Half-graben Basins Research Articles

Early Jurassic extensional basin formation in the Daqing Shan segment of the Yinshan belt, northern North China Block, Inner Mongolia

Structural and stratigraphic studies of the western Daqing Shan segment of the Yinshan belt have recognized an Early Jurassic extensional episode supported by several lines of evidence. First, normal faults cut the lowermost Jurassic sequence and are overlapped by younger Lower Jurassic rocks. Second, Lower Jurassic rocks include growth strata in small-scale graben at the base of the Jurassic basin. Third, rapid lateral facies changes are mapped from boulder conglomerates along the basin-bounding faults to lacustrine and meandering fluvial rocks in the basin center. Fourth, paleodrainage systems provided sediment input from three sides of the basin, two transverse and one axial. Finally, there is a strongly asymmetric distribution of coarse proximal and fine distal facies within the basin. The Early Jurassic extensional episode was responsible for formation of an east-trending half-graben basin in the western Daqing Shan in which at least 1800 m of syn-extensional nonmarine sediments accumulated and are preserved. Previous studies in the Triassic and Jurassic of other parts of northwest and north-central China have concluded that the early Mesozoic was a time of continental amalgamation and contractile deformation. The recognition of an Early Jurassic extensional episode along the northern margin of the North China Block is problematic in this context, at least superficially. We propose two possible explanations for the Early Jurassic extension: transtension associated with strike-slip tectonics, or gravitational collapse of a pre-existing Late Paleozoic–Early Mesozoic contractile orogenic belt. The first possibility, transtensional deformation, is problematic because specific strike-slip faults have not been identified that could control extension in the western Daqing Shan. However, several lines of evidence allow the possibility of such a driving mechanism, including: documented Early Jurassic transtensional systems in the southwestern North China Block, several candidate strike-slip faults along the China–Mongolia border region, structural discontinuity between the Daqing Shan and southern Mongolia, and along strike-changes in structural style within the Yinshan belt. The second possibility, orogenic collapse, is similarly difficult to establish because of the limited amount of data concerning the regional distribution and orientation of pre-Jurassic contractile structures and Early Jurassic extensional structures. However, documented Late Paleozoic–Triassic contractile deformation, as well as the close temporal and spatial association, and parallelism between Early Jurassic extensional structures and older contractile structures in the western Daqing Shan requires consideration of gravitational collapse as a driving mechanism for Early Jurassic extension in the Yinshan belt.

Formation, evolution, and inversion of the middle Tertiary Diligencia basin, Orocopia Mountains, southern California

The Diligencia basin, located in the eastern Orocopia Mountains of southern California, contains 1500–2000 m of Oligocene−Miocene continental, siliciclastic sedimentary rocks and subordinate limestone and evaporite deposits, intercalated with basaltic lavas. These rocks are locally intensely folded and faulted, defining in present-day coordinates an elongate east-west−trending basin, and are unconformably overlain by flat-lying late Pliocene(?) and Pleistocene alluvial deposits. The sedimentological, stratigraphic, and structural history of the basin is compatible with late Oligocene−early Miocene formation as a half-graben basin produced by orthogonal, Basin and Range−type extension, and latest Miocene to Pliocene basin inversion in either a localized transpressional Transverse Range regime, or a more diffuse compressional regime associated with north-south shortening of the entire Mojave block. Facies associations in the lower part of the Diligencia basin display a distinctly asymmetric distribution across the basin, indicating deposition in a half-graben with a steep, possibly fault controlled, south-facing northern escarpment and a more gentle north-facing southern slope. Paleocurrent data, particularly from high-energy deposits on the northern basin margin, indicate stream flow toward the southeast to south-southeast, oblique to the deformed basin margins, and suggest an approximate northeast to east-northeast strike for the basin before deformation. Previously published paleomagnetic data, however, indicate that the elongate, currently east-west−trending, fault block containing the Diligencia basin has rotated clockwise by as much as 110° about a vertical axis (angle depends on data and model used) since deposition ceased. If correct, this would suggest that the basin may have originally opened to between the northeast and east-northeast, subparallel to well-documented extension directions in Miocene age basins in the Mojave Desert to the east. Clockwise rotation on east-west fault blocks exposed in this region has previously been bracketed as between ca. 10 and 4.5 Ma. Structural and paleomagnetic data indicate that inversion of the Diligencia basin occurred after block rotation, implying a latest Miocene to Pliocene age for inversion. We speculate that basin inversion within a north-south (and still active) compressive stress field resulted from the locking, and subsequent internal deformation, of this previously passively rotating elongate crustal block.

Lower Miocene Nukhul Formation, Gebel el Zeit, Egypt: Model for structural control on early synrift strata and reservoirs, Gulf of Suez

The Aquitanian-early Burdigalian (lower Miocene) Nukhul Formation at Gebel el Zeit, Egypt, was deposited during early stages of Gulf of Suez rifting. The unit dips 8-15° less than underlying prerift strata, indicating that significant rotation and extension preceded subsidence of the Gebel el Zeit fault block. The Nukhul Formation at Gebel el Zeit is up to 75 m thick in outcrop and consists of a lower sandstone and an upper carbonate unit. The formation varies considerably along strike because of syndepositional differential movement of small fault-bounded blocks. The lower clastic unit at South Gebel el Zeit contains poorly sorted, conglomeratic, marly sandstone that commonly displays grading and Bouma sequences. Beds were deposited below storm base by sediment gravity flows. Thicker intervals are inferred to fill small, structurally controlled, submarine gullies that funneled sand and gravel southwestward to a half-graben basin. In contrast, an inferred correlative, thin, basal conglomeratic unit in North Gebel el Zeit was deposited in a shallow-marine setting. The presence of basement clasts in Nukhul strata indicates early syndepositional uplift due to structural tilting. The upper carbonate unit consists of bioclast, peloid, and intraclast packstone, wackestone, and grainstone with minor floatstone, rudstone, and coral-algal boundstone. Carbonate strata were deposited variously in deep-marine, low-energy peritidal and subtidal, and reefal environments. Deeper submerged blocks were the site of carbonate resedimentation or deeper shelf deposition. Reefs and shallow-marine bioclast shoals formed on higher submerged blocks. Nukhul strata show that synrift reservoir prediction in the Gulf of Suez, the Red Sea, and presumably in other rifts requires mapping of synrift cross faults and fault block by fault block facies analysis. Robert Winn is a consulting geologist. He received a Ph.D. from the University of Wisconsin-Madison in 1975 and joined Marathon Oil Company as a research geologist in 1977. He left Marathon in 1994 for the University of Papua New Guinea, where he was senior lecturer and then associate professor. He was head of the Geology Department from 1996 to 2000. His primary interests are in clastic sedimentology and sequence stratigraphy.Paul Crevello is a consulting geologist and technical director of PetrexAsia, which he formed in 1997. He received an M.S. degree from the University of Miami (1978) and a Ph.D. from Colorado School of Mines (1989). He was employed by Marathon Oil as a research geologist from 1978 to 1994 and by the University of Brunei as senior lecturer from 1994 to 1997. His specialties are in sequence stratigraphy and sedimentology of carbonate and turbidite systems and the integrated calibration of 3-D reservoir models. William Bosworth is employed by Marathon Oil Company. He joined Marathon in 1984 and has worked principally on international exploration and development projects. From 1980 to 1984 he taught structural geology and tectonics at Colgate University in Hamilton, New York. His research interests are principally in extensional tectonics, continental stress field evolution, and the paleogeodynamics of the African plate.

Tectonique synsedimentaire d'age cretace superieur en Tunisie nord orientale; blocs bascules et reorganisation des aires de subsidence

Abstract Structural analysis of late Cretaceous sequences from the northeastern Tunisian Atlas, led to conclude on an active basin floor instability. Regional tectonics resulted in tilted blocks with a subsidence reorganization, since the Campanian time. These structural movements are controlled both by N140 and N100-120 trending faults. The Turonian-Coniacian and Santonian sequences display lateral thickness and facies variation, due to tectonic activity at that time. During Campanian-Maastrichtian, a reorganization of the main subsidence areas occurred, the early Senonian basins, have been sealed and closed and new half graben basins developed on area which constituted previously palaeohigh structures. These syndepositional deformations are characterized by frequent slumps, synsedimentary tilting materials, sealed normal faults and progressive low angle unconformities. These tilted blocks combined to a subsidence axis migration were induced by a NE-SW trending extensional regime. This extension which affects the Tunisian margin during the Upper Cretaceous, is related to the Tethyan and Mesogean rifting phase which resulted from the combined movements of the African and European plates.

A magnetotelluric model of the Mana Pools basin, northern Zimbabwe

The Mana Pools sedimentary basin lies within the Zambezi mobile belt in northern Zimbabwe. New and preexisting magnetotelluric data and the available seismic reflection data are used to constrain the basin structure and the depth to the electrical basement. Long‐period magnetotelluric (LMT) data were collected at five stations along a 60 km north‐south profile across the Mana Pools basin and onto the southern escarpment. These data augment an existing audiofrequency (AMT) data set from 11 sites in the same area. The subsurface apparent resistivities measured at periods sampling the basin are very low (a few Ωm). After processing both data sets, the estimated impedance tensor is decomposed, showing that the resistivity structure of the Mana Pools basin can be modeled two dimensionally. The ρ+ algorithm is used to show that there is no systematic offset in magnitude between the AMT and LMT data sets before they are combined. Minimum structure resistivity models of the Mana Pools basin compare well with the information from reflection seismic data and support its previous description as a half graben basin of ∼7 km depth. The excellent conductor in the Mana Pools basin is quite different to those seen elsewhere in the orogenic belt in that it is a feature of the sedimentary fill rather than the basement. The resistivity of the basement is low but no localized good conductor is observed; these low resistivities may result from a high degree of either chemical or tectonic alteration to the underlying rocks due to metamorphic processes and tectonic disruption during rift formation.

Stratigraphic and structural evolution of the Selenga Delta Accommodation Zone, Lake Baikal Rift, Siberia

Seismic reflection profiles from the Lake Baikal Rift reveal extensive details about the sediment thickness, structural geometry and history of extensional deformation and syn-rift sedimentation in this classic continental rift. The Selenga River is the largest single source of terrigenous input into Lake Baikal, and its large delta sits astride the major accommodation zone between the Central and South basins of the lake. Incorporating one of the world's largest lacustrine deltas, this depositional system is a classic example of the influence of rift basin structural segmentation on a major continental drainage. More than 3700 km of deep basin-scale multi-channel seismic reflection (MCS) data were acquired during the 1989 Russian and the 1992 Russian–American field programs. The seismic data image most of the sedimentary section, including pre-rift basement in several localities. The MCS data reveal that the broad bathymetric saddle between these two major half-graben basins is underlain by a complex of severely deformed basement blocks, and is not simply a consequence of long-term deltaic deposition. Maximum sediment thickness is estimated to be more than 9 km in some areas around the Selenga Delta. Detailed stratigraphic analyses of the Selenga area MCS data suggest that modes of deposition have shifted markedly during the history of the delta. The present mode of gravity- and mass-flow sedimentation that dominates the northern and southern parts of the modern delta, as well as the pronounced bathymetric relief in the area, are relatively recent developments in the history of the Lake Baikal Rift. Several episodes of major delta progradation, each extending far across the modern rift, can be documented in the MCS data. The stratigraphic framework defined by these prograding deltaic sequences can be used to constrain the structural as well as depositional evolution of this part of the Baikal Rift. An age model has been established for this stratigraphy, by tying the delta sequences to the site of the Baikal Drilling Project 1993 Drill Hole. Although the drill hole is only 100 m deep, and the base of the cores is only ∼670 ka in age, ages were extrapolated to deeper stratigraphic intervals using the Reflection-Seismic-Radiocarbon method of Cohen et al. (1993). The deep prograding delta sequences now observed in the MCS data probably formed in response to major fluctuations in sediment supply, rather than in response to shifts in lake level. This stratigraphic framework and age model suggest that the deep delta packages developed at intervals of approximately 400 ka and may have formed as a consequence of climate changes affiliated with the northern hemisphere glaciations. The stratigraphic analysis also suggests that the Selenga Basin and Syncline developed as a distinct depocentre only during the past ∼2–3 Ma.

Tectonic and geomorphic controls on the fluvial styles of the Eslida Formation, Middle Triassic, Eastern Spain

The Eslida Fm is an alluvial unit of the Buntsandstein Facies of the Iberian Ranges deposited in a half-graben basin over a period of 2–2.5 Ma during the lower Anisian (Middle Triassic). It can be subdivided into six cycles that can be correlated along the basin. The overall architecture and cyclicity were controlled by the episodic activity of the basin boundary fault. Two proximal source areas of minor importance and a distal one of major importance are identified. The petrological composition of the sandstones is arkosic to subgrauwackic, indicating source areas containing both low-metamorphic rocks and gneiss, and granitic rocks. Alluvial fans, very common in rift basins, were not developed because of slow fault scarp retreat and the lack of large antecedent rivers cutting across the rift shoulders. Paleocurrents point to the SE, except for a zone in the middle part of the basin where interference with a NE–SW fault system deflected the channels to the S. The infilling of the basin is asymmetric in a transverse section, with channel facies near the basin boundary fault to the NE and overbank facies in the distal SW part.

Role of a ductile décollement in the development of pull-apart basins: Experimental results and natural examples

Traditional models describe pull-apart basins as graben or half-graben basins with normal or normal-oblique slip master faults, analogous to Death Valley, California. Yet many pull-aparts are characterized by asymmetric basins with strike-slip master faults indicating that not all pull-apart basins conform to the simple Death Valley models. We present analogue modelling results that show developmental sequences and structural styles of pull-aparts are dramatically different when overburden rides over a ductile horizon, and that thickness of the ductile horizon exerts control on basin development. In our models, synthetic and antithetic strike-slip faults control basin geometries, while localized normal faulting and local oblique slip on strike-slip faults accommodate basin subsidence. Faults evolve from initial strike-slip to normal-oblique and normal dip slip to form a system of either isolated sub-basins in the case of thick ductile layers, or coalescing sub-basins in the case of thin ductile layers. These results demonstrate distinct differences between non-ductile and ductile décollement pull apart structures. Basin boundaries dominated by normal faults suggest a décollement within or at the base of a non-ductile layer similar to Death Valley, California. Basin bounding faults dominated by strike-slip and oblique-slip faults indicate basin formation over a ductile layer, similar to the Gulf of Elat (Aqaba) or Gulf of Paria (Venezuela and Trinidad).

Using basement-hosted clastic dykes as syn-rifting palaeostress indicators: an example from the basal Stoer Group, northwest Scotland

Clastic infills of fractures (here termed clastic veins) in basement rocks immediately underlying sedimentary cover sequences can be used to date fault movements if these demonstrably occurred at the time of infilling or prior to the lithification of the entrained clastic material. This allows reconstruction of the syn-rifting palaeostress system using stress inversion techniques. During Riphean intracontinential rifting of Laurentia, the Torridonian Stoer Group sediments of northwest Scotland were deposited in half-graben basins controlled by faults, e.g. the Coigach and Clachtoll faults. At Clachtoll, northeast–southwest oblique sinistral normal faulting in the underlying basement is associated with extensive development of shear, hybrid and tensile clastic veins filled with Stoer Group sediment, infilled and deformed prior to sediment lithification. Clastic veins initially formed by gravitational infilling of sediment from above, followed by tectonically-driven, forceful hydraulic injection of fluidized sand into new fractures and reactivated pre-existing basement faults. Palaeostress axes, determined from fault lineation data and tensile fracture extension directions in the Clachtoll Fault zone, indicate west-northwest–east-southeast directed extension during rifting. On a regional scale, this implies oblique-dextral extension on the north- to north-northeast-trending Coigach Fault during Stoer Group deposition. Similar orientations, age relationships and kinematics have been obtained from pre-Torridon Group fault arrays developed in the Lewisian basement near Gairloch and Loch Maree. Overall, the faulting patterns reflect a three-dimensional strain (k≠1) formed by east-southeast–west-northwest-directed extension during deposition of the Stoer Group. More speculatitively, asymmetric density patterns of sinistral and dextral faults may indicate that rifting occurred in a regional zone of broadly north–south-oriented dextral transtension.

Tectonic and sediment supply control of deep rift lake turbidite systems: Lake Baikal, Russia

Research Article| February 01, 1999 Tectonic and sediment supply control of deep rift lake turbidite systems: Lake Baikal, Russia C. Hans Nelson; C. Hans Nelson 1U.S. Geological Survey, Menlo Park, California 94025, USA Search for other works by this author on: GSW Google Scholar Evgeny B. Karabanov; Evgeny B. Karabanov 2Department of Geological Science, University of South Carolina, Columbia, South Carolina 29208, USA Search for other works by this author on: GSW Google Scholar Steven M. Colman; Steven M. Colman 3U.S. Geological Survey, Woods Hole, Massachusetts 02543, USA Search for other works by this author on: GSW Google Scholar Carlota Escutia Carlota Escutia 4Ocean Drilling Program, Texas A & M University, College Station, Texas 77845, USA Search for other works by this author on: GSW Google Scholar Geology (1999) 27 (2): 163–166. https://doi.org/10.1130/0091-7613(1999)027<0163:TASSCO>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation C. Hans Nelson, Evgeny B. Karabanov, Steven M. Colman, Carlota Escutia; Tectonic and sediment supply control of deep rift lake turbidite systems: Lake Baikal, Russia. Geology 1999;; 27 (2): 163–166. doi: https://doi.org/10.1130/0091-7613(1999)027<0163:TASSCO>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search nav search search input Search input auto suggest search filter All ContentBy SocietyGeology Search Advanced Search Abstract Tectonically influenced half-graben morphology controls the amount and type of sediment supply and consequent type of late Quaternary turbidite systems developed in the active rift basins of Lake Baikal, Russia. Steep border fault slopes (footwall) on the northwest sides of half-graben basins provide a limited supply of coarser grained clastic material to multiple small fan deltas. These multiple sediment sources in turn laterally feed small (<10 km diameter) unchannelized sublacustrine sand-rich aprons that are deposited at the slope base on the lake floor. Gradual slopes of the southeastern ramp margins (hanging wall) of the lake basins, conversely, feed finer grained sediment from larger drainages into two different types of channelized turbidite sublacustrine fan systems: (1) small (5–20 km) laterally fed sand-rich fans sourced by local rivers, often originating from glaciated valleys; and (2) large (>65 km) axially fed elongate mud-rich fans sourced by regional exterior drainage of the Selenga River that supplies large quantities of silt. Basin plain turbidites in the center of the linear basins and axial channels that are controlled by rift-parallel faults are fed from, and interfinger with, aprons and fans. The predictability of the turbidite systems in Lake Baikal provides the best example yet studied of how tectonics and sediment supply interact to control the development of a wide variety of coeval turbidite systems on a single basin floor. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not currently have access to this article.

Tectonic and sediment supply control of deep rift lake turbidite systems: Lake Baikal, Russia: Comment and Reply

Tectonically influenced half-graben morphology controls the amount and type of sediment supply and consequent type of late Quaternary turbidite systems developed in the active rift basins of Lake Baikal, Russia. Steep border fault slopes (footwall) on the northwest sides of half-graben basins provide a limited supply of coarser grained clastic material to multiple small fan deltas. These multiple sediment sources in turn laterally feed small ( 65 km) axially fed elongate mud-rich fans sourced by regional exterior drainage of the Selenga River that supplies large quantities of silt. Basin plain turbidites in the center of the linear basins and axial channels that are controlled by rift-parallel faults are fed from, and interfinger with, aprons and fans. The predictability of the turbidite systems in Lake Baikal provides the best example yet studied of how tectonics and sediment supply interact to control the development of a wide variety of coeval turbidite systems on a single basin floor.

New Early Cretaceous palaeomagnetic pole from Córdoba Province (Argentina): revision of previous studies and implications for the South American database

SUMMARY A continental sequence of red beds and interbedded basaltic layers crops out in the Sierra Chica of Cordoba Province, Argentina (31.5°S, 64.4°W). This succession was deposited in a half-graben basin during the Early Cretaceous. We have carried out a palaeomagnetic survey on outcrops of this basin (147 sites in seven localities). From an analysis of IRM acquisition curves and detailed demagnetization behaviour, three diVerent magnetic components are identified in the volcanic rocks: components A, B and X are carried by single- or pseudo-single-domain (titano) magnetite, haematite and multidomain magnetite, respectively. Component A is interpreted as a primary compo- nent of magnetization because it passes conglomerate, contact, tilt and reversal tests. The carrier of the primary magnetization, fine-grained (titano)magnetite, is present in basalts with a high degree of deuteric oxidation. This kind of oxidation is interpreted to have occurred during cooling. Components B and X are discarded because they are interpreted as recent magnetizations. In the sedimentary rocks, haematite and magnetite are identified as the carriers of remanence. Both minerals carry the same component, which passes a reversal test. The calculated palaeomagnetic pole, based on 55 sites, is Lat. 86.0°S, Long. 75.9° E( A 95 =3.3, K=35). This palaeomagnetic pole supersedes four with anomalous positions reported in previous papers.

Comparative sequence stratigraphy of low-latitude versus high-latitude lacustrine rift basins: seismic data examples from the East African and Baikal rifts

Abstract Lakes Baikal, Malawi and Tanganyika are the world's three largest rift valley lakes and are the classic modern examples of lacustrine rift basins. All the rift lakes are segmented into half-graben basins, and seismic reflection datasets reveal how this segmentation controls the filling of the rift basins through time. In the early stages of rifting, basins are fed primarily by flexural margin and axial margin drainage systems. At the climax of syn-rift sedimentation, however, when the basins are deeply subsided, almost all the margins are walled off by rift shoulder uplifts, and sediment flux into the basins is concentrated at accommodation zone and axial margin river deltas. Flexural margin unconformities are commonplace in the tropical lakes but less so in high-latitude Lake Baikal. Lake levels are extremely dynamic in the tropical lakes and in low-latitude systems in general because of the predominance of evaporation in the hydrologic cycle in those systems. Evaporation is minimized in relation to inflow in the high-latitude Lake Baikal and in most high-latitude systems, and consequently, major sequence boundaries tend to be tectonically controlled in that type of system. The acoustic stratigraphies of the tropical lakes are dominated by high-frequency and high-amplitude lake level shifts, whereas in high-latitude Lake Baikal, stratigraphic cycles are dominated by tectonism and sediment-supply variations.

Tectonic and volcanic controls on Early Jurassic rift-valley lake deposition during emplacement of Karoo flood basalts, southern Namibia

The Karoo Igneous Province of southern Africa is one of the classic Mesozoic flood basalt provinces of the world. In the case of the early Jurassic Kalkrand Formation of Namibia the succession comprises three major flood basalt units that are separated by two stratigraphically important fluvio-lacustrine interlayers. These horizons preserve a record of the complex interplay between sedimentation, effusion of Karoo flood basalts and extensional tectonics that predated and accompanied the break-up of Gondwanaland. Both sediment layers start with the dominantly local derivation of weathered and eroded lava debris, followed by the emplacement of subaqueous mass flows and subsequent deposition of chemical sediments. The latter are characterised by interbedded stromatolitic carbonates, grass-like structured gypsum, and plane-bedded sandstones and mudstones containing euhedral displacive gypsum crystals that grew in the subsurface as well as rosettes which nucleated on the sediment surface. The central parts of the lacustrine units are overlain by thin deltaic sandstones showing bottomset, foreset, and topset layering and, finally, braided fluvial, trough cross-bedded sandstones. Evidence of subsidence synchronous with the formation of lake bodies can be explained by two principal mechanisms. The first acted in localised areas only and is reflected by the development of small, centrally subsiding basins. From the repeated occurrence of onlapping geometries within such a pool, a multiphase history of sagging is deduced, being most likely related to periodic magma withdrawal and reduction in magmatic pressures in subsurface lava feeders beneath the basin floor. Abundant syn-subsident fracture features at lava–sediment contacts, such as sediment-, hydrothermal calcite-, and sometimes basic lava-filled fissure systems indicate the pronounced interaction between the underlying volcanics and these small areas of pronounced subsidence. Fluids passing through the volcanic pile exhaled into the lake, giving it the characteristics of alkaline lake systems described from more recent flood basalt areas associated with the modern African Rift System. On the regional scale, however, northerly trending extensional fault systems controlled half-graben basin geometries and both facies and thickness variations across faults indicate that tectonism operated contemporaneously with volcanism and lacustrine sedimentation. The analysis of faults and associated structures, such as regularly aligned sediment-filled fissures, sets of micro-faults, folds and basaltic dykes constrains the extensional opening direction for the Karoo graben structures in this area that heralded the opening of the South Atlantic and thus provides a basis to discuss the extensional history of the Namibian coastal margin within the regional tectonic framework.

Toroda Creek half graben, northeast Washington: Late-stage sedimentary infilling of a synextensional basin

The Toroda Creek half graben is one of several north-northeast–trending Eocene extensional basins associated in time and space with the Okanogan and Kettle metamorphic core complexes in the southern Omineca crystalline belt of Washington and British Columbia. Sedimentary rocks of the middle Eocene Klondike Mountain Formation are the youngest stratified, nonvolcanic rocks in the Toroda Creek half graben. These sedimentary rocks preserve details of the character of Toroda Creek half-graben basin development in a humid temperate setting, help constrain the timing of brittle faulting in the basin, and provide insight into approximate rates of Okanogan core-complex uplift. Facies distributions and provenance determinations indicate that the Klondike Mountain Formation accumulated in a localized basin. There is no evidence that the Klondike Mountain Formation strata in the Toroda Creek half graben were once part of a more widespread deposit that later was segmented by faulting into its present configuration. Klondike Mountain sedimentary strata consist of sedimentary, gravity-dominated rock-avalanche, slide, and debris-flow deposits that accumulated on alluvial fans that delivered their sediments into either a single elongate lake or a series of possibly interconnected lakes. Provenance determinations and facies distribution patterns suggest that Klondike Mountain detritus primarily was derived from steep, unstable highlands exposed in the footwall of the Bodie Mountain fault on the east side of the basin. Sediments at the southern end of the basin were shed from an upland created by a west-northwest–trending transfer fault (Granite Creek fault). Rates of footwall uplift (and/or basin subsidence) estimated from unroofing of plutonic and presumed mylonitic rocks suggest rapid and significant basin margin uplift.

The Erris Ridge: a major geological feature in the NW Irish Offshore Basins

The Erris Ridge is a narrow structural high that separates the Erris and Rockall Troughs. It has a composite internal character and its orientation is thought to be controlled by a pre-Caledonian structural fabric which was frequently reactivated during Caledonian to Cenozoic time. In the north it is fault-bounded and probably contains rocks of Palaeozoic and older age. It formed a positive topographic feature which was progressively onlapped during Permo-Triassic to Cretaceous times. Fault reactivation occurred along its western boundary in Tertiary times and it was submerged by approximately latest Miocene times. Further south the Ridge is more complex. The Palaeozoic and older core is flanked to the SE by interpreted Permo-Triassic to Lower Jurassic strata which constitute a fault-controlled, uplifted, remnant of a half-graben basin. This composite structure was probably inverted during Late Jurassic times and remained as a topographic feature until Cretaceous times. It was progressively onlapped during Cretaceous and Tertiary times, with a minor phase of fault reactivation during the Early Tertiary. The Ridge acted as a rigid block which facilitated the focusing of several phases of crustal extension in the Erris Trough and in the Rockall Trough.

Infill Architecture of a Nonmarine Half-Graben Triassic Basin (Central Spain)

ABSTRACT Upper Permian to Middle Triassic continental red beds were studied using architectural-element analysis in a selected area in Central Spain where the outstanding quality of outcrops allows a three-dimensional study. These red beds were deposited in a tectonically active half-graben basin with episodically changing rates of basin subsidence. Seventeen architectural elements (channel belt and overbank elements) were recognized, described in detail, and interpreted. Twelve channel-belt elements and five overbank elements are present in the Buntsandstein. They combine in nine different alluvial systems with diverse bed-load channel type and discharge regime (gravel bed-load braided, mixed-load braided, three different sandy braided, intermediate sinuosity, high sinuosity, ephemeral, and alluvial fan). Not only the sizes of the different elements but also the scale of the complete alluvial systems were emphasized in this study. The differential tectonic, vertical and lateral evolution in the area was the main control on basin asymmetry, channel interconnection, paleocurrent pattern, and consequently the fluvial architecture. he asymmetry of the basin, together with the presence of antithetic and synthetic fault systems, caused the fluvial system to migrate away from the footwall scarp, to a more eastern position, where the lowest area was located. As the effect of the initial increase in subsidence was compensated by the sedimentation, the axial southeastern trend then became reestablished.

Neocomian Half Graben in the Western San Jorge Basin, Argentina: Petroleum Systems, Origin and Tectonic Inversion: ABSTRACT

During late Jurassic-early Cretaceous times, the extension related to the Gondwana break-up generated many isolated small half-graben basins, with a NW-SE structural trend, in the central part of Patagonia Terrane. The sedimentary record, which overlays an igneous metamorphic prejurassic basement, is made up by three megasequences that represent different stages in rifting evolution. The main source rock, composed of Neocomian lacustrine dark organic-rich shales, with type I kerogen, rests on a late Jurassic volcanic and sedimentary complex, and it is found within Megasequence I (early and late rift). This megasequence ends with fluvial deltaic and tidal sandstones as the oldest reservoir rocks. The main reservoirs, made up of upper Cretaceous braided and meander sandstones, are found within Megasequence II (early and late sag), which ends with thick pyroclastic overbank deposits as the regional seal. Megasequence III, composed of Tertiary marine and continental rocks, lacking source and reservoirs levels, solely acts as overburden rock. After a W-E Tertiary compression, these basins were partially inverted, to different degrees, according to the orientation of pre-existing faults. Likewise, the thermal maturity history of the source rock, the migration pathways and the traps vary in each of these basins. This study sets out to showmore » how analogous basins with the same stratigraphic record became independent petroleum systems due to a different tectonic evolution.« less

Rapid subsidence and stacked Gilbert-type fan deltas, Pliocene Loreto basin, Baja California Sur, Mexico

Pliocene nonmarine to marine sedimentary rocks exposed in the Loreto basin, Baja California Sur, provide a record of syntectonic subsidence and sedimentation in a transform-rift basin that developed along the western margin of the Gulf of California. A thick sequence of twelve Gilbert-type fan deltas, having a total measured thickness of about 615 m, accumulated near the fault-bounded southwestern margin of this basin. Based on stratal geometries and lithofacies associations, sedimentary rocks are divided into Gilbert-delta topset, foreset and bottomset strata, shell beds and background shallow-marine shelf deposits. Topset strata of each Gilbert-type delta cycle are capped by laterally persistent molluscan shell beds containing diverse assemblages of bivalves, pectens, oysters, gastropods and echinoids. These shell beds are interpreted to be condensed intervals that record sediment starvation during abandonment of the fan-delta plain. Delta abandonment may have been caused by large episodic faulting events, which submerged each pre-existing fan-delta plain, substantially slowed detrital input by drowning of alluvial feeder channels, and created new accommodation space for each new Gilbert-type fan delta. Alternatively, it is possible that delta-plain abandonment was caused by upstream avulsions and autocyclic lateral switching of fan-delta lobes during relatively uniform rates of slip along the basin-bounding fault.Two contrasting, plausible basin models are proposed for the Loreto basin: (1) asymmetric subsidence along a high-angle oblique-slip normal fault, producing a classic half-graben basin geometry with vertically stacked Gilbert-type fan deltas; or (2) lateral stacking and horizontal displacement of strata away from a relatively fixed depocenter due to fault movement in the releasing bend of a listric strike-slip fault. We favor the first model because field relations and simple geometric constraints suggest that most of the total measured section represents a true vertical stratigraphic profile. Assuming vertical sediment accumulation and using ages of interbedded tuffs obtained from high-precision 40Ar/ 39Ar dating of plagioclase and biotite, quantitative decompaction and geohistory analysis was carried out for the Loreto basin sequence. Tuff ages range from 2.61 ± 0.01 Ma in the lower part of the basinal sequence to 1.97 ± 0.02 Ma near the top, with two intermediate tuffs dated at 2.46 ± 0.06 and 2.36 ± 0.02 Ma that are separated by 782 m of measured section. Basin subsidence initially took place at moderate rates of 0.43 ± 0.17 mm/yr and accelerated dramatically at 2.46 Ma to 8.1 ± 5.1 mm/yr. This phase of extremely rapid subsidence lasted for only about 100 ka, and it produced much of the total accomodation space and sedimentary thickness in the basin. Accumulation of Gilbert-type fan deltas took place only during the short pulse of very rapid subsidence, between 2.46 and 2.36 Ma. Prior to this time interval, alluvial-fan and shelf-type fan-delta depositional systems prevailed; afterwards no fan deltas of any kind were deposited, and the basin evolved to a slowly subsiding low-energy carbonate shelf setting. This suggests that very rapid subsidence, combined with rapid sediment input, may be required to maintain steep basin-margin slopes and continually create new accommodation space, conditions that seem necessary for the development of thick sequences of stacked Gilbert-type fan deltas.

Reconstruction of Pre-Rift Pyrenean Relief in the Oligo-Aquitanian Camargue Basin (Gulf of Lion Passive Margin, SE France). Implications on Thermal History of Basins: ABSTRACT

Fault reconstruction techniques commonly assume horizontal pre-rift level datum to calculate fault geometry from hanging-wall geometry or viceversa. Example from Camargue basin shows that neglecting pre-rift relief may lead to important errors in calculating the fault and hanging-wall geometries, and the total extension. These errors have direct implications on reconstruction of the thermal history of basins. The Camargue basin results front NW-SE extension and rifting of the Gulf of Lion passive margin. More than 4000m of Oligo-Aquitanian syn-rift series unconformably overlie a crust previously thickened during Pyrenean orogeny. The half-graben basin is controlled by the SE-dipping listric Nimes basement fault which generated a typical roll-over. As both fault and hanging-wall geometries are constrained, the pre-rift surface topography can be restored, using three reconstruction techniques. Either the constant-bed-length and constant-heave techniques produce a depression in the axis of the basin and a relief (1500m and 12(X)m respectively) atop the roll-over. The simple-shear (a=60{degrees}) technique generates a 1500m topography atop the roll-over, more coherent with regional data. Testing the hypothesis of a pre-rift horizontal datum leads to a roll-over 1400m too deep. Pre-rift surface elevation corresponds to the residual topography herited from the Pyrenean orogeny. Consequently, there has been some 1000m subsidencemore » more than predicted by the syn-rift sedimentary record.« less