Differential Compaction Research Articles

Fossil seep structures of the Monterey Bay region and tectonic/structural controls on fluid flow in an active transform margin

The subsurface plumbing system of ancient seeps of hydrocarbon-rich gases and fluids is partially preserved by seep deposits in Miocene rocks of the Coast Ranges and on the seafloor of the Monterey Bay region in central California. The fossil seep-related deposits are of two types: authigenic carbonates and intrusions of oil-saturated sandstones. Both types occur in fractured and faulted siliceous mudstone and porcelanite layers of the organic-rich Miocene Santa Cruz Mudstone. They are associated with tectonic structures indicating deformation, and are located in the proximity of faults. The morphology of the seep carbonates is characterized by strong asymmetries and by holes, pipes and other types of fluid conduits with orientations parallel to the directions of the local fracture- and fault-sets, thus suggesting structural control over pathways of fluid migration. The dominant direction of the conduits in the seep carbonates and of the intrusive dikes (northeast–southwest) differs from the dominant orientation of the tectonic structures of the San Andreas Fault System (northwest–southeast). Previous interpretations of these ancient seep-related deposits suggested that lateral changes of the thickness of the Cenozoic marine host sediments favored fluid migration from basin depocenters located offshore toward basin margins. Different driving forces for fluids have been hypothesized including hydrocarbon generation, differential compaction, and tectonic compression. Most of the fossil seeps are located on structural highs of the basement and adjacent to offshore basins that form elongated depressions oriented northwest–southeast. Previously unidentified structures that offset the basement along northeast–southwest trends (cross-setting faults) paralleling the orientations of the conduits of the seep carbonates and the dikes of oil-saturated sandstones were also recognized. Tectonic deformation, block rotations (documented by local clockwise rotations of paleomagnetic directions in the Coastal Ranges) and cross-faults were probably produced during the Late Miocene and Pliocene to accommodate the shear stress between conjugate faults during eastward shifting of the activity of the San Andreas Fault System. Comparison with previous studies on ancient seep deposits, regional geology and offshore tectonic data, suggest that the fluids that sourced the seeps were driven by episodes of inverse tectonics, during which previously extensional depositional basins were involved in episodes of tectonic compression. Tectonic deformation created cross-faults in rotated blocks, which could have acted as important conduits for the flow of hydrocarbon-rich fluids and gases and fostered the injection of liquefied sands.

Depositional record of an avulsive fluvial system controlled by peat compaction (Neogene, Most Basin, Czech Republic)

AbstractThis paper illustrates the response of a fluvial depositional system to the interplay between peat compaction and clastic sediment supply, at a range of spatial and temporal scales, as documented by extensive exposures in an open‐cast mine in the Most Basin, part of the Oligo‐Miocene Ohře Rift (Eger Graben) basin system in the Czech Republic. The Most Basin is characterized by the occurrence of a number of phenomena resulting from syn‐ and post‐depositional interactions between clastic sedimentary systems and the underlying accumulation of organic material that was the precursor of the main lignite seam of up to 45 m thickness. The studied strata are interpreted as deposits of an avulsive, mixed‐load fluvial system. The large‐scale depositional architecture documents an existence of at least five stratal packages up to 1500 m wide and up to several tens of metres thick, representing a record of long‐term evolution of a clastic floodplain bordered by accumulating peat. Within each of the packages, several small‐scale channel‐belts were documented. Individual packages are separated by carbonaceous mudstones indicating a period of reduced clastic input and interpreted as due to avulsion of the fluvial channels out of the floodplain limit. Two main, mutually linked, processes controlled the evolution of the studied fluvial system: (i) syndepositional compaction of the underlying peat and (ii) avulsions of the channels away from the original floodplain, resulting in formation of a new floodplain. The processes which caused the channels of the Hrabák fluvial system to reach the avulsion threshold were: (i) decrease of rate of creation of accommodation leading to increased sinuosity and thus to a decreased channel slope, and (ii) cross‐floodplain tilting of the channel belt caused by differential compaction of underlying organic‐rich substratum.

Quantifying the contribution of tectonics vs. differential compaction in the development of domes along the Mid‐Norwegian Atlantic margin

AbstractA sequential restoration based on combined backstripping and unfolding methods affords the opportunity to study the Cenozoic evolution of two low amplitude domes in the Mid‐Norwegian extensional margin, the Helland Hansen Arch and the Vema Dome. The integration of growth strata geometries observed in both flanks of the domes demonstrate that the structures grew by a variable combination of tectonics and differential compaction mechanisms. Sequential restoration shows that the Helland Hansen Arch grew between Early Oligocene and earliest Late Pliocene times (33–1.9 Ma). During the first phase of growth (33–9 Ma), the tectonic compression accounted for a minimum of 27% of the total dome amplitude. During Late Miocene to Pliocene times (9–1.9 Ma), differential compaction was the mechanism for dome growth. During Late Pliocene times, the Helland Hansen Arch grew with the highest rates coinciding with initial deposition of prograding wedges (3.6–1.9 Ma). In contrast, the Vema Dome started to develop in Early Eocene times and grew at a fairly constant rate up to Early Pliocene times at 3.6 Ma. The amplification of the Vema Dome took place through both differential compaction and tectonics between Early Eocene and Late Miocene times (54.8–7 Ma). The tectonic contribution accounted for a minimum of a 37% of the total dome amplitude. During Pleistocene times, the progradation of clastic wedges led to a decrease of the amplitudes of both the Helland Hansen Arch and the Vema Dome. The different timing of tectonic growth for analysed domes and arches suggest that a small and protracted phase of compression affected the Mid‐Norwegian Margin. This agrees with well‐known widespread contractional deformation affecting the Atlantic Margin of the European Plate during the Tertiary.

Geological processes that control lateral and vertical variability in coal seam moisture contents—Latrobe Valley (Gippsland Basin) Australia

A study throughout the Latrobe Valley coal measures of coal moisture distribution using downhole bore data and 3D digital models of large bore data sets indicates lateral and vertical moisture variability is controlled by a number of factors. These include burial, type of overburden, age of the seam, marine influence, coal lithotype and lateral compression on folds and monoclines. The same rules appear to apply to the adjacent Alberton coal measures in the south Gippsland Basin, suggesting these factors may be widespread in other brown coal basins where moisture contents are still equilibrating through differential compaction. In Gippsland average moisture down seam decreases by 1–2% per seam and at a rate of approximately 1% every 20 m. Weight average moisture content for each seam decreases on average at a rate of 0.5% every 1 million years. A stepwise decrease in moisture content between the ∼100 m thick major seams of 1–7% reflects large intervals of time are represented by the relatively thinner (1–10 m thick) interseam sediments. Compression by monoclinal folding can decrease moisture contents up to 7%. For any given depth, moisture content appears higher on anticlines due to a lowered intensity of compression than in adjacent synclines. It is considered likely that coals folded over anticlines are more fractured, thereby containing greater amounts of free water. Use of the lower moisture coals and monoclinal coals may be preferable for future power station developments. Despite an increase in overburden/coal ratios, there would be significant savings in a lowered coal volume to calorific value, and a consequent reduction in greenhouse gas emission.

Buried impact craters: A topographic analysis of quasi‐circular depressions, Utopia Basin, Mars

Many quasi‐circular depressions (QCDs) revealed in the Mars Orbiter Laser Altimeter data set have no obvious structural representation in visible images of Mars. It has been suggested that these QCDs are surface representations of buried impact craters, and on the basis of this assumption, cumulative count versus diameter plots of QCDs in the northern lowlands have been constructed to date the buried lowland floor. If the QCDs are, in fact, buried impact craters, their surface relief should scale directly with their diameter; this relationship holds true for 115 QCDs with diameters ranging from 7 to 100 km within roughly 1500 km of the center of the Utopia Basin. The coefficient of determination (R2) of the relationship more closely approaches 1 when QCDs in zones of comparable cover thickness are evaluated. The surface relief versus diameter relationship of 55 randomly selected “pseudo‐QCDs” was also determined; their poor correlation indicates that the direct relationship of surface relief to diameter of actual QCDs is due to differential compaction of a cover material over buried impact craters and is not just a random effect of QCD diameter. The slope of the trend of the QCDs' surface relief versus diameter, when divided into zones of comparable cover thickness, can be used to determine which zones have thicker or thinner cover.

Differential compaction and subsidence in sedimentary basins due to silica diagenesis: A case study

The conversion of opal A to opal CT is a thermochemical diagenetic process that can cause kilometer-scale differential compaction and differential subsidence within clastic sedimentary basins. This is demonstrated for an opal A to opal CT conversion imaged by three-dimensional seismic data on the northeastern Atlantic margin. This diagenetic has a circular to polygonal ridge-depression morphology. The ridges have a relief of 10–50 m and separate circular to polygonal depressions that have diameters of 1–2 km, some of which link up to form interconnected networks. This morphology initially developed because the host sediments mantled an underlying polygonal fault system and the front tracked deformed stratigraphy. Ridges formed above the tips of the underlying faults, and depressions formed between them. Ridge relief and width then progressively increased due to earlier opal A to opal CT conversion above ridges as well as conversion laterally along beds or bed sets. This caused compaction and concomitant subsidence in the overburden and the development of a circular to polygonal network of overburden troughs aligned with the ridge system. A positive feedback loop was established where additional overburden sediment above the growing ridges reached the depth of conversion ahead of adjacent areas, driving more compaction and differential subsidence. This paper provides the first insights into the large-scale morphological characteristics of opal A to opal CT diagenetic fronts. It demonstrates the potential for a local stratigraphic control on diagenetic front development and morphology and lastly highlights the potential utility of seismic reflection data for understanding diagenetic processes.

Paleocene deep-marine sandstone plays in the Siri Canyon, offshore Denmark–southern Norway

The Paleocene Siri Canyon extends for more than 120 km from the Stavanger Platform to the Tail End Graben along the Danish–Norwegian North Sea boundary. Initial formation of the canyon system took place in the Early Paleocene (Danian) and was related to major, submarine sliding in the uppermost chalk section caused by uplift of the Scandinavian hinterlands. The trend of the canyon partly follows salt structures along the southern edge of the Norwegian–Danish Basin (part of the Northern Zechstein Basin). The canyon fill consists of deep-marine, highly glauconitic sandstones interbedded with hemipelagic and turbidite marls and mudstones. As for many Paleocene deep-marine sandstones in the North Sea, the sandstones of the Siri Canyon are massive and blocky. Frequent occurrence of injected sandstone sills/dykes, injection breccias, as well as erosion and associated rip-down clasts along the upper boundaries of in situ sandstones, indicate thorough, post-depositional modification. Fluidization is, thus, likely to be the main reason for the general scarcity of primary sedimentary structures and massive appearance of the sandstones. All sandstones are basically free of detrital clays and most likely formed from non-cohesive, concentrated density flows probably developed as turbidity currents. The sand flows were confined by the canyon and the depositional pattern indicates a complex interaction of turbidity currents, salt-induced seafloor topography and differential compaction. Release of sandy flows was probably caused by seismic shocks resulting in a collapse of unstable shelf sands concentrated on the Stavanger Platform. Transport distances of the sands were up to 120 km and thereby contrast with many Paleocene submarine sandstones in the North Sea. The deep-marine play in the Siri Canyon relies on long distance migration of hydrocarbons, up to 75 km from mature Jurassic source rocks in the Central Graben. This was facilitated by the elongate and confined geometry of the sandstones forming a well-connected system for updip migration.

Control of differential compaction on the geometry of sediments onlapping paleoescarpments: Insights from field geology (Central Apennines, Italy) and numerical modeling

Basin strata onlapping pelagic carbonate platform margins in the Central Apennines commonly dip considerably (as much as 20°–30°) toward the basin when platform strata (Calcare Massiccio) are retrodeformed to horizontal in order to undo the tilting related to Apenninic compressional tectonics. These angular relationships are commonly ascribed to normal drag associated with synsedimentary faulting. We show that such angular relationships are fully consistent with results of numerical simulations that aim at modeling first-order geometric features associated with sedimentation and compaction of deposits onlapping the escarpment at the margin of basins. Therefore, growth of an underlying normal fault is not generally required to generate them. This conclusion is also supported by field investigations of the contacts at pelagic carbonate platform margins and by paleoescarpment analysis, which strongly suggest post–early Liassic tectonic quiescence.

3D seismic reflection mapping of the Silverpit multi-ringed crater, North Sea

Research Article| March 01, 2005 3D seismic reflection mapping of the Silverpit multi-ringed crater, North Sea S.A. Stewart; S.A. Stewart 1BP Azerbaijan, c/o Chertsey Road, Sunbury on Thames, Middlesex TW16 7LN, UK Search for other works by this author on: GSW Google Scholar P.J. Allen P.J. Allen 2Production Geoscience Ltd, Banchory AB31 5YR, UK Search for other works by this author on: GSW Google Scholar Author and Article Information S.A. Stewart 1BP Azerbaijan, c/o Chertsey Road, Sunbury on Thames, Middlesex TW16 7LN, UK P.J. Allen 2Production Geoscience Ltd, Banchory AB31 5YR, UK Publisher: Geological Society of America Received: 17 Feb 2004 Revision Received: 22 Jun 2004 Accepted: 29 Jun 2004 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (2005) 117 (3-4): 354–368. https://doi.org/10.1130/B25591.1 Article history Received: 17 Feb 2004 Revision Received: 22 Jun 2004 Accepted: 29 Jun 2004 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation S.A. Stewart, P.J. Allen; 3D seismic reflection mapping of the Silverpit multi-ringed crater, North Sea. GSA Bulletin 2005;; 117 (3-4): 354–368. doi: https://doi.org/10.1130/B25591.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The first comprehensive mapping of the Silverpit multi-ringed crater is presented, based on five adjoining 3D seismic surveys that together provide 78% (216 km2) areal coverage of the structure with spatial and depth sampling of ∼10 m. The top Cretaceous surface shows an excavated crater 3 km in diameter surrounded by 10–15 concentric ring structures out to a radius of 9.4 km. Seismic sequence stratigraphy and drilled wells indicate the lower age bound is a disconformity spanning 74–54 Ma. The upper bound is Eocene termination of concentric ring fault growth (ca. 45 Ma). The inner rings are extensional faults defining rotated fault blocks that would be considered part of the crater in an impact structure interpretation, giving a final impact crater diameter of 8 km. The outer rings show a surprising variation in style, from graben in the west to compressional structures in the east. This may be explained by postcratering gravity spreading on easterly dipping intra-Cretaceous detachments. Within the crater, the top Cretaceous seismic reflector is disrupted and blocky, but defines a central uplift with a slight crestal depression. Below this uplift, the prominent base Cretaceous seismic reflector shows Jurassic strata rising into a cone 300 m high and 1 km wide, characterized by radial faults. This central uplift within the crater may structurally balance the inner ring fault blocks via a slumping model, but we also identify low seismic velocity in the uplifted Jurassic, so fracturing may have contributed to growth of the central peak. At a radius of 2.5 km, base Cretaceous shows a dish-shaped depression that we interpret as marking either the extent of slumping toward the crater at this structural level or the limit of shockwave-induced porosity collapse. Seismic reflectors above the top Cretaceous excavated crater show differential compaction during the Eocene, accommodated by drape structure plus two conical faults that define narrow ring graben and represent the final phase of crater-related tectonics. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Origin and development of plains-type folds in the mid-continent (United States) during the late Paleozoic

Plains-type folds are local, subtle anticlines formed in the thin sedimentary package overlying a shallow, crystalline basement on the craton. They are small in areal extent (usually less than 1–3 km2 [0.4–1.2 mi2]), and their amplitude increases with depth (usually tens of meters), which is mainly the result of differential compaction of sediments (usually clastic units) over tilted, rigid, basement fault blocks. The development of these structural features by continuous but intermittent movement of the basement fault blocks in the late Paleozoic in the United States mid-continent is substantiated by a record of stratigraphic and sedimentological evidence.The recurrent structural movement, which reflects adjustment to external stresses, is expressed by the change in thickness of stratigraphic units over the crest of the fold compared to the flanks. By plotting the change in thickness for different stratigraphic units of anticlines on different fault blocks, it is possible to determine the timing of movement of the blocks that reflect structural adjustment. These readjustments are confirmed by sedimentological evidence, such as convolute, soft-sediment deformation features and small intraformational faults.The stratigraphic interval change in thickness for numerous structures in the Cherokee, Forest City, and Salina basins and on the Nemaha anticline of the mid-continent United States was determined and compared for location and timing of the adjustments. Most of the adjustment occurred during and after time of deposition of the Permian–Pennsylvanian clastic units, which, in turn, reflect tectonic disturbance in adjacent areas, and the largest amount of movement on the plains-type structures occurred on those nearest and semiparallel to major positive features, such as the Nemaha anticline.Depending on the time of origin and development of plains-type folds, they may control the entrapment and occurrence of oil and gas.

Large‐scale hydraulic conductivities inferred from three‐dimensional groundwater flow and 4He transport modeling in the Carrizo aquifer, Texas

Through a series of groundwater flow and 4He transport simulations, this study illustrates the conceptual and practical gains achieved by expanding a two‐dimensional (2‐D) model to a true 3‐D one through an application in the Carrizo aquifer and surrounding formations in southwestern Texas. The 3‐D simulations allow for a more detailed and accurate definition of the heterogeneities of the system by specifically identifying and differentiating processes that directly impact the three‐dimensional hydraulic conductivity field. It is shown that while hydraulic conductivity decreases exponentially along the regional groundwater flow direction, such decrease is better described as a function of depth rather than recharge distance. This relationship reflects the combined influences of differential compaction of the media as well as downdip lithological change. The intrinsic permeability derived from this relationship agrees with field information and with previous findings obtained for the continental crust for depths ≤2 km, suggesting that for large scales, decrease rate of permeability with depth is independent of the media. Results also suggest that the solution for groundwater flow simulations based on calibration of hydraulic heads depends on the ratio between hydraulic conductivities of different formations, indicating that an infinite number of solutions are available for calibration of 3‐D groundwater flow models. Understanding how geological processes directly affect the 3‐D hydraulic conductivity field at the regional scale is essential not only to hydrogeological applications but also at improving our understanding of the Earth's crust and mantle dynamics by allowing for a more accurate quantification of helium and heat fluxes.

Appalachian Mine Soil Morphology and Properties

Surface coal mining and reclamation methods in the Appalachians have changed dramatically since the passage of the Surface Mining Control and Reclamation Act (SMCRA) of 1977 and subsequent improvements in mining and reclamation technology. In this study, 30 pre‐SMCRA mine soil profiles (4–20 yr old) were examined and sampled in 1980 and compared with 20 mine soil profiles (8–13 yr old) described in the same area in 2002 after it had been completely remined by modern deep cut methods. Mine soils in both sampling years had high rock fragment content (42–81%), relatively well‐developed A horizons, and generally exhibited A‐C or A‐AC‐C horizonation. Although six Bw horizons were described in 1980, only two met all requirements for cambic horizons. The 1980 mine soils developed in overburden dominated by oxidized, preweathered material due to relatively shallow mining cuts. The 1980 mine soils had lower rock fragment content, finer textures, lower pH, and tended to be more heterogeneous in horizonation, morphology, and texture than soils observed in 2002, which had formed primarily in unweathered overburden from deeper cuts. Half the pedons sampled in both years had densic materials within 70 cm of the surface. Four poorly to very poorly drained soil profiles were described in each sampling year containing distinct hydric soil indicators in surface horizons. While older pre‐SMCRA mine soils do have many properties in common with newer mine soils, their properties are highly influenced by the fact that they generally have formed in more weathered overburden from higher in the geologic column. Overall, Appalachian mine soils are much more complex in subsoil morphology than commonly assumed, and differential compaction greatly complicates their internal drainage and limits their overall productivity potential.

English

Surface coal mining and reclamation methods in the Appalachians have changed dramatically since the passage of the Surface Mining Control and Reclamation Act (SMCRA) of 1977 and subsequent improvements in mining and reclamation technology. In this study, 30 pre-SMCRA mine soil profiles (4-20 yr old) were examined and sampled in 1980 and compared to 20 mine soil profiles (8-13 yr old) described in the same area in 2002 after it had been completely re- mined by modern deep cut methods. Mine soils in both sampling years had high rock fragment content (42 to 81%), relatively well-developed A horizons, and generally exhibited A-C, or A-AC-C horizonation. Although six Bw horizons were described in 1980, only two met all requirements for cambic horizons. The 1980 mine soils developed in overburden dominated by oxidized, pre-weathered material due to relatively shallow mining cuts. The 1980 mine soils had lower rock fragment content, finer textures, lower pH, and tended to be more heterogeneous in horizonation, morphology, and texture than soils observed in 2002, which had formed primarily in unweathered overburden from deeper cuts. Half the pedons sampled in both years had densic materials within 70 cm of the surface. Four poorly to very poorly drained soil profiles were described in each sampling year containing distinct hydric soil indicators in surface horizons. While older pre-SMCRA mine soils do have many properties in common with newer mine soils, their properties are highly influenced by the fact that they generally have formed in more weathered overburden from higher in the geologic column. Overall, Appalachian mine soils are much more complex in subsoil morphology than commonly assumed, and differential compaction greatly complicates their internal drainage and limits their overall productivity potential.

Geochemical and textural evidence for early (shallow) diagenetic growth of stratigraphically confined carbonate concretions, Upper Devonian Rhinestreet black shale, western New York

Laterally persistent carbonate concretionary horizons are conspicuous to the Upper Devonian (Frasnian) Rhinestreet black shale of the western New York State Appalachian Plateau. Field observations, including randomly tilted concretions and differential compaction of host sediment laminae around concretions, are consistent with early diagenetic growth in unconsolidated sediment. Further, estimates of pre-cementation host sediment porosity based on the volume percentage of CaCO 3 cement (74–93%) and, perhaps most importantly, the preservation of a cardhouse clay fabric in mudstones collected from concretion pressure shadow regions suggest that concretionary growth occurred rapidly within perhaps a meter of the seafloor. Petrographic examination (SEM) of matrix carbonate samples collected from six concretions reveals a relatively porous (∼4–7%) framework of calcite micrite and microsparite, euhedral pyrite (most abundant near concretion rims), disseminated pyrite framboids and randomly oriented detrital clay platelets and domains. C isotopic compositions of Rhinestreet concretion matrix carbonate range from −13.9‰ to +1.7‰ PDB suggesting derivation of C from a mixed source that included depleted and isotopically heavy C. Slightly to moderately depleted O isotopic compositions (−3.8‰ to−6.8‰ PDB) do not corroborate the shallow depth of concretion growth suggested by field and textural observations. The vertically confined nature of most Rhinestreet concretions, their large size, oblate ellipsoidal shape, variable C isotope compositions and absence of any evidence that concretions were sited at local concentrations of organic matter suggest an origin by anaerobic CH 4 oxidation. Early pre-concretion sulfate reduction and associated pyrite framboid formation that occurred throughout the host sediment was renewed in thin, shallow zones of uncompacted sediment—the concretionary horizons—by CH 4 and isotopically heavy dissolved carbonate diffusing upward from the zone of biogenic methanogenesis to the base of the sulfate reduction zone, probably little more than a meter below the seabed. Local variations in the mix of isotopically light CH 4 and heavier dissolved carbonate may have been responsible for the wide range of δ 13C isotopic compositions of Rhinestreet concretions. Crucial to this mechanism is a pause in deposition that would have held the zone of carbonate precipitation at a fixed distance below the ocean floor long enough for the large concretions to grow. Evidence that sedimentation slowed or even stopped during concretion growth includes (1) laminae that can be traced through concretions with no observed systematic changes in thickness and (2) minimal concretion center-to-edge variations in estimated pre-cementation porosity. A resumption of sedimentation and related burial brought concretion growth in each horizon to an end. The moderately depleted δ 18O values of Rhinestreet concretions probably reflect the “resetting” of O isotopic compositions by warm fluids migrating through the permeable concretions during burial.

Formation of double‐ring circular grabens due to volumetric compaction over buried impact craters: Implications for thickness and nature of cover material in Utopia Planitia, Mars

Throughout the polygonal terrain of Utopia Planitia are circular grabens, inferred to overlie buried crater rims. Thirty‐four of the 37 circular grabens discovered in Viking and Thermal Emission Imaging System (THEMIS) images southwest of the Utopia Basin (21°–36°N, 240°–265°W) have two nested rings, while the remaining three have a single ring. All 37 circular grabens bound topographic depressions whose surface relief scales directly with diameter, suggesting differential compaction of cover material within the polygonal terrain. The distance of doubled circular grabens from the center of the Utopia Basin scales directly with the spacing between the two rings, implying a relation between ring spacing and cover thickness. Numerical models of horizontal shrinkage during volumetric compaction of material over crater rims show two maxima of horizontal normal tension on the surface. These maxima, where we would expect grabens to form, move away from each other with increasing cover thickness. The modeled spacings of stress maxima within 1–2 km thick cover are comparable to the graben spacing observed in southwest Utopia Planitia, implying that cover thickness in this polygonal terrain may be 1–2 km thick. Where cover material is thin, the proximity of the two normal stress maxima may only produce a single circular graben, accounting for the observation that the three single‐ringed grabens are most distal to the center of the basin. We contend that the correlation of shrinkage model results and observed ring spacing supports a wet sedimentary origin of polygonal terrain materials, consistent with the past existence of standing water in the Martian lowland.

Structural Evolution of a Complex 3D Fault Array in the Cretaceous and Tertiary of the Porcupine Basin, Offshore Ireland

Abstract A high-quality 3D seismic survey, located in the northwest Porcupine Basin (Irish Atlantic Margin), has been used to investigate the geometry and origin of pervasively developed and complexly distributed linked extensional fault arrays, present within Late Cretaceous and Early Tertiary sequences. The faults show a downwards transition from relatively simple, planar fault segment geometries (~N-S-trending) within younger Early Eocene sand-dominated clastic sequences, into complex conjugate arrays in the underlying older Early Eocene to Late Cretaceous shale-dominated sequences. Rectilinear to polygonal structural configurations are developed at the deeper levels. Most of the fault array ultimately terminates downwards into the Late Cretaceous, where structural accommodation may have taken place by localized or more regional bedding plane slip and/or by volume changes resulting from compaction of fine-grained sequences. Locally, reactivated Jurassic syn-rift extensional faults are locally seen to link upwards into the shallow fault array and appear to have controlled both the intensity and facing direction of the shallower faults on a km scale. The seismic data also clearly show that early upslope-throwing faults are cross-cut by later, downslope-throwing faults. Such geometries are comparable to those formed in sandbox models where gravitational collapse of a tilted sequence is the dominant process controlling fault development. Overall, the fault array geometries seen in the Cretaceous and lower Tertiary successions in this area are interpreted to have resulted from gravitational collapse processes during basin subsidence and sediment compaction, and where the main deformation mechanism was non-rigid block rotation. Differential compaction of Cretaceous and lower Tertiary sediments over pre-Cretaceous rift topography and selective reactivation of the Jurassic fault array are also considered important influences on the resultant fault distribution in 3D.

Initial Deformation in a Subduction Thrust System: Polygonal Normal Faulting in the Incoming Sedimentary Sequence of the Nankai Subduction Zone, Southwestern Japan

Abstract 3D seismic data from the Nankai margin provide detailed imagery documenting the onset of deformation at an active sediment-dominated accretionary prism, including a previously unmapped network of normal faults. The Nankai margin off southwest Japan is characterized by active subduction, seismogenesis, and a large accretionary prism with fold-and-thrust belt structure. Imbricate thrusting is the dominant structural style of the outer 20 km of the prism. This structural domain develops at the prism toe, where an incipient imbricate thrust displays significant along-strike variability in dip, offset, and development of hangingwall anticlines. Compressional deformation is preceded by normal faulting that initiates seaward of the trench axis. Seismic data in this area reveal a complex, intersecting pattern of normal faults within the incoming hemipelagic sediments. Underlying the faulted section is a high-amplitude reflector interpreted as representing oceanic basement. This reflector contains elongate horsts and grabens oriented perpendicular to the margin interpreted as relict spreading centre fabric. Analysis of the orientation of normal faults within the Shikoku basin sequence shows a correlation between fault geometry and basement structure. This faulting is notably similar to layer-bound compaction faults, documented in the North Sea and elsewhere, attributed to both hydrofracturing and volumetric contraction of fine-grained sediments. Mapped normal faults may thus be the result of a combination of differential compaction of sediments above irregular, dipping oceanic basement and compactional dewatering seaward of the toe of the accretionary prism.

Depositional remnants, part 2: Examples from the Western Interior Cretaceous basin of North America

The Western Interior Cretaceous basin contains numerous examples of depositional remnants (partially preserved portions of originally extensive depositional systems). Emphasis is placed on examples of accommodation remnants (i.e., a type of depositional remnant whose overlying erosion surface displays little relief and is preserved in an area of locally increased accommodation). Variations in spatial accommodation have been attributed to synsedimentary tectonics, erosional topography, differential compaction, and salt dissolution and are commonly associated with basement faults. Remnants range in size from small to very large (basin-scale), and larger remnants commonly contain several smaller remnants. Remnants of shallow-marine sandstone encased in shale are an important and common type of stratigraphic trap. Failure to recognize deposits as only remnants of depositional systems has led to misinterpretations of depositional environment and inaccurate paleogeographic reconstructions.

Depositional remnants, part 1: Common components of the stratigraphic record with important implications for hydrocarbon exploration and production

The preservation of sedimentary strata below erosion surfaces can be highly variable laterally, and commonly only small remnants of originally more extensive deposits remain. The concept of remnants is not new, but their abundance and the fact that they commonly develop below low-relief erosion surfaces, as well as below those that incise deeply, may not be fully appreciated. Spatial variations in accommodation because of paleotopography, synsedimentary and penecontemporaneous tectonics, and differential compaction occur on a variety of scales in all settings. In low-accommodation and shallow-shelf settings, spatial variations in accommodation commonly result in the formation of remnants ranging in size from small (few to tens of square kilometers) to basin scale (hundreds of square kilometers). Remnants of porous and permeable facies encased in low-permeability facies are an important and relatively common type of stratigraphic trap. Conceptual models for three scales of remnants are used to illustrate their significance to petroleum exploration and production.The recognition of these deposits, herein referred to as depositional remnants, is commonly critical to systems analysis because stratigraphic terminations caused by erosional truncation must be interpreted differently from those caused by lithofacies change. In addition, the preserved geometry of remnants can be quite different from the geometry of the original deposit, and because geometry is heavily relied upon in interpretations of ancient systems, interpreting a preserved geometry as a geometry can result in significant misunderstandings. Depositional remnants are common in the stratigraphic record and represent an important concept to open up new hydrocarbon plays in mature basins.

Quantification of high-frequency sea-level fluctuations in shallow-water carbonates: an example from the Berriasian–Valanginian (French Jura)

When quantifying sedimentary processes on shallow carbonate platforms, it is important to know the high-frequency accommodation changes through time. Accommodation changes in cyclic successions are often analysed by simply converting cycle thickness to Fischer plots. This approach is not satisfactory, because it does not account for differential compaction, possible erosion, sea-level fall below the depositional surface, or subtidal cycles. An attempt is made here to reconstruct a realistic, high-frequency accommodation and sea-level curve based on a detailed facies and cyclostratigraphical analysis of Middle Berriasian to Lower Valanginian sections in the French Jura Mountains. The general depositional environment was a shallow-marine carbonate platform on a passive margin. Our approach includes the following steps: (1) facies interpretation; (2) cyclostratigraphical analysis and identification of Milankovitch parameters in a well-constrained chronostratigraphic framework; (3) differential decompaction according to facies; (4) estimation of depth ranges of erosion and vadose zone; (5) estimation of water-depth ranges at sequence boundaries and maximum flooding intervals; (6) estimation of mean subsidence rate; (7) classification of depositional sequences according to types of facies evolution: ‘catch-up’, ‘catch-down’, ‘give-up’, or ‘keep-up’; (8) classification of depositional sequences according to long-term sea-level evolution: ‘rising’, ‘stable’, ‘falling’; (9) calculation of ‘eustatic’ sea-level change for each depositional sequence using the parameters inferred from these scenarios, assuming that sea-level cycles were essentially symmetrical (which is probable in Early Cretaceous greenhouse conditions); (10) calculation of a sea-level curve for each studied section; (11) comparison of these curves among each other to filter out differential subsidence; (12) construction of a ‘composite eustatic’ sea-level curve for the entire studied platform; (13) spectral analysis of the calculated sea-level curves. Limitations of the method are those common to every stratigraphic analysis. However, the method has the potential to improve the original cyclostratigraphical interpretations and to better constrain the high-frequency sea-level changes that control carbonate production and sediment fluxes.