Syndepositional Faults Research Articles

Footwall progradation in syn-rift carbonate platform-slope systems (Early Jurassic, Northern Apennines, Italy)

The so-called Umbria–Marche Domain of Northern Apennines represents a vast depositional system, also stretching across the Adriatic Sea subsurface, that was characterized by dominantly pelagic sedimentation through most of its Jurassic to Oligocene/Early Miocene history. The pelagic succession is underlain by Hettangian shallow-water carbonates (Calcare Massiccio Fm.), constituting a regional carbonate platform that was subjected to tectonic extension due to rifting of the Adria/African Plate in the earliest Jurassic. While tectonic subsidence of the hangingwalls drove the drowning of the platform around the Hettangian/Sinemurian boundary, the production of benthic carbonate on footwall blocks continued parallel to faulting, through a sequence of facies that was abruptly terminated by drowning and development of condensed pelagites in the early Pliensbachian. By then rifting had ceased, so that the Pliensbachian to Early Cretaceous hangingwall deposits represent a post-rift basin–fill succession onlapping the tectonically-generated escarpment margins of the highs. During the early phases of syndepositional faulting, the carbonate factories of footwall blocks were still temporarily able to fill part of the accommodation space produced by the normal faults by prograding into the incipient basins. In this paper we describe for the first time a relatively low-angle (<10°) clinoform bed package documenting such an ephemeral phase of lateral growth of a carbonate factory. The clinoforms are sigmoidal, and form low-relief (maximum 5–7m) bodies representing a shallow-water slope that was productive due to development of a Lithocodium-dominated factory. Continued faulting and hangingwall subsidence then decoupled the slope from the platform top, halting the growth of clinoforms and causing the platform margin to switch from accretionary to bypass mode as the pre-rift substrate became exposed along a submarine fault escarpment. The downfaulted clinoform slope was then buried by base-of-escarpment proximal turbidites, forming a bypass wedge. Such a contact would be imaged along a seismic section as an unconformity, suggestive of shut-off of the local carbonate factory and onlap by pelagic mud. The composition of the turbidites, however, at least initially duplicates that of the clinoforms, indicating that the footwall top was still productive, yet the mechanisms of sediment shedding into the basin had changed due to the modifications of submarine topography induced by synsedimentary tectonics.

A facies model for an Early Aptian carbonate platform (Zamaia, Spain)

The Cretaceous (Early Aptian, uppermost Bedoulian, Dufrenoyia furcata Zone) Zamaia Formation is a carbonate unit, up to 224 m thick and 1.5 km wide, which formed on a regional coastal sea bordering the continental Iberian craton. A high-resolution, facies-based, stratigraphic framework is presented using facies mapping and vertical-log characterization. The depositional succession consists of a shallow estuarine facies of the Ereza Fm overlain by shallow-water rudist limestones (Zamaia Fm) building relief over positive tectonic blocks and separated by an intraplatform depression. The margins of these shallow-water rudist buildups record low-angle transitional slopes toward the adjacent surrounding basins. Syn-depositional faulting is responsible for differential subsidence and creation of highs and lows, and local emplacement of limestone olistoliths and slope breccias. Two main carbonate phases are separated by an intervening siliciclastic-carbonate estuarine episode. The platform carbonates are composed of repetitive swallowing-upward cycles, commonly ending with a paleokarstic surface. Depositional systems tracts within sequences are recognized on the basis of facies patterns and are interpreted in terms of variations of relative sea level. Both Zamaia carbonate platform phases were terminated by a relative sea-level fall and karstification, immediately followed by a relative sea-level rise. This study refines our understanding of the paleogeography and sea-level history in the Early Cretaceous Aptian of the Basque-Cantabrian Basin. The detailed information on biostratigraphy and lithostratigraphy provides a foundation for regional to global correlations.

Temporal and lateral variation in the development of growth faults and growth strata in western Niger Delta, Nigeria

This study examines eight syndepositional faults and syntectonic sediments in five major fault blocks in western Niger Delta, offshore Nigeria, on three-dimensional seismic data. The initiation, lateral growth and retreat, periods of activity and quiescence, and decay of faulting around these blocks can be ascertained by analyzing a series of time-structure and isopach maps. The study area can be subdivided into three structural zones. The first structural zone is a northwestern zone characterized by a major counterregional growth fault in the deep subsurface. This deep-seated structure is superposed by an array of younger regional growth faults displacing a kilometer-thick sedimentary overburden that accumulated on the former footwall. The second structural zone is a central to eastern zone that seems mostly unaffected by young deltaic faulting. This zone is characterized by the thinnest sedimentary record of the study area. The third structural zone is a southeastern zone that is dominated by a large, listric, backstepping master fault zone associated with a kilometer-scale rollover system. Regional structural and stratigraphic analyses document an apparently strong relationship between syntectonic sedimentation and syndepositional fault activity in that phases of significant fault activity, lateral fault growth, and fault migration concur with major depositional phases; in turn, areas and intervals characterized by the least sediment accumulation also record the lowest fault activity. However, one particularity of the studied system is that it underwent at least one period of seaward fault progression that coincided with a backstepping of faulting on the landward side. Although the forward stepping of faulting near the delta front can be interpreted as the consequence of the progressive loading during delta progradation, the contemporaneous backstepping of faulting further inboard likely reflects the sustained lateral growth of mature deltaic faults into previously undeformed proximal parts of the depocenter. The results of this study, thus, document that although an apparent correlation with the superimposed depositional system exists on a regional scale, inboard deltaic faults may persist to grow irrespective of sedimentary loading. The recognition of such fault trends is particularly important for estimating the influence of late-stage fault movement on hydrocarbon migration or the discovery of subtle fault-controlled hanging-wall reservoirs.

Late Eocene–Neogene sedimentary geology of the Girne (Kyrenia) Range, northern Cyprus: A case history of sedimentation related to progressive and diachronous continental collision

Sedimentary geology can shed light on the processes of continental collision. The easternmost Mediterranean provides an exceptional opportunity to study sediments deposited during progressive and diachronous continental collision, with implications for comparable settings elsewhere. Here, we focus on sediment sources, pathways and deposition of mostly deep-sea clastic sediments exposed in northern Cyprus. The sedimentary basin forms part of the northern continental margin of the Southern Neotethys ocean and is represented by a ca. 200km-long, E–W-trending, arcuate lineament, encompassing northern Cyprus and its submarine link with Turkey. The Upper Eocene–Upper Miocene basin was divided into northerly and southerly sub-basins during Mid- to Late Miocene time by an E–W-trending, inferred syn-depositional fault lineament. The sequence begins with non-marine to shallow-marine basal conglomerates (Late Eocene) that abruptly fine upwards into siliciclastic turbidites (Oligocene), followed by biogenic calciturbidites and marls (Aquitanian–Langhian). During Mid–Late Miocene the northerly sub-basin was characterised by thin- to medium-bedded calciturbidites and marls (Serravallian), overlain by thick-bedded, medium-grained lithic sandstones (Tortonian). In contrast, the southern sub-basin is dominated by siliciclastic turbidites with abundant sole structures (Burdigalian–Tortonian). Palaeocurrent evidence indicates dominantly westward sediment transport, especially during the Late Miocene. The Serravallian–Tortonian sandstones of the northern sub-basin are relatively rich in carbonates, while the southern sub-basin contains more siliciclastic and ophiolite-derived detritus indicating differences in sediment supply. In general, the Upper Eocene basal conglomerates were mainly derived from the north, from within the Girne (Kyrenia) Range and its inferred northward extension. In response to rapid subsidence (reflecting possible southward subduction zone retreat) clastic sediment sources to the north then submerged while clastic sediment was instead mainly supplied from the South Neotethyan suture zone to the east and northeast in southern Turkey. Reflecting diachronous collision, deep-water deposition persisted in the easternmost Mediterranean basin further west until this was terminated in northern Cyprus by thrusting/folding during latest Miocene–earliest Pliocene. Quaternary collision-related uplift ensued. Similar sedimentary development is to be expected in collisional settings elsewhere (e.g. Tethyan closure in the Himalayas; Iapetus closure in the Appalachians).

Quartz arenites of the Cambro-Ordovician Kamouraska Formation, Quebec Appalachians, Canada: I. Deep-water depositional processes in a continental-slope environment

The Kamouraska Formation is an uppermost Cambrian – lowermost Ordovician quartz-arenite-dominated unit of controversial origin deposited on the southeastern slope of Laurentia bordering the Iapetus Ocean. It is exposed in the Quebec Appalachians on the south shore of the St. Lawrence Estuary. The formation consists of basal polymictic conglomerate and overlying massive sheet-like quartz arenite. The conglomerate beds are reversely and reversely to normally graded. The quartz arenite beds are generally massive, although they may show coarse-tail grading. Beds containing full or partial Bouma sequences are rare. Paleoflow directions from ripple-cross lamination, ripple marks on bed surfaces, and sole marks point towards southeast, south, and southwest. The clastic sediments of the Kamouraska were transported into the deep sea by sediment gravity flows that evolved from hyperconcentrated to concentrated density flows, and then to turbidity currents. The depositional environment is interpreted to have been a southwest-trending meandering submarine canyon. The exposed part of the canyon deposits is slightly oblique to the strike of slope. If correct, our interpretation establishes the preservation of continental-slope deposits in more distal deep-water siliciclastic sedimentary rocks of the Taconian orogen in Quebec, which traditionally have been interpreted as submarine-fan and (or) basin-plain deposits. The orientation of a canyon near parallel-to strike of the slope may have been controlled by syn-depositional growth faults. The coarsest hyperconcentrated flows, which deposited the conglomerate, were restricted to the deepest parts of the canyon during its early stages of development, whereas the concentrated density flows that deposited the massive quartz-arenite beds covered a wider area.

Soft‐sediment deformation structures in Cretaceous non‐marine deposits of southeastern Gyeongsang Basin, Korea: Occurrences and origin

AbstractA variety of soft‐sediment deformation structures formed during or shortly after deposition occurs in the Cretaceous Seongpori and Dadaepo Formations of the southeastern Gyeongsang Basin exposed along coastal areas of southeastern Korean Peninsula for 0.5–2 km. These are mostly present in a fluvial plain facies, with interbedded lacustrine deposits. In this study, the features of different kinds of soft‐sediment deformation structures have been interpreted on the basis of sedimentology of structure‐bearing deposits, comparison with normal sedimentary structures, timing and mechanism of deformation, and triggering mechanisms. The soft‐sediment deformation structures can be classified into four morphological groups: (i) load structures (load casts, ball‐and‐pillow structures); (ii) soft‐sediment intrusive structures (dish‐and‐pillars, clastic dykes, sills); (iii) ductile disturbed structures (convolute folds, slump structures); and (iv) brittle deformation structures (syndepositional faulting, dislocated breccia). The most probable triggering mechanisms resulting in these structures were seismic shocks. These interpretations are based on the following field observations: (i) location of the study area within tectonically active fault zone reactivated several times during the Cretaceous; (ii) deformation structures confined to single stratigraphic levels; (iii) lateral continuity and occurrences of various soft‐sediment deformation structures in the deformed level over large areas; (iv) absence of depositional slope to indicate gravity sliding or slumping; and (v) similarity to the structures produced experimentally. The soft‐sediment deformation structures in the study areas are thus interpreted to have been generated by seismic shocks with an estimated magnitude of M &gt; 5, representing an intermittent record of the active tectonic and sedimentary processes during the development and evolution of two formations from the late Early Cretaceous to the Late Cretaceous.

Tectonic evolution of the southern Levant margin since Mesozoic

Recognition of syndepositional faults and mechanical analysis of fault-slip data are used to reconstruct a high resolution tectonic evolution of Israel, and to characterize the stress fields associated with the major tectonic deformation events of this area since Mesozoic times. Syndepositional normal faults recognized in Mesozoic and Paleogene rocks indicate three extensive deformation phases that were active at least during Early Jurassic, Campanian, and Eocene times. Inversion of fault-slip data indicate that a NE–SW extension characterizes the two first events, whereas orientation of extension during the Eocene was N–S to NNE–SSW. Several scenarios are proposed to explain these newly documented normal faults. Measured faults also document synchronous strike-slip and reverse regimes. Unambiguous chronological relationships between fault-data and bedding attest that a WNW–ESE to NW–SE compression is associated with the Syrian Arc folding event during the Late Cretaceous times. This compression pursued during the development of the Dead Sea transform plate boundary but its relationship with Neogene structures is unclear. Fault-data suggest that folding during the Neogene was driven by a regional NNW–SSE compression. A minor NE–SW compression was also recognized but no compatible large-scale structures were found to date. We also show that the post-Pliocene normal faults developed under a NE–SW extension.

Submarine channel response to intrabasinal tectonics: The influence of lateral tilt

Lateral tilting is a common deformation style in extensional basins; its influence on subaerial channels is, to a degree, understood and may be significant, controlling the style of channel development and the resultant sand-body architecture. Growth faulting and lateral tilting in turbidite channel systems have been demonstrated from three-dimensional seismic data, but the resultant architecture of channels within these settings has not yet been documented. In the Carboniferous of northern England, a sand-rich slope channel, developed within a basin undergoing late-stage extension, underwent progressive and unidirectional migration toward a topographic low on a laterally tilting block. The resultant sandstone body is wedge shaped in cross section and composed dominantly of sigmoidal lateral accretion deposits. The channel returned to an axial course before undergoing lateral migration in the same direction, creating a multistory, multilateral channel sandstone body. The repeated unidirectional migration combined with evidence of syndepositional deformation suggests that active tectonism strongly influenced channel evolution and deposition. A model of submarine channel evolution in extensional basins is presented; in systems where large displacements occur, the channel system may avulse, creating isolated sand ribbons, which are connected updip; where the lateral dip is always more influential than the regional dip, the system may pond in the hanging-wall syncline. The model is compared to a subsurface channel within the Pliocene of the Nile Delta slope, which was influenced by syndepositional fault movement; application of the outcrop-derived model allows some simple architectural interpretations to be made.

Depositional environment controls on petroleum potential of the Eocene in the North of Tunisia

Outcrop data as well as petrographical, tectonic and geochemical data were used to reconstruct the depositional environments of Ypresian–Lower Lutetian-aged carbonate systems bearing Nummulites. They are shelf deposits which pertain to the Metlaoui Group from the northwest and central-east Tunisia, constitute exploration targets for hydrocarbons and form the reservoirs of some important oil-fields at Ashtart and Sidi El Iteyem. Cyclostratigraphy (sedimentary figures, syndepositional faulting, sequence stratigraphy and framework) and laboratory investigations (petrography, geochemical evaluation) were made of these chalky series, to develop a sequence model for the Metlaoui group and clarify the main relationships between global sea-water level changes, lithofacies repartition and source-rock potential. The rock units that include three main third-order sequences, were deposited in a variety of shelf to platform environments with distinct associations of phosphorite/clay/evaporite and carbonate facies. Associated with Nummulitid facies of two main outer shelves, are Globigerinid facies, Discocyclinid subfacies, pack–grainstones enriched in glauconite and phosphate, and Bivalve and Algae-bearing packstones and grainstones. Due to tectonic activity, channels were often cut into the shelf and materials eroded from paleohighs often bypassed the shelf and were deposited in the deep basins. Basin deposits include two main cross-sections: (1) Globigerinid-rich limestones and cherts of the Boudabbous formation and (2) a lateral equivalent with interbedded limestones, clayey limestones, marls and microbreccias which exhibit abundant pyroclastic fragments that testify to an intense Ypresian volcanism. The deposits display substantial heterogeneity resulting from depositional, diagenetic and structural processes. Porosity development in Nummulitid-bearing strata is principally a result (1) of meteoric diagenesis due to uplift and emersion (tests dissolution) and locally dolomitization at the end of short-term cycles (Milankovich cycles) and (2) frequent stylolitization and microfracturation due to late compaction concurrent with transpressional tectonics. Elevated amounts of organic matter were determined in these series, and lithofacies which present high petroleum indices would provide sourcing for seals where the rocks have been deeply buried, notably beneath the Oligocene–Miocene thick series.

Subsidence history from a backstripping analysis of the Permo‐Mesozoic succession of the Central Southern Alps (Northern Italy)

ABSTRACTSeven tectonic subsidence curves, based on outcrop data, have been calculated in order to constrain the geodynamic evolution of the Permian–Mesozoic sedimentary succession (up to 10 km thick) of the Central Southern Alps basin (Italy). The analysis of the tectonic subsidence curves, covering a time span of about 200 Ma, allowed us to quantify the subsidence rates, to document the activity of syndepositional fault systems and calculate their slip rates. Different stages, in terms of duration and magnitude of subsidence‐uplift trends, have been identified in the evolution of the basin. The fault activity, reconstructed by comparing subsidence curves from adjacent sectors, resulted as highly variable both temporally and spatially. Strike‐slip tectonics was coeval to Permian sedimentation, as suggested by the strong differences in the subsidence rates in the sections. The evolution and subsidence rates suggest a continental shelf deposition from Early Triassic to Carnian, when subsidence came to a stop. A rapid resumption of subsidence is observed from the Norian, with a subsidence pulse in the Late Norian, followed by the regional uplift, in the Late Rhaetian. The following Early Jurassic subsidence is characterized by tectonic subsidence similar to that of the Norian. The Norian and Early Jurassic pulses were characterized by the highest slip rates along growth faults and are identified as two distinct tectonic events. The Norian–Rhaetian event is tentatively related to transtensional tectonics whereas the Early Jurassic event is related to crustal extension. The Early Jurassic subsidence records a shift in space an time of the beginning of the extensional stage, from Late Hettangian to the east to Late Pliensbachian–Toarcian to the west. From the Toarcian to the Aptian, the curves are compatible with regional thermal subsidence, later followed (Albian–Cenomanian) by uplift pulses in a retrobelt foreland basin (from Cenomanian onward).

Syndepositional fault controlling on depositional filling of the third member of Shahejie Formation in Beitang sag

In the continental lake basin whose structures were extraordinarily active, tectonism is an important factor in controlling the sequence and the depositional filling of the basin. This article reports the assemble patterns of syndepositional fault in the third member of Shahejie (沙河街) Formation in Beitang (北塘) sag. The results show that the comb-shape fracture system and the fracture transformation zone were developed in Beitang sag. These assemble patterns obviously controlled the sand-body and spatial distribution of sedimentary system. However, the steep slope belt of fault terrace, the multistage slope belt and the low uplift gentle slope belt controlled the development of sequence styles. Analyses of the spatial-temporal relationship of the assemble pattern of syndepositional faults and the sedimentary system help predict the favorable exploration zone.

Insights into syndepositional fault movement in a foreland basin; trends in seismites of the Upper Cretaceous, Wahweap Formation, Kaiparowits Basin, Utah, USA

ABSTRACTThe Upper Cretaceous Wahweap Formation accumulated in the active Cordilleran foreland basin of Utah. Soft‐sediment deformation structures are abundant in the capping sandstone member of the Wahweap Formation. By comparing with well‐established criteria, a seismogenic origin was determined for the majority of structures, which places these soft‐sediment deformation features in a class of sedimentary features referred to as seismites. A systematic study of the seismite trends included their vertical and horizontal distribution and a semi‐quantitative intensity analysis using a scale from 1 to 5 that is based on magnitude, sedimentary structure type, and the predominance of inferred process of hydroplastic deformation, liquefaction or fluidization. In addition, orientations of soft‐sediment fold axes were recorded. Construction of a northwest‐to‐southeast stratigraphic and seismite intensity cross‐section demonstrates: (1) reduction in stratigraphic thickness and percentage of conglomerates to the southeast, (2) the presence of lower seismite, middle nonseismite, and upper seismite zones within the capping sandstone (permitting subdivision of the capping sandstone member), and (3) elimination of the nonseismite zone and amalgamation of the lower and upper seismite zones to the southeast. Regional isoseismal contour maps generated from the semi‐quantitative analysis indicate a decrease in overall intensity from northwest to southeast in the upper and lower seismic zones and in sandstone within 5 m stratigraphically of the contact between the upper and capping sandstone members. In addition, cumulative seismite fold orientations support a west–northwest direction towards regional epicentres. Isoseismal maps are used to distinguish the effects of intrabasinal normal faulting from those of regional orogenic thrusting. Thus, this study demonstrates the utility of mapping seismites to separate the importance of regional vs. local tectonic activity influencing foreland basin sedimentation by identifying patterns that delineate palaeoepicentres associated with specific local intrabasinal normal faults vs. regional trends in soft‐sediment deformation related to Sevier belt earthquakes.

Quaternary transpression and lacustrine sedimentation in the San Lorenzo area (Sant'Arcangelo Basin, Italy)

Quaternary lacustrine sedimentation of the San Lorenzo Succession represents one of the more recent steps in the evolution of the northern part of the Sant'Arcangelo Basin. The latter is a large Pliocene to Pleistocene wedge-top depozone of the southern Apennines foreland-basin system that developed close to the front of the thrust belt in southern Italy. The lower to middle Pleistocene lacustrine deposits are composed of siltstone and claystone interbedded with sandstone, carbonate and volcaniclastic beds, arranged in fining-upward sequences. The overall stratigraphical, sedimentological, and structural data collected demonstrate that the San Lorenzo lacustrine deposits formed when the northern sector of the Sant'Arcangelo Basin was undergoing tectonic deformation. In particular, new data reveal the presence of strike-slip syndepositional faults. These faults belong to positive flower-type structures, which developed in a left-lateral transpressive tectonic regime characterized by a E–W horizontal component of shear stress. The lacustrine deposits recorded the activity of these faults showing growth structures represented mainly by gentle synclines, as well as other minor folds. The tectonic activity of the main faults formed an endorheic area with at least two different depocentres accumulating lacustrine successions.

Syndepositional fault control on lower Frasnian platform evolution, Lennard Shelf, Canning Basin, Australia

Syndepositional faulting was a major control on internal platform stratigraphy in a Frasnian reef complex on the southeastern Lennard Shelf, northern Canning Basin, Australia. By combining platform (mostly backreef) facies distributions with key stratal surfaces and biostratigraphic data, we have developed a temporal framework for the Hull Range area. Platform evolution was controlled by normal faults via an initial tilt block geometry and subsequent differential subsidence and accommodation across the platform. Three third-order, flooding surface–bounded platform phases are recorded. Basal shallow marine siliciclastic facies were deposited in topographic lows adjacent to the hanging wall and close to internal faults. Early carbonate deposition in dominantly deep subtidal environments suggests an overall ramp-style setting that deepened toward the southeast. This phase is capped by a sequence boundary represented by multiple paleokarst surfaces in the northwest that pass laterally into a major flooding surface to the southeast, above which the platform expanded toward the Mount Elma–Painted Rocks fault system. The overall stacking pattern above the sequence boundary–flooding surface is aggradational to progradational with higher-frequency shoaling trends, and locally developed exposure surfaces, related to higher-order relative sea-level changes. This second phase of platform growth ended with major flooding and a pronounced backstep of the leeward margin.

Evidence for Late Messinian seismites, Nijar Basin, south‐east Spain

AbstractThe Feos Formation of the Nijar Basin comprises sediments deposited during the final stage of the Messinian salinity crisis when the Mediterranean was almost totally isolated. Levels of soft‐sediment deformation structures occur in both conglomeratic alluvial sediments deposited close to faults and the hyposaline Lago Mare facies, a laminated and thin‐bedded succession of whitish chalky marls and intercalated sands alternating with non‐marine coastal plain deposits. Deformation structures in the coarse clastics include funnel‐shaped depressions filled with conglomerate, liquefaction dykes terminating downwards in gravel pockets, soft‐sediment mixing bodies, chaotic intervals and flame structures. Evidence for soft‐sediment deformation in the fine‐grained Lago Mare facies comprises syndepositional faulting and fault‐grading, sandstone dykes, mixed layers, slumping and sliding of sandstone beds, convolute bedding, and pillar and flame structures. The soft‐sediment deformed intervals resemble those ascribed elsewhere to seismic shaking. Moreover, the study area provides the appropriate conditions for the preservation of deformation structures induced by seismicity; such as location in a tectonically active area, variable sediment input to produce heterolithic deposits and an absence of bioturbation. The vertical distribution of soft‐sediment deformation implies frequent seismic shocks, underlining the importance of seismicity in the Betic region during the Late Messinian when the Nijar Basin became separated from the Sorbas Basin to the north. The presence of liquefied gravel injections in the marginal facies indicates strong earthquakes (M ≥ 7). The identification of at least four separate fissured levels within a single Lago Mare interval suggests a recurrence interval for large magnitude earthquakes of the order of millennia, assuming that the cyclicity of the alternating Lago Mare and continental intervals was precession‐controlled. This suggestion is consistent with the present‐day seismic activity in SE Spain.

Basin architecture and syndepositional fault activity during deposition of the Neoproterozoic Mackenzie Mountains supergroup, Northwest Territories, CanadaNorthwest Territories Geoscience Office Contribution 0040.

The early Neoproterozoic Mackenzie Mountains supergroup (MMSG; ∼4 km thick, &lt;1083 Ma, &gt;779 Ma) was deposited in a large but poorly understood basin. Analysis of new and existing stratigraphic data reveals unexpected, abrupt changes in thickness and lithofacies patterns in formation scale and smaller units over short distances parallel and perpendicular to depositional strike. These isopach and lithofacies patterns depict irregularly subsiding syndepositional sub-basins. These basins may have developed during basin extension in a lower-plate segment of an evolving passive margin, as the result of episodic, excessive subsidence along growth faults parallel to the basin margin, and crustal-scale (transfer) faults at a high angle to the margin. The extent of the lower-plate area is inferred to correspond to the distribution of the MMSG (∼500 km strike length); upper-plate segments are inferred to flank the MMSG embayment to the northwest and southeast. A lower-plate model for the arcuate MMSG embayment has important implications for the Mackenzie Mountains zinc district. The present-day regional distribution of known carbonate-hosted base-metal showings suggests that the configuration of the thinned, faulted Neoproterozoic lower-plate segment, together with later reactivation of basement-rooted transfer faults during Neoproterozoic–Paleozoic rifting or Laramide compression, may have been important in focussing metalliferous fluids into overlying units.

Understanding the 1720–1640 Ma Palaeoproterozoic Willyama Supergroup, Curnamona Province, Southeastern Australia: Implications for tectonics, basin evolution and ore genesis

The ∼1720–1640 Ma Willyama Supergroup was deposited in an epi-continental rift basin accompanied by felsic and mafic magmatism, and hosts the largest known Pb–Zn–Ag accumulation at Broken Hill. The Supergroup occupies the southern portion of the Curnamona Province, a largely buried ∼50,000 km 2 crustal block, possibly the southernmost part of the Eastern Australian Pb–Zn belt (which includes Mt Isa). Additionally, the Province is a component of the early Mesoproterozoic IOCG ‘super-province’ that includes the Gawler Craton (Olympic Dam), Eastern Succession of the Mt Isa region (Ernest Henry) and perhaps parts of NW North America (Wernecke Mountains). The Curnamona Province, but a small part of the original basin, is divided into domains on depositional, tectonic and magmatic criteria with variably distributed stratigraphic units reflecting syn-depositional faulting and onlapping relationships resulting from rifting during the earlier extensional stage of basin evolution. Rifting controlled the variable distribution and thickness of the succession. In the Olary Domain to the west, the palaeo-flank exposes the ∼1720–1715 Ma Curnamona Group, but overlying sediment packages are but sporadically developed. To the east and closer to the rift axis, sedimentation in the Broken Hill Domain was more complete, comprising ∼1710–1705 Ma migmatitic-metasediments, and the overlying ∼1705–1695 Ma Thackaringa, ∼1695–1685 Ma Broken Hill and ∼1685–1670 Ma Sundown Groups. The ∼1670–1640 Ma Paragon and Strathearn Groups represent sag-phase deposition with a change in basin configuration and provenance. Earliest syn-depositional magmatism is recorded in the Olary Domain by ∼1720–1710 Ma A-type felsic volcanics and intrusives and restricted basaltic lavas. The locus of magmatism then moved to the Broken Hill Domain with partial melting of either the lower parts of the Willyama Supergroup, or underlying rocks of similar composition, producing S-type granitic sills of the ∼1705–1685 Ma Silver City Suite. Magmatism climaxed in the upper Broken Hill Group with emplacement of Fe-rich tholeiite sills and the ∼1685 Ma volcaniclastic felsic Hores Gneiss hosting the Broken Hill Pb–Zn–Ag orebody. Polyphase deformation and high T, low P metamorphism, ranging from greenschist to lower granulite grade, resulted from the ∼1620–1590 Ma Olarian Orogeny. Early layer-parallel schistosity was deformed progressively, first by large ductile recumbent and sheath-style folds, and later by upright folding. The mainly S-type granites of the ∼1595–1580 Ma Ninnerie Supersuite were intruded late in the deformation history. An extensive network of post-1580 Ma retrograde shear zones dismembered the Willyama Supergroup and was partially reactivated during the ∼500 Ma Delamerian Orogeny. Of the wide range of mineralisation styles in the Curnamona Province, Broken Hill-type Pb–Zn–Ag deposits and epigenetic Cu–Au and U are currently of greatest interest. Broken Hill-type mineralisation probably formed in a shallow depositional environment at or below the sea-floor, with maximum development approximating cessation of magmatic activity. Cu–Au (±Co, Mo) deposits (e.g. Kalkaroo, Portia) are located in chemical and structural traps near a regionally extensive redox boundary. By sharing geological aspects of the eastern Gawler Craton, the Curnamona Province has excellent prospectivity for Olympic Dam-style IOCG mineralisation. Uranium deposits, some economic, are located both in basement rocks and as roll-front concentrations in overlying Tertiary channel sands.

Platform margins, reef facies, and microbial carbonates; a comparison of Devonian reef complexes in the Canning Basin, Western Australia, and the Guilin region, South China

Devonian reef complexes were well developed in Western Australia and South China, but no detailed direct comparison has been made between reef building in the two regions. The regions differ in several respects, including tectonic, stratigraphic and palaeoceanographic–palaeogeographic settings, and the reef building styles reflect minor differences in reef builders and reef facies. Similarities and differences between the two reef complexes provide insights into the characteristics of platform margins, reef facies and microbial carbonates of both regions. Here we present a comparison of platform margin types from different stratigraphic positions in the Late Devonian reef complex of the Canning Basin, Western Australia and Middle and Late Devonian margin to marginal slope successions in Guilin, South China. Comparisons are integrated into a review of the reefal stratigraphy of both regions. Reef facies, reef complex architecture, temporal reef builder associations, 2nd order stratigraphy and platform cyclicity in the two regions were generally similar where the successions overlap temporally. However, carbonate deposition began earlier in South China. Carbonate complexes were also more widespread in South China and represent a thicker succession overall. Platforms in the Canning Basin grew directly on Precambrian crystalline basement or early Palaeozoic sedimentary rocks, but in South China, carbonate complexes developed conformably on older Devonian siliciclastic strata. Pre-Frasnian reef facies in South China had more abundant skeletal frameworks than in Canning Basin reefs of equivalent age, and Famennian shoaling margins containing various microbial reefs may have been more common and probably more diverse in South China. However, Late Devonian platform margin types have been documented more completely in the Canning Basin. Deep intra-platform troughs (deep depressions containing non-carbonate pelagic sediments — Nandan-type successions) that developed along syndepositional faults characterize Devonian carbonate platforms in South China, but have no equivalent on the Lennard Shelf, Canning Basin where inter-reef areas were more shallow. The South China platform-to-depression pattern was generally continuous from the Lower to Upper Devonian, indicating that many pre-Devonian tectonic features continued to exercise considerable effect through deposition. Localized, fault-controlled subsidence was an important factor in both regions, but similarities in 2nd order aggradation–progradation cycles suggest that eustasy was also an important control on the larger scale stratigraphic development of both regions.

Three‐dimensional restoration of original sedimentary geometries in deformed basin fill, onshore Brunei Darussalam, NW Borneo

ABSTRACTThe western flank of the Jerudong Anticline, onshore Brunei Darussalam, provides a rare opportunity to analyse the base of a major Miocene mud‐rich delta in outcrop, including kilometre‐scale prograding clinoforms, delta‐front turbidites and large‐scale syndepositional faults. The lateral continuation of this system in the subsurface of the Belait Syncline is documented on two‐dimensional (2D) reflection seismic data and wireline logs. In order to link geological observations at surface with corresponding geophysical subsurface signatures, we constructed a combined, quantitative 3D surface–subsurface model of onshore Brunei Darussalam. This 3D model is used to analyse and discuss the relation between field geology and geophysical subsurface interpretation, and provides the base for a quantitative kinematic restoration of the Miocene Belait delta to its original shape before folding. Final decompaction of the balanced rock volume allows the reconstruction of the palaeo‐relief of a series of Miocene clinoforms, indicating a close relation between delta‐lobe activity, clinoform morphology and the generation of delta‐front turbidites.

Differential subsidence driving the formation of mounded stratigraphy in deep-water sediments; Palaeocene, central North Sea

Deep-water Palaeocene strata of the East Central Graben, UK North Sea, are characterised by elongate depressions and elevations parallel with the palaeotransport direction and with underlying structural lineaments. Palaeozoic and mesozoic faults underlying the basin were briefly re-activated during the Palaeocene, affecting the seabed topography in the form of gentle folding, faulting and slope instability. Two types of mounded features formed in response to the tectonically induced seabed structuration: (i) deformational mounded morphology, a product of syndepositional faulting and mass wasting, (ii) non-deformational mounded morphology, interpreted to result from changing flow dynamics as flows travelled over the deformed sea floor, accentuating the structural topography by means of localised erosion and deposition. This study shows how differential subsidence acted as the ultimate control behind a range of processes leading to mounded stratigraphy. The models of the origin of the mounded morphologies are novel in the context of the North-Sea Palaeocene and have implications for the prediction of lithology in mounded basinal deposits, in the North Sea and elsewhere.