Late Cretaceous Faults Research Articles

Preservation of thermal signature of elevated syn-rift heat flow during multiphase extension: a case study from the Duntroon Sub-basin, Great Australian Bight

Quantifying the thermal histories of rift basins is important for evaluating their resource and CO2 storage potential because temperature controls hydrocarbon generation, and the diagenesis of reservoir rocks. However, in many rift basins, it is difficult to obtain evidence for elevated heat flow accompanying rifting, since paleotemperature data from drilled sections typically record heating related to post-rift burial. Here we integrate geochemical, geophysical and petrophysical data from the Duntroon Sub-basin, Great Australian Bight, that show how strain-migration during multiphase extension can preserve the signature of syn-rift elevated geothermal gradients. During the late Jurassic–early Cretaceous, rifting was focussed along ~ESE-striking normal fault systems in the northern part of the Duntroon Sub-basin. During the late Cretaceous, strain migrated to the southwest through the development of normal faults which accommodated the deposition of Upper Cretaceous strata. The Echidna-1 well was drilled into a basement high, in the footwall of a late Cretaceous fault system, penetrating ~2.5 km of Lower Cretaceous strata. Paleotemperature proxies define an early Cretaceous paleogeothermal gradient of ~60°C km−1, substantially higher than the present-day gradient. Our results indicate that preserved Lower Cretaceous strata were more deeply buried by ~1 km of additional section, which was likely eroded during an episode of mid-Cretaceous exhumation associated with the migrating locus of rifting; this enabled the preservation of thermal signature of elevated syn-rift heat flow. Similar evidence is also observed in the Otway Basin, demonstrating the regional extent of elevated syn-rift heat flow along the southern Australian margin.

Multiphase deformation history of the Porcupine Basin, offshore west Ireland

AbstractThe Porcupine Basin is a large underexplored sedimentary basin located offshore west of Ireland within the structurally complex European North Atlantic Margin. The basin has evolved through multiple Jurassic–Recent phases of deformation and although the overall plate tectonic context of the margin is well‐documented, there are still uncertainties regarding the phases of tectonic activity, and their associated strain distribution and fault kinematics. Based on the analysis of large volumes of 2D and 3D seismic data from the Porcupine Basin, we provide an overview of the nature and origin of multiple fault systems, both tectonic and nontectonic, with links to regional tectonic events where possible. Three distinct basin‐wide phases of tectonically induced extensional faulting are recognized: (a) Late Jurassic N‐S to NE‐SW trending rift faults, (b) Late Cretaceous E‐W to ENE‐WSW trending normal faults and (c) Mid Eocene N‐S trending faults. The Jurassic faults were active over a period of 11.5 Myr between the Oxfordian and Kimmeridgian but with an intervening 4.5 Myr period of quiescence providing a two‐stage rift evolution. The Late Cretaceous faulting in the Porcupine Basin broadly correlates with extension in the Celtic Sea basins and is tentatively attributed to the rotational spreading of the Bay of Biscay in the south. The Mid Eocene phase of extension, which coincides with the onset of spreading between Europe and Greenland (Atlantic spreading), resulted in partial reactivation of the Jurassic faults. A series of non‐tectonic fault systems occur within specific stratigraphic intervals suggesting a compactional or gravitationally driven origin, including basinward dipping normal faults within a Palaeocene–early Eocene deltaic sequence, and multiple tiers of polygonal faults within Cretaceous, late Eocene and Neogene strata.

PETROLEUM GEOLOGY OF THE NOGAL BASIN AND SURROUNDING AREA, NORTHERN SOMALIA, PART 2: HYDROCARBON POTENTIAL

Seismic reflection profiles and well data show that the Nogal Basin, northern Somalia, has a structure and stratigraphy suitable for the generation and trapping of hydrocarbons. However, the data suggest that the Upper Jurassic Bihendula Group, which is the main source rock elsewhere in northern Somalia, is largely absent from the basin or is present only in the western part. The high geothermal gradient (∼35–49 °C/km) and rapid increase of vitrinite reflectance with depth in the Upper Cretaceous succession indicate that the Gumburo Formation shales may locally have reached oil window maturity close to plutonic bodies. The Gumburo and Jesomma Formations include high quality reservoir sandstones and are sealed by transgressive mudstones and carbonates.ID petroleum systems modelling was performed at wells Nogal‐1 and Kalis‐1, with 2D modelling along seismic lines CS‐155 and CS‐229 which pass through the wells. Two source rock models (Bihendula and lower Gumburo) were considered at the Nogal‐1 well because the well did not penetrate the sequences below the Gumburo Formation. The two models generated significant hydrocarbon accumulations in tilted fault blocks within the Adigrat and Gumburo Formations. However, the model along the Kalis‐1 well generated only negligible volumes of hydrocarbons, implying that the hydrocarbon potential is higher in the western part of the Nogal Basin than in the east. Potential traps in the basin are rotated fault blocks and roll‐over anticlines which were mainly developed during Oligocene–Miocene rifting.The main exploration risks in the basin are the lack of the Upper Jurassic source and reservoirs rocks, and the uncertain maturity of the Upper Cretaceous Gumburo and Jesomma shales. In addition, Oligocene‐Miocene rift‐related deformation has resulted in trap breaching and the reactivation of Late Cretaceous faults.

Late Palaeozoic – Mesozoic tectonostratigraphic development of the eastern Faroe-Shetland Basin: New insights from high-resolution 3D seismic and well data

Abstract The Faroe-Shetland Basin (FSB) belongs to the sedimentary basins that encompass outer parts of the NE Atlantic continental margin. The FSB contains several kilometres thick Cenozoic and Cretaceous strata, which overlie poorly understood pre-Cretaceous strata. Regional high-resolution 3D seismic reflection data used in this study covers the eastern half of the basin, generally devoid of thick basaltic successions. The pre-Cretaceous successions are preserved as erosional remnants of originally more extensive deposits throughout the FSB. Devono-Carboniferous and Permo-Triassic rifting probably took place in the FSB, but the magnitude and regional extent of each phase is less constrained than in the neighbouring inner Hebrides-Shetland rift basins. Major rifting took place in Jurassic times and generated a fault-controlled marine basin. During the Apto-Albian and Cenomanian, distributed extension occurred throughout the FSB, suggesting wide rift mode of extension. The subsequent Campanian - Maastrichtian extensional phase, on the contrary, was localized in the Flett Sub-basin controlled by the Westray and Corona faults along its western margin and oppositely dipping faults along the eastern basin margin. The fault-controlled Cretaceous deposition in the FSB is a distinct feature that is documented in the basins along the mid-Norwegian margin but rarely recorded in other large Cretaceous basins along the NE Atlantic margin. In the FSB, Late Cretaceous faults frequently reactivated earlier structures, suggesting that a structural inheritance played an important role in basin development. The existence of Permo-Triassic evaporites is postulated to explain geometries of Jurassic and Lower Cretaceous strata and fault patterns in the Foula Sub-basin. A partial mantle serpentinization during Early Cretaceous times proposed for other outer basins along the NE Atlantic margin is unlikely to have occurred in the FSB.

Structural evolution of a normal fault array in the Gambier Embayment, offshore Otway Basin, South Australia: insights from 3D seismic data

ABSTRACTThis study was undertaken to determine the structural evolution of a normal fault array using detailed kinematic analysis of normal fault tip propagation and linkage, adding to the growing pool of research on normal fault growth. In addition, we aim to provide further insight into the evolution of the offshore Otway Basin, Australia. We use three-dimensional (3D) seismic reflection data to analyse the temporal and spatial evolution of a Late Cretaceous–Cenozoic age normal fault array located in the Gambier Embayment of the offshore Otway Basin, South Australia. The seismic reflection data cover a NW–SE-oriented normal fault array consisting of six faults, which have grown from the linkage of numerous, smaller segments. This fault array overlies and has partial dip-linkage to E–W-striking, basement-involved faults that formed during the initial Tithonian–Barremian rifting event in the Otway Basin. Fault displacement analysis suggests four key stages in the post-Cenomanian growth history of the upper array: (1) nucleation of the majority of faults resulting from resumed crustal extension during the early Late Cretaceous; (2) an intra-Late Cretaceous period of general fault dormancy, with the nucleation of only one newly formed fault; (3) latest Cretaceous nucleation of another newly formed fault and further growth of all other faults; and (4) continued growth of all faults, leading to the formation of the Cenozoic Gambier Sub-basin in the Otway Basin. Our analysis also demonstrates that Late Cretaceous faults, which are located above and dip-link to basement-involved faults, display earlier nucleation and greater overall throw and length, compared with those which do not link to basement-involved faults. This is likely attributed to increased rift-related stress concentrations in cover sediments above the upper tips of basement faults. This study improves our understanding of the geological evolution of the presently under-explored Gambier Embayment, offshore Otway Basin, South Australia by documenting the segmented growth style of a Late Cretaceous normal fault array that is located over, and interacts with, a reactivated basement framework.

Great Plains polygonal fault system as expressed in Saskatchewan: Late Cretaceous fault initiation and graben formation

An extensive polygonal fault system (PFS) has been recognized in fine-grained Late Cretaceous sediments of the Western Interior Seaway of North America. Polygonal fault systems are pervasive organizations of nontectonic faults with fault traces that coalesce to form distinctive polygonal fault patterns. Interpretation of a three-dimensional seismic dataset from southeast Saskatchewan provides insight into fault initiation, timing, and geometry for the Great Plains PFS (GPPFS). Faulting initiates in the Niobrara Formation, with the largest fault throws occurring over Early Cretaceous Viking Formation sandstone accumulations, suggesting that drape compaction over the channel sand initiated some of the faulting. Above this, faulting increases in vertical offset, and the predominant fault strike angles change in the Lea Park, Belly River, and Bearpaw formations (all homotaxial to the Pierre Shale) throughout Campanian time. By late Bearpaw time, the initially almost random fault strike orientations change to well-defined northwest–southeast- and west–east-striking grabens. These grabens have up to 20 m of throw and can be 125 m wide and 900 m long at ∼400 m current depth. Predominant graben faults are the continuation of some of the deeper PFS faults. Moreover, the grabens are present over a Campanian clinoform bed and may be interpreted to indicate Bearpaw time extension tectonics that is local or regional in scale. The PFS helps to explain near-surface faulting observed in Late Cretaceous sediments in the Western Interior Seaway and could be used as a model to help explain Late Cretaceous geology, subsurface groundwater flow, and shallow natural gas reservoir continuity.

Crestal fault geometries reveal late halokinesis and collapse of the Samson Dome, Northern Norway: Implications for petroleum systems in the Barents Sea

This paper uses 2D and high-quality 3D seismic reflection data to assess the geometry and kinematics of the Samson Dome, offshore Norway, revising the implications of the new data to hydrocarbon exploration in the Barents Sea. The study area was divided into three (3) zones in terms of fault geometries and predominant strikes. Displacement-length (D-x) and Throw-depth (T-z) plots showed faults to consist of several segments that were later dip-linked. Interpreted faults were categorised into three families, with Type A comprising crestal faults, Type B representing large E-W faults, and Type C consisting of polygonal faults. The Samson Dome was formed in three major stages: a) a first stage recording buckling of the post-salt overburden and generation of radial faults; b) a second stage involving dissolution and collapse of the dome, causing subsidence of the overburden and linkage of initially isolated fault segments; and c) a final stage in which large fault segments were developed. Late Cretaceous faults strike predominantly to the NW, whereas NE-trending faults comprise Triassic structures that were reactivated in a later stage. Our work provides scarce evidence for the escape of hydrocarbons in the Samson Dome. In addition, fault analyses based on present-day stress distributions indicate a tendency for ‘locking’ of faults at depth, with the largest leakage factors occurring close to the surface. The Samson Dome is an analogue to salt structures in the Barents Sea where oil and gas exploration has occurred with varied degrees of success.

Faulting and folding beneath the Canterbury Plains identified prior to the 2010 emergence of the Greendale Fault

Prior to the 2010–2011 earthquake sequence, several fault and fold structures were mapped beneath the Canterbury Plains using seismic reflection surveys and surface observations and depicted on the Christchurch and Aoraki 1:250,000 scale geological maps. Localised grabens associated with east-southeast-striking normal faults formed largely during the Late Cretaceous. South of Rakaia River, some graben-bounding faults show minor normal offset extending into the late Cenozoic. Near Ashley River, proximity to a Late Cretaceous–Paleogene graben suggests that the active, predominantly contractional, east-striking Ashley Fault is at least in part a rejuvenated pre-existing normal fault. The easterly strike of the previously unknown Greendale Fault implies that it too may be a reactivated Late Cretaceous fault. Northeast-striking, southeast-facing reverse faults and fault-propagation folds beneath the western and northern parts of the plains are primarily late Cenozoic features. Variation in the distributions of Miocene sedimentary strata strongly suggests that contractional faulting was initiated as early as the Miocene. The overall late Cenozoic tectonic pattern is extension beneath the southern Canterbury Plains and contraction farther north.

Characteristics of late cretaceous faults of the eastern segment of the northern Qaidam basin, NW China

The Late Cretaceous tectonic upheaval was an important event during the evolution of the Qaidam Basin, resulting in the omission of the Upper Cretaceous in the whole basin and unconformities between the Paleogene sequence and pre-K 2 strata. Integrating geological and geophysical data, two different groups of Late Cretaceous faults were recognized in the study area, one group consisting of E-W extending strike-slip faults (e.g., the Maxian and Yema-Jinan faults in the Mahai area, which caused an E-W omission zone of Mesozoic), while the other one has NW-SE thrust faults, resulting in NW-SE fold-and-thrust belts. Considering the different strikes and scale of these two groups, a simple-shear model has been employed to explain this structural phenomenon. The NW-SE thrust faults were thought to be subsidiary to the E-W strike-slip faults. Putting this into the framework of the Cretaceous paleogeographic environment of central Asia, it is inferred that this tectonic event of the Qaidam Basin is a response to the continuous northward drifting of the India plate.

Geochronological constraints on the evolution of the Periadriatic Fault System (Alps)

Fault rocks from various segments of the Periadriatic fault system (PAF; Alps) have been directly dated using texturally controlled Rb–Sr microsampling dating applied to mylonites, and both stepwise-heating and laser-ablation 40Ar/39Ar dating applied to pseudotachylytes. The new fault ages place better constraints on tectonic models proposed for the PAF, particularly in its central sector. Along the North Giudicarie fault, Oligocene (E)SE-directed thrusting (29–32 Ma) is currently best explained as accommodation across a cogenetic restraining bend within the Oligocene dextral Tonale-Pustertal fault system. In this case, the limited jump in metamorphic grade observed across the North Giudicarie fault restricts the dextral displacement along the kinematically linked Tonale fault to ~30 km. Dextral displacement between the Tonale and Pustertal faults cannot be transferred via the Peio fault because of both Late Cretaceous fault ages (74–67 Ma) and sinistral transtensive fault kinematics. In combination with other pseudotachylyte ages (62–58 Ma), widespread Late Cretaceous–Paleocene extension is established within the Austroalpine unit, coeval with sedimentation of Gosau Group sediments. Early Miocene pseudotachylyte ages (22–16 Ma) from the Tonale, Pustertal, Jaufen and Passeier faults argue for a period of enhanced fault activity contemporaneous with lateral extrusion of the Eastern Alps. This event coincides with exhumation of the Penninic units and contemporaneous sedimentation within fault-bound basins.

Mesozoic fault systems, deformation and fault block rotation in the Andean forearc: a crustal scale strike-slip duplex in the Coastal Cordillera of northern Chile

In this paper we discuss the evolution and tectonic significance of the Mesozoic trench-parallel fault systems which affected the Coastal Cordillera and their relation to magmatism and crustal rotation. The oldest, extensional, fault system separates basement from rift-related Late Triassic and younger sedimentary units. This system [I] subsequently developed into a wider extensional fault system which acted as the locus of magma ascent and emplacement of the Coastal Batholith during much of the Jurassic to earliest Cretaceous period. This extensional fault system defined the forearc sliver during this period and was the consequence of a retreating subduction boundary. During the Early Cretaceous (c. 132–125 Ma) the kinematics of this fault system changed to transtension [II] and accommodated a major component of left-lateral strike-slip motion, the principal fault being the Atacama Fault Zone along which plutons continued to be emplaced. The final phase of pluton emplacement within the Coastal Cordillera appears to be c. 106 Ma, after which this magmatic arc and fault system was abandoned. An Late Cretaceous arc and fault system [III] developed some 20 Ma later and located some 50 km to the east in what is now the Central Valley of northern Chile. This paper seeks to show that the Coastal Cordillera was deformed as a whole by this Late Cretaceous fault system [III] which formed a crustal-scale left-lateral transpressional duplex. During this deformation the thermally weakened crust was dissected into a series of large-scale blocks bounded by NW-trending left-lateral strike-slip faults which merge into a NNE–SSW fault zone which forms the eastern boundary to the duplex. We term this eastern boundary zone the Central Valley Fault Zone (CVFZ) and this together with the NW-trending faults defines the duplex system which we refer to as a whole as the Coastal Cordillera Fault System (CCFS) [III]. We have traced the CCFS duplex between 25°S and 29°S and suspect that it continues northward. The timing of the deformation is constrained to be post 106 Ma, the age of Coastal Cordillera arc abandonment, and pre-Tertiary based on the deformation and pluton emplacement in and along the Central Valley Fault Zone. Palaeomagnetic data from the fault bound blocks within the CCFS duplex indicate 35°–45° of post-Early Cretaceous clockwise rotation with no substantial latitudinal motion. We suggest that the observed fault kinematics of the CCFS are consistent with this crustal-scale duplex model where rotations would have occurred in response to left-lateral transpression.

The transition from passive to active margin tectonics: a case study from the zone of Samedan (eastern Switzerland)

The Zone of Samedan is part of a fossil, early Mesozoic rift system originally situated in the distal, Lower Austro-Alpine domain of the Adriatic passive continental margin. An early Mesozoic configuration of asymmetrical rift basins bounded by relative structural highs compartmentalized Late Cretaceous active margin tectonics; Jurassic half-grabens were folded into arcuate synclines, whereas relative structural highs engendered thin, imbricated thrust sheets. West-directed thrusting and folding initiated at the surface and continued to depths favoring mylonitization under lower greenschist-facies conditions. At this time Liguria-Piemontese ophiolites were accreted to Lower Austro-Alpine units directly underlying the Zone of Samedan. Late Cretaceous orogenic collapse of the Adriatic active margin involved the reactivation of west-directed thrusts as low-angle, top-to-the-east, normal faults. These faults accommodated extensional uplift of Liguria-Piemontese ophiolites and Lower Austro-Alpine units beneath and within the Zone of Samedan. During Paleogene collision, some Late Cretaceous faults in the Zone of Samedan were reactivated under lower anchizonal conditions as north-directed thrusts. The latter stages of this early Tertiary thickening were transitional to brittle, high-angle normal faulting associated with top-to-the-east extension and spreading above the warm, uplifting Lepontine dome.

A multiphase plate tectonic history of the southeast continental margin of Oman

Abstract The major basins of the western Indian Ocean formed during the Late Jurassic separation of India-Madagascar-Antarctica from Africa-Arabia. From north to south these are the Owen, North Somali, West Somali, and Mozambique Basins. Mesozoic magnetic anomalies confirm that these latter two regions have remained tectonically unchanged; by contrast, anomalies have yet to be identified in the Owen and North Somali Basins, leaving open the possibility that one or both have been overprinted by subsequent spreading events. Nonetheless, many researchers have chosen to assume that the basement within the Owen Basin is an unaltered fragment of Jurassic to Early Cretaceous oceanic crust, and that SE Oman has been a passive margin since its rifting from India. Marine geophysical data acquired aboard R/V Robert D. Conrad during 1986 provides new information that suggests a more complex regional plate history must be developed. Seismic reflection profiles collected across the Owen Basin reveal structure that is not consistent with a Jurassic age. First, we observe that basement in the Owen Basin is more than 1 km shallower than should be expected from thermal subsidence since Jurassic time. Second, the age of sediments in the Owen Basin can be estimated by tracing them seismically to DSDP Site 224 on the Owen Ridge. Both of these procedures argue for the Owen Basin being no older than Late Cretaceous. Seismic profiles along the SE Oman Margin near 18°N show complex deformational features that are incompatible with a simple passive margin history. A thick sedimentary basin bounded by basement-involved faults was mapped beneath the continental slope. Offsets reach to very near the seafloor, suggesting they have been active in the recent past. We acquired numerous crossings of a basement feature 75 km SW of Ra’s Madrakah and along the strike of ophiolites exposed there and at Masirah Island. The magnetic, seismic, and structural characteristics of this feature are all consistent with its being a buried, along-strike extension of this same slice of oceanic crust. To reconcile these observations with well-established facts concerning the opening history of the NW Indian Ocean, we propose the following scenario. First, a proto-Owen Basin formed in Jurassic time when India-Madagascar-Antarctica separated from Africa-Arabia. However, following the Late Cretaceous separation of India from Madagascar, the new spreading ridge was offset along the SE Oman margin, not at the Owen Ridge as is assumed by many. During this stage of tectonic development, the proto-Owen Basin crust was carried northeastward towards eventual collision and subduction with Asia. The result was a strike-slip Oman margin adjacent to a Late Cretaceous Owen Basin. Ophiolites now found along the SE coast of Oman are fragments of this Late Cretaceous Owen Basin crust, and were probably emplaced as a result of transpressional motion along this plate boundary. This arrangement persisted until yet another spreading centre opened the Gulf of Aden in Neogene time. The latter reorganization of plate geometries led to major compressive stresses across the Owen Basin that resulted in elevation of the Owen Ridge (readily seen by way of stratal relationships in profiles tied to DSDP Site 224), and reactivation of Late Cretaceous faults along the Oman Margin. We anticipate that verification of this multiple-phase history will emerge from future comparisons between post-Late Cretaceous tectonism within SE Oman and our marine seismic and drilling data.

A preliminary assessment of the hydrocarbon potential of the Larsen Basin, Antarctica

The Larsen Basin, on the northwest margin of the Weddell Sea, formed as a Mesozoic ensialic basin during Gondwana breakup. Deposition was either in half grabens on the extending Weddell Sea margin, or in a restricted back-arc basin. At the northern end of this basin 5–6 km of sedimentary rock crop out on James Ross Island, exposing elements of a large potential hydrocarbon system. Aeromagnetic and outcrop data suggest that the basin structure inferred from James Ross Island can be recognised at least as far south as 70°S. Upper Jurassic anoxic marine strata, deposited prior to the main phase of arc development, form a rich potential source (T.O.C. up to 3.5%) with both marine and terrestrial kerogens. Arc-derived volcaniclastic sediments of Barremian — Oligocene age form a regressive megasequence. Basal strata represent slope apron and rudaceous submarine fan deposits proximal to the margin; fan conglomerates form lenticular bodies hundreds of metres thick and tens of kilometres across, enveloped in slope-apron mudstones. Late Cretaceous fault reactivation and uplift led to dramatic shallowing of the basin, with deposition of shelf facies. Although there are many potentially attractive reservoir targets, there may be problems of pore occlusion due to the abundant labile volcanic grains. However, there is evidence of more quartzose sandstone towards the top of the section, and, inferentially, toward the basin centre. In the Larsen Basin, there is moderate potential for oil generated from Upper Jurassic source rocks and reservoired in Cretaceous and Tertiary sandstones and conglomerates, in large stratigraphic or structural traps caused by partial basin inversion during deposition.

Structural geology and accretionary history of the Northwest Cascades system, Washington and British Columbia

Pre-Tertiary rock units on the western flank of the Cascades in Washington and southern British Columbia constitute the Northwest Cascades system (NWCS). This litho-tectonic package is characterized by predominantly oceanic lithologies subjected to high P/T metamorphism and structurally mixed to form a regional melange. Although the focus of this report is on rocks of the NWCS as originally defined, survey of the regional geology suggests broadening of the limits of this system to include the San Juan Islands, the Pacific Rim complex, and pre-Tertiary rocks on the western flank of the central and southern Washington Cascades. The Shuksan Metamorphic Suite, a relatively coherent and broadly exposed unit of the NWCS, contains a regionally consistent stretching lineation which provides evidence of east-northeast–directed underthrusting (subduction) during the Early Cretaceous blue-schist metamorphism. Metamorphic F2 folds are approximately normal to the stretching lineation and appear to have developed late in the same deformational event. The melange structure postdates the high-pressure regional metamorphism. It is characterized by a tectonic assemblage of diverse and unrelated rock units occurring as fragments as much as 10 or more kilometres in breadth and separated by a network of low- to high-angle north-northwest–striking faults. Pre-fault restoration is impossible. Argillaceous or serpentinitic material commonly occurs in deformation zones between more competent lithologies. Mylonite zones as much as several metres thick are widely developed and display stretched clast lineations, indicating a north-northwest–south-southeast transport direction for both low- and high-angle faults. The high-angle faults are therefore strike-slip in nature. Shear-sense indicators are mixed in thrust fault zones, but many indicate dextral movement for the strike-slip faults. K/Ar whole-rock ages of mylonites range from 20–127 Ma, but two Late Cretaceous ages of 87 and 91 ± 3 Ma are considered as best representing the time of faulting. The inferred shift in direction of tectonic transport from east-northeast–west-southwest during Early Cretaceous metamorphism to north-northwest–south-southeast during Late Cretaceous faulting and melange formation is correlated with a mid-Cretaceous change in plate interaction between the North American and Farallon/Kula plates from high- to low-angle convergence. The NWCS melange is interpreted to have formed during large dextral transform motion along the continental margin and to have reached final emplacement by thrusting against the southern end of Wrangellia and the Coast Plutonic Complex.

Cenozoic faulting in the vicinity of the Charleston, South Carolina, 1886 earthquake

Multichannel seismic-reflection profiles in the meizoseismal area of the 1886 Charleston earthquake and nearby offshore area show evidence of Cenozoic faulting. Deformation observed on two land profiles defines a northeast-striking reverse(?) fault zone named the Cooke fault. A Jurassic-age basalt layer at 750 m depth has 50 m vertical displacement, southeast side down. Overlying Upper Cretaceous and Cenozoic beds have decreasing displacement with decreasing depth. These displacements, coupled with larger (∼ 190 m) displacements of reflectors below the basalt, indicate continuing Cenozoic movement of a post–basalt-flow, pre–Late Cretaceous fault into Eocene time or later. The fault extends into a cluster of 1973–1978 epicenters; this suggests a possible causal relationship with seismicity. Marine multichannel seismic-reflection data combined with single-channel, high-resolution reflection profiles reveal Cenozoic faulting beneath the continental shelf near Charleston. A reverse fault 12 km offshore, named the Helena Banks fault, extends for at least 30 km and is observed as shallow as 10 m below the sea bottom, with the most recent movement in post-Miocene or Pliocene time. There is no known seismicity in the area of this fault. The reverse movement on the Helena Banks and Cooke faults is consistent with a northwest-southeast compressional stress regime. Elucidation of the relation, if any, between reactivated Triassic or older structures and/or a decollement (defined by diffractions at 11.4 ± 1.5 km depth) on the one hand and seismicity on the other will be important in assessing earthquake risk in many areas of similar tectonic setting outside of the Charleston region in the Atlantic coastal and continental margin area.

A transgressive shelf sequence exhibiting hummocky stratification; the Cape Sebastian Sandstone (Upper Cretaceous), southwestern Oregon

ABSTRACT In the tectonically active Circum-Pacific Belt, thick transgressive sequences are not uncommon, in contrast with their rarity in the Cretaceous of the Western Interior. Thick transgressive sediment packages reflect rapid sedimentation rates but even more rapid rates of relative sea-level rise. A well-exposed example is the Cape Sebastian Sandstone, a 200-m thick, fining-upward sequence representing foreshore to offshore deposition. Progressively increasing depth of deposition is indicated by both physical and biogenic sedimentary structures in the Cape Sebastian Sandstone. Four facies make up the Cape Sebastian Sandstone. The lowest unit is a basal, shelly, boulder conglomerate overlain by trough cross-bedded pebbly sandstone, plane-laminated coarse sandstone, and crudely graded conglomerates. A single type of subvertical trace fossil is locally abundant. These sediments represent beach to nearshore deposition. The middle and thickest part of the formation comprises hummocky-bedded sandstone. divided into a lower hummocky-bedded facies and an upper hummocky-bedded and burrowed facies. Grain size, frequency of pebble lenses, and thickness of hummocky laminae all decrease upward through this part of the sequence. Conversely, burrowed zones, diversity of burrows, plane-laminated zones, plant debris, and symmetrical-ripple preservation increase upward. These sediments record storm-influenced, inner-shelf sedimentation. The uppermost part of the formation consists of alternating laminated, very fine sandstone and progressively thicker, burrowed sandy siltstone. Increased trace-fossil size and number and abundant plant debris characterize these sediments, which represent outer-shelf deposition. Modern examples of the structures described above have been observed on the Oregon and California shelves, supporting the hypothesis that the Cape Sebastian Sandstone represents a transgressive shelf sequence. The same structures have also been described in progradational (regressive) sequences in the Cretaceous of the Western Interior, where thick transgressive sequences are rare or absent. Evidence for Late Cretaceous faulting in southwestern Oregon supports the proposition that thick transgressive sequences may be deposited in tectonically active regions.

Retrogradational Shelf Sequence--Upper Cretaceous (Campanian-Maestrichtian) Cape Sebastian Sandstone, Southwestern Oregon: ABSTRACT

In the tectonically active Circum-Pacific belt, thick retrogradational (transgressive) sequences are not uncommon, in contrast with the Cretaceous of the Western Interior. Retrogradational sediment packages reflect rapid sedimentation rates but even more rapid rates of relative sea-level rise. A well-exposed and convincingly documented example is the Cape Sebastian Sandstone, a 250-m-thick, fining-upward sequence representing foreshore to offshore accumulation. Progressively increasing depth of deposition is reflected in both physical and biogenic sedimentary structures in the Cape Sebastian Sandstone. The basal, shelly boulder conglomerate is overlain by trough-cross-bedded pebbly sandstones, plane-laminated coarse-grained sandstones, and graded conglomerates. A single, subvertical trace-fossil type is locally abundant. These sediments represent foreshore to nearshore depositional environments. The middle and bulk of the formation comprises hummocky cross-stratified sandstones. Grain size, frequency of pebble lenses, and thickness of hummocky laminae decrease upward. Burrowed zones, diversity of burrows, plane-laminated zones, plant debris, and oscillation-ripple preservation increase upward. These sediments reflect storm-influenced, inner-shelf sedimentation. The uppermost part of the formation consists of alternating laminated, fine-grained sandstones and progressively thicker, burrowed sandy siltstones. Increased trace-fossil size and diversity, as well as abundant plant debris, characterize these sediments, which represent an outer-shelf sedimentary environment. Modern examples of the described structures have been observed off the Oregon and California coasts, corroborating the hypothesis that the Cape Sebastian Sandstone represents a retrogradational shelf sequence. Also, evidence for Late Cretaceous faulting in southwestern Oregon supports the proposition that thick, retrogradational sequences may be deposited in tectonically active regions. End_of_Article - Last_Page 422------------