new 14C ages from foraminifera show that sandy sedimentation was ongoing in what is now an ox-bow cut-off ~7 ka, while terraces up to 200 m above the thalweg have been blanketed by fine-grained sediments and sands since 9 ka. A core in the mid shelf canyon shows that sedimentation there has been rapid since at least 1000 yrs ago, and may have involved significant recycling, possibly from the depocenter at the canyon head. Nd and Sr isotopes now allow us to see that sediment in the canyon is of the same composition as that in the river mouth at the time of sedimentation. This raises the possibility that the river was supplying sediment to the canyon since at least 5 ka, shortly after eustatic sealevel stopped rising. This contrasts with the western shelf clinoform where sediment recycling from glacial age shelf sediment and long-shore current flow is important. Our data indicate that despite sealevel rise sediment supply to the canyon was not cut-off during the deglaciation, although the volume of the flux was reduced. We evaluate the roles played by sea level variations, sediment supply, and cyclones in feeding sediment into the canyon and assess the continuity of sandy channel fills. We suggest that enhanced sediment supply, driven by strong monsoon rains onshore compensated for the rising sealevel and allowed the connection between river and canyon to be maintained.
New high resolution sparker seismic data acquired in Prince William Sound (PWS) and the adjacent Gulf of Alaska provide insights into the post-glacial depositional history of the coastal areas of southern Alaska. Our data suggest Holocene strata lie directly upon Tertiary bedrock below most coastal waterways, the result of active tectonics and extensive glaciation that has uplifted and scoured pre-Holocene strata. We identify two regional unconformities within PWS that we presume to represent the onset of post “last glacial maxima” deposition approximately 15 ka and a neoglacial period estimated at 3.5 ka. Reflector stratification and the general decrease in sediment volume away from Hinchinbrook Entrance suggest prodelta sediments derived from the Copper River dominates late Holocene deposition within eastern PWS. This observation is consistent with the Alaska coastal currents carrying sediment into PWS from the eastern Gulf of Alaska through the Hinchinbrook Entrance. The only outlet for modern sediment back to the Gulf of Alaska from within PWS is through Montague Strait. Below the neoglacial unconformity and above Tertiary bedrock, the distribution and presence of poorly stratified sediments suggest Hinchinbrook Entrance did not supply sediment to PWS during early to middle Holocene times and that sediments deposited within PWS were derived from local sources. Bathymetric data point to sea floor bedrock, active faults, and glacial structures. Subbottom reflector truncations suggest subduction related active reverse faults are abundant throughout southern Alaska. The focus of uplift from both the 1964 M9.2 earthquake and prior great earthquakes is along the western margin of PWS and adjacent Gulf of Alaska.
The study area is located within the Mad Dog Field development area in the northern Gulf of Mexico. Several previous papers have been published reporting results from integrated studies carried out in the area. Of particular interest is a study that identified fan sands behind and in front of the escarpment using coherence and seismic amplitudes. The submitted paper focuses on integrating such seismic attributes and age dates to infer the depositional history of key deposystems.
The detailed architectural and facies analysis of the deposystems were interpreted using 3D high resolution seismic imaging and seismic attribute analysis. An integration of the seismic facies analysis with biostratigraphic ages was used to infer the prevalent geologic processes and thus the depositional history of the systems.
The deposystems identified in this study include a channel-lobe complex, channel-like features, and mass transport complexes (MTC).
The study revealed that the mapped depositional elements were mainly controlled by the interaction of sea level changes, sedimentation, and salt movement. Analysis of the channel-lobe complex and MTC reveal successive periods of deposition which suggest periods of high sediment supply. Age dates of bounding markers reveal that these periods coincide with low stand to rising sea level.
Other contributory factors to the depositional history within the study area have been discussed in detail in previous OTC papers. The reader should note that the age dating methods used in this study have since been refined and more data have been acquired to better understand the geological processes in the area.
The study area is located within the Mad Dog Development Area in the Gulf of Mexico. Several studies have been carried out in the area including the use of coherence and seismic amplitudes to identify fan sands behind the prominent Sigsbee Escarpment. This paper focuses on integrating seismic attributes and age dates to infer the depositional history of key deposystems landward of the Escarpment. The assessment was achieved by using data sets donated to the University of Houston by BP America, Inc.
The detailed architectural and facies analysis of the deposystems were interpreted using 3D high resolution seismic imaging and seismic attribution. Integration of the seismic facies analysis with biostratigraphic ages was used to infer the prevalent geologic processes and thus the depositional history of the systems. Deposystems identified in this study include a channel-lobe complex, channel/glide-track features, and mass transport complexes (MTCs).
The study revealed that the mapped depositional elements were mainly controlled by the interaction of sea level changes, sedimentation, and salt movement. Analysis of the channel-lobe complex and MTCs reveal successive periods of deposition which suggest periods of high sediment supply. Age dates of bounding markers reveal that these periods coincide with low stand to rising sea level. The influence of salt movement is not discussed in this paper though it was inferred from the study.
Late Pleistocene and early Holocene landscapes exposed during the last glaciation have since been inundated by rising sea levels, resulting in their submergence and often subsequent burial by sediment accretion. On the outer continental shelf (OCS) in the northwestern Gulf of Mexico (GOM) the formerly exposed landscape coincides with the presence of Paleoindian and Early Archaic human populations in the region. Unlike submerged prehistoric sites in other parts of the world the emphasis in the northwestern GOM is not on artifacts or even sites, but rather on the identification of the landscapes in which archaeological sites would have been located prior to sea-level rise. The first step in identifying and verifying these features, and any subsequent sites, consists of geophysical remote sensing, especially acoustic profiling, to identify subseafloor horizons and depositional events considered conducive for human habitation. This paper will look at current methods for identifying prehistoric sites on the GOM OCS, challenges faced in identifying prehistoric sites using the most prevalent technologies, and will briefly explore alternate technologies that are available or in development that could improve our current capabilities.
Maximizing recovery in oil and gas fields relies on geological models that realistically portray the spatial complexity, composition, and properties of reservoir units. Present day arid climate coastal systems, like the coastline of Qatar provide analogues for depositional and diagenetic processes that control reservoir quality in ancient reservoirs. Many major reservoirs in Qatar and the Middle East formed under conditions that are remarkably similar to those shaping the Qatari coastlines of today.
Major controls on coastal sedimentation patterns are: 1) coastline orientation, 2) wind, wave and tidal energy, 3) climate, 4) relative sea level, 5) depositional relief, and 6) sediment sources.
Strong NW prevailing winds (Shamal winds) drive shallow marine circulation patterns, creating four very distinct depositional profiles: windward, leeward, oblique, and protected. Windward coastlines are marked by reef development and intertidal sheet and beach sands. The leeward coast profile is dominated by an eolian sediment supply, as sand dunes are blown into the sea. Along windward and oblique coastlines, shoreface hardgrounds stabilize circulation patterns, creating mud-prone areas of stromatolites and mangroves. Protected coastlines are characterized by finer-grained peneroplid sands and low-relief beaches. Grain size, composition, and dimensions of coastal sands vary due to wave energy.
Coastal deposits are equally affected by high-frequency oscillations in sea level. Approximately 6,000 years ago, sea level was about 2 to 4 meters higher than it is currently and the Qatari coastline was up to 10km inland. Most coastal deposits and sabkhas are relicts of this ancient highstand in sea level. Punctuated sea-level drops to present day level have led to the formation of seaward-stepping beach spit systems.
Sedimentation patterns and their diagenetic overprint were studied in detail at the coastal sabkha of Mesaieed, which represents an oblique coastal system relatively to the predominant wind direction. Detailed field mapping, radiocarbon age dating analyses, and the integration of geotechnical borehole data, as well as data from numerous shallow pits allowed reconstructing the thickness of the Holocene, the dating and spatial reconstruction of the progradational pattern of the beach spits relative to the varying sea level, and the mapping of the amount and distribution of porosity destroying gypsum.
The observed spatial complexity and heterogeneity of modern coastal systems are important aspects to be considered for conditioning three-dimensional geological models. Modern depositional systems along the Qatar coastline, like the one studied at the Mesaieed sabkha, are particularly useful as analogs for conditioning subsurface data sets in geologic (static) and reservoir (dynamic) models.
The peninsula of Qatar is located approximately 25 degrees north of the equator and measures roughly 190km in north-south and 90km in east-west direction. Strong, seasonal northwesterly winds locally called Shamal winds, drive marine circulation patterns. Other factors that control sedimentation patterns in coastal areas include: relative sea level, climate, depositional relief, and sediment sources (Jameson et al., 2009; Jameson et al., 2010). Together, these factors combine to produce four distinct coastal environments: 1) windward coastline: northern coastline (Al-Ruwayis area), oblique coastline: northeastern to eastern coastline (Al-Thakhira and Mesaieed areas), leeward coastline: southeastern coastline (Khor Al-Adaid area), and protected coastline: western coastline (Bir Zekreet and Al Zareq areas). These coastal areas together with the inland sabkhas of Dukhan (east of the Dukhan anticline) and Sawda Nathil are the focus of our research (Fig. 1).
Modern sabkhas are recognized as analogues to ancient evaporitic reservoirs and as Earth analogues to Martian paleo-environments. Sabkhas are normal marine coastal sediments modified by groundwater precipitation of evaporites and carbonates. Previous work on Holocene sabkhas has focused largely on dolomitisation in carbonate-evaporite systems. Little attention has been given to understanding the origins of evaporites in mixed clastic-carbonate systems and their influence on reservoir quality. Extensive and detailed geomorphological and sedimentological characterization of depositional environments in Qatar provides a framework within which to understand processes controlling the origins of evaporites, their spatial distribution and likely evolution through time.
Mesaieed sabkha is a 4-6 km wide coastal plain which consists of an onlap wedge of Holocene sediments some 3-6 m thick reaching a maximum of 15 m, which onlaps onto Eocene bedrock. Within the sabkha, gypsum is the most abundant diagenetic mineral, reaching 20-50% of the sediment volume over several square kilometres, with minor calcite, dolomite, anhydrite and halite. Gypsum cementation is pervasive above and below the water table in the proximal sabkha, in sediments dated c.6,000 years before present (yr BP), whilst in the central part (c. 4,000 yr BP) gypsum is restricted to surface crusts and water table cements, and is largely absent in the distal (coastal) sabkha (= 2,000 yr BP).
Preliminary analysis of hydrological and geochemical data suggests evaporative pumping of groundwater from the underlying aquifer is an important source of solutes in the upper part of the sabkha, whilst seawater recharges the lower sabkha via the porous and permeable Eocene carbonates. Evaporation close to the water table results in fluids reaching gypsum saturation, and active precipitation of gypsum is evidenced by depletion of calcium and sulphate in the shallow brines. This is most marked in the middle part of the sabkha where salinity is highest. These increased density fluids reflux downwards from the Holocene, to mix within the Eocene aquifer, where reaction with the Eocene carbonates results in relative enrichment of calcium.
Many ancient sedimentary systems, particularly those deposited at low latitude, include units deposited in non-evaporitic settings but within which evaporitic minerals occlude significant volumes of porosity such as the Permian Zechstein Formation, the Permo-Triassic Khuff Formation, and the Jurassic Arab Formation. Much of the pore-filling or nodular anhydrite that is common at a wide range of burial depths may be secondary, precipitated from pore fluids rich in Ca2+ and SO42- (Kendall and Walters, 1978). Anhydrite is the product of dehydration of a gypsum precursor, which is the most abundant primary CaSO4 mineral (Warren, 2006). However, the sources of solutes forming these evaporites, flow pathways of fluids transporting these solutes and the stability of resulting evaporite precipitates has received little attention.
Studies of ancient rock seldom attempt to distinguish between CaSO4 that was precipitated as gypsum prior to dehydration and CaSO4 precipitated at depth as primary anhydrite. Hence by using a modern arid environment to develop a better understanding of the distribution of early diagenetic CaSO4, and thus contribute to reconciling potential sources of secondary anhydrite formed during burial.