Early Jurassic Rifting Research Articles

The Mercedario rift system in the principal Cordillera of Argentina and Chile (32° SL)

Recent studies carried out in the High Andes of central-western Argentina in the provinces of San Juan and Mendoza have established its stratigraphic and structural evolution. This paper presents new data on the Triassic–Early Jurassic rift system, the depositional sequences, and a synthesis of the tectonic evolution of the region, along with a correlation with the Chilean continental margin. The paleogeographic evolution of the Cordillera Principal at these latitudes is controlled by the development of the Mercedario rift system. This rift began with the sedimentation of synrift deposits of the Rancho de Lata Formation, during the Rhetian (about 190 Ma). Subsidence was driven by normal faults, locally preserved in spite of the severe tectonic inversion of the Andes during the Cenozoic. Different authors have emphasized that an important extension dominated the transition between the Triassic and Jurassic periods along the magmatic arc in the Coastal Cordillera of Chile on the western side of the Andes. Extension was related to the bimodal magmatism that characterized the evolution of this segment (30°–33° SL). The granitic plutonism and the associated mafic volcanism indicate that they were controlled by extension during 220–200 Ma. The first subduction related granitoids at these latitudes are 170 Ma old (Bathonian). The geometry of the Mercedario rift system may be reconstructed by the pattern of the normal faults. Rifting was followed by a thermal subsidence that expanded the original area of sedimentation and controlled the paleogeography of the Los Patillos Formation during Pliensbachian to early Callovian times. This period of cooling and thermal subsidence is correlated with magmatic quiescence in the continental margin. The evolution of the basin closely matches the magmatic history of the Chilean continental margin. Subduction at the continental margin began in the Bathonian, together with deposition of the upper section of Los Patillos Formation. Arc magmatism shifted to the Cordillera Principal during the Kimmeridgian, where it is represented by the volcanic and volcaniclastic deposits of Tordillo Formation. Early Mesozoic evolution of the Andean system at these latitudes is, thus, reconstructed by a comparative analysis of these two adjacent regions, driven by a common tectonic regime, but through different geological processes.

Mesozoic extensional tectonics in the southeast Iberian Chain

AbstractThe Desert de les Palmes area, in the southeast Iberian Chain, belongs to a Mesozoic NE–SW high which separated the early Cretaceous basins of the Maestrat and Aliaga-Penyagolosa from the little Orpesa basin. Its structure is characterized by the development of a system of NE–SW to ENE–WSW extensional listric faults detached in a shallow upper crustal level (1.7–2.2 km), mostly affecting the pre-Upper Cretaceous rocks. These faults record two well-differentiated rifting periods: (1) a first late Triassic–early Jurassic rifting period that divided the Desert de les Palmes high in several blocks; (2) a second early Cretaceous rifting period, only developed in the eastern margin of the Desert de les Palmes high, which was related to the opening of the Maestrat, Aliaga-Penyagolosa and Orpesa basins. Based on the comparison of the main features of this Mesozoic structure with an analysis of the structural and subsidence data already known in the neighbouring Mesozoic basins (Maestrat, Aliaga-Penyagolosa and Columbrets), a geodynamic scenario for the crustal evolution of the eastern Iberian Chain is also suggested. This involves four evolutionary stages: (1) Triassic rift (late Permian–Hettangian); (2) early and middle Jurassic postrift (Sinemurian–Oxfordian); (3) late Jurassic and early Cretaceous rift (Kimmeridgian–middle Albian), which includes a short Hauterivian postrift period; and (4) late Cretaceous postrift (late Albian–Maastrichtian).

Tectonic Evolution and Hydrocarbon Potential of the Southern Moesian Platform and Balkan-Forebalkan Regions of Northern Bulgaria

The major tectonic elements of northern Bulgaria are the east-west-trending Balkan-Forebalkan fold belt and the Moesian platform. Moderate hydrocarbon exploration potential exists in trapping geometries generated during the tectonic evolution of the region coupled with reservoir/seal pairs and source rocks within Mesozoic strata. The tectonic evolution of the region includes Early Triassic to Early Jurassic intracratonic rifting followed by multiphase compression that contracted the rift basin and produced a north vergent fold and thrust belt along the southern margin of the stable Moesian platform. Compression began during the Early Cretaceous, continued during the Paleocene, and concluded during the middle Eocene. Trap types generated during the tectonic evolution include normal fault-bounded rotated blocks in the autochthonous section and elongate, asymmetric anticlines in the allochthonous section. Triassic to Upper Jurassic Marine facies were deposited in an east-west-trending rift. Sediments deposited in a shallow foredeep, which evolved during Lower cretaceous compression, overlay the rift sequence. The Early Mesozoic rift sequence provides the depositional settings for Middle Triassic and lower Middle Jurassic source rock shales and sandstone/carbonate reservoirs ranging from Middle Triassic to Lower Cretaceous. Carbonate reservoirs generally are porous dolomites with intercrystalline, moldic, and vugular pore types interbedded with nonporous limestones. Clastic reservoirsmore » are quartz-rich sandstones with pore types that are reduced intergranular, dissolution, and microporosity. These heterogeneous reservoir targets exhibit poor to good reservoir characteristics and are overlain with sealing lithologies of variable thicknesses.« less

Neptunian dykes and associated breccias (Southern Alps, Italy and Switzerland): role of gravity sliding in open and closed systems

ABSTRACTNeptunian dykes and sills of Middle Jurassic pelagic limestone within Lower Jurassic shallow‐water carbonate host rocks occur at many localities in the Southern Alps of Italy and Switzerland, especially on what were the upper slopes of tilted half‐grabens created during the Early Jurassic rifting stage of a passive margin that faced the Middle and Late Jurassic Tethyan Ocean. The host rocks were dilated by cracking, folding, and brecciation during movements of shallow‐based gravity‐driven slides and slumps of semibrittle platform strata, commonly along décollement contacts between layers of different competence. In most places, the network of cavities in the dilated strata connected to the sea floor, and pelagic sediments trickled from above into the open spaces. In other places, the brittle strata were overlain by somewhat impermeable sediments that formed a partial seal. Sudden dilation of the brittle beds resulted in forceful injection of the overlying weakly consolidated or plastic sediments into open spaces. The filling in both open and closed systems was commonly episodic, resulting in complex internal‐sediment stratigraphy and cross‐cutting dykes. Stable isotopic data on internal sediments and early‐formed cement lie within the field of normal sea water, and none of the sedimentological or stable isotopic data supports a subaerial, dissolution (karst) origin for the Jurassic neptunian dykes of this region.

Evolution of the San Jorge Basin, Argentina (1)

The San Jorge basin, although small, is the most important hydrocarbon-producing basin in Argentina. Remaining untested potential is high because of the presence of good source rock, favorable structural complexity, and multiple reservoirs. Reservoir quality is commonly low because of the highly tuffaceous sandstones. The sedimentary fill of the basin is closely related to its tectonic history. Northwest-southeast-trending grabens formed and filled during a Triassic and Early Jurassic early rift phase, climaxing with a pervasive Middle Jurassic volcanic episode; continued growth and filling of the basin occurred during a Late Jurassic-earliest Cretaceous late rift phase and Cretaceous early and late sag phases. Late Cretaceous-early Tertiary extension set up many of the present-day structural traps along normal faults. Middle Tertiary Andean compression produced the narrow, north-south San Bernardo structural belt, which exhibits reversed movement on older, normal, graben-bounding faults and on local, low-angle thrust faults. Marked early to middle Tertiary erosion produced a significant unconformi y within Cretaceous beds around basin margins. Origin of Upper Jurassic and lowermost Cretaceous sedimentary fill is primarily lacustrine or fluvial in origin. Lacustrine, organic-rich black shales are fringed by oolitic and other limestones and fluvial-deltaic sandstones derived mostly from the north. A significant southern source of sand existed during the Valanginian. Interbedded marine shales occur mostly to the west toward a presumed marine seaway connection to the northern Magallanes basin. Middle to Upper Cretaceous sedimentary rocks, sourced mostly from the north, are mainly fluvial sandstone-shale successions with some minor lacustrine influence. Reservoir-quality glauconitic sands were deposited during a Late Cretaceous-early Tertiary marine incursion from the Atlantic. After development of a lower Tertiary regional unco formity, relatively undisturbed younger Tertiary sediments completed the basin fill. Lower Cretaceous lacustrine shales of the D-129 Formation are the best source rocks yet identified. However, most hydrocarbon production occurs above the source in middle and Upper Cretaceous sandstones along the basin flanks. The most obvious hydrocarbon emplacement mechanism is vertical migration along reactivated graben faults.

Décollement in the Alpine system: an overview

Decollement in the Alpine system is implied in the «nappe» concept. Jura decollement, originally proposed byBuxtorf (1907), is the latest addition north of the Alps. The rheology of evaporites shows easy feasibility of Jura decollement which is required by material balance. In eastern Switzerland the front of the Alpine foreland thrust belt in the Molasse is a typical triangle structure (tectonic wedge), an important decollement structure also in other places and scales. The Helvetic nappes are flat- and-ramp structures similar to the Jura but translated by greater amounts and deformed under larger overburden. Corresponding basement deformation was largely ductile (basement lobes) and lagged behind. The Penninic nappes are usually metamorphosed in various degrees, except some cover nappes transported far into the foreland. Coordination between basement and displaced cover is uncertain. The Austroalpine cover nappes of Switzerland and adjacent Austria as a rule are only weakly metamorphosed and resemble the southern Alps in both stratigraphy and its expression in tectonics. The basement part of both are essentially upright slivers not more than five kilometers thick with a frontal ramp fold. Splitting apart of the sedimentary sequence particularly in the Carnian with tectonic wedging is frequent in both domains. Extensional decollement, occasionally accompanied by tectonic denudation and high cooling rates, occurred during several episodes, most notably during early Jurassic rifting and between the compressional phases (Gosau, Oligocene — early Miocene episodes). The main strike-slip elements of the transpressive Neoalps are the Insubric fault system with its dextral Riedel shears and the dextrally arranged brachyanticlines of the late Alpine windows which again imply decollement.

Lacustrine Strata of Early Jurassic Age, Hartford and Deerfield Rift Basins, Massachusetts and Connecticut: Deposition, Maturation of Black Mudstones, Sandstone Diagenesis, and Hydrocarbon Occurrences: ABSTRACT

Lacustrine cycles occur in the 100-m Shuttle Meadow, 170-m East Berlin, and lower part of the 1,200-m Portland Formations in the Hartford basin and in the 600-m Turners Falls Sandstone in the Deerfield basin. Eleven exposed cycles range in thickness from 3 to 10 m. Black mudstones are mostly 1-2 m thick, deposited in the deeper parts of oligomictic perennial lakes. Gray sandstones accumulated on delta lobes, fan deltas, and from sheetfloods that spread across the valley floor to cover exposed lacustrine littoral muds. Both rift-basin fills are dominated by alluvial-fan, fluvial, and playa red beds. The lacustrine sandstones are arkoses and lithic arkoses (72 modal analyses). Omitting minor cements, diagenetic events are (1) compaction, (2) quartz and albite overgrowths with albitization of detrital plagioclase, (3) dolomite and ferroan dolomite cements, and (4) hydrocarbon migration with creation of secondary porosity. Consistently large intergranular volumes in the lacustrine sandstones, as high as 40%, suggest that cementation began prior to significant compaction. Porosity in the sandstones ranges from 0 to 11%, averaging 2%. Organic geochemical data for black mudstones in the East Berlin, Portland, and Turners Falls Formations, combined with published and USGS open-file data, show that organic richness and oil pronenessmore » increase southward in the Hartford basin whereas thermal maturity increases northward. Oil generation has occurred in the East Berlin Formation, as evidenced by relatively large amounts of extractable bitumen, high-molecular-weight hydrocarbons, and hydrocarbons present in the sandstones. The Deerfield basin is overmature for oil.« less

Tectonics of the Dolomites (southern alps, northern Italy)

In post-Variscan times the Dolomites underwent a number of tectonic events, which may be summarized as follows: Permian and Triassic rifting phases broke the area into NS trending basins with different degrees of subsidence. A Middle Triassic transpressive event then deformed the region along a N70°E axis, generating flower structures within the basement. Volcano-tectonic domal uplift and subsequent caldera formation occurred at the same time as the Late Ladinian magmatism. Early Jurassic rifting also controlled the subsidence which increased eastward. This long period of deformation was followed by a pre-Neogene (Late Cretaceous-Palaeogene ?) EW (ENE-WSW) compression which generated a W-vergent belt, possibly equivalent to the folded foreland of the Dinaric chain. A 70 km EW section of the Dolomites indicates shortening of at least 10 km. During the Neogene the Dolomites, as far north as the Insubric Lineament, were the innermost part of a S-vergent thrust belt: the basement of the Dolomites was thrust southwards along the Valsugana Line onto the sedimentary cover of the Venetian Prealps for at least 10 km. This caused a regional uplift of 3–5 km. The Valsugana Line and its backthrusts on the northern side of the central Dolomites generated a 60 km wide pop-up in the form of a synclinorium within which the sedimentary cover adapted itself mainly by flexural-slip often forming triangle zones. The shortening linked to this folding is about 5 km with Neogene thrusts faulting and folding pre-existing thrust-planes. On the north-eastern side of the Dolomites, Neogene deformation is apparently more strictly controlled by the transpressive effects of the Insubric Lineament and shortening of the sedimentary cover may be greater than in the central Dolomites. Minor deformation linked to the Giudicarie belt is present in the western Dolomites. The present structure of the Dolomites is thus the result of a number of tectonic events of different significance and different strike. Only a 3-dimensional restoration can unravel the true structure of the Dolomites.

Sedimentary Evolution of Passive Margins of Mesozoic Tethys: ABSTRACT

The Alpine mountain chains of the Circum-Mediterranean area and the Near East are the result of oblique convergence between Africa and Eurasia. This convergence was, from the Late Triassic to Early Cretaceous, preceded by an oblique divergence during the opening of the Jurassic-Early Cretaceous Atlantic-Tethyan ocean. Rifting and spreading of this ocean are discordantly superimposed onto the preexisting late Paleozoic paleogeography of Pangea and the paleo-Tethys. Kinematic considerations suggest that in the Alpine-Mediterranean area the opening of Tethys was dominated by sinistral transform movements. Whereas, in the central Atlantic and western Mediterranean area, Late Triassic to Early Jurassic rifting occurred in a continental environment and was accompanied by alkaline volcanicity and evaporite deposition, east of the central Mediterranean the zones of rifting which eventually led to the opening of the oceanic Tethys did not follow the complex pattern of Triassic seaways, but occurred across the marine carbonate belts of the Gondwanian margin of paleo-Tethys. As a consequence, there are hardly any siliciclastic sediments associated with the Early Jurassic phase of rifting, and evaporite deposits of Jurassic age are conspicuously lacking along the rift zone. Depositional geometry of the synrift sediments, at places, suggests listric normal faulting as a possible mechanism of cru tal thinning, and the resulting pattern of tilted fault blocks compares very well with that found along undeformed margins, e.g., the Cretaceous Iberian or Armorican margins. In the Tethys, this rifting phase initiated a new paleogeography along the developing continental margins with Bahamian-type carbonate platforms, submarine plateaus, basins, and marginal highs. With the onset of spreading in the oceanic areas, subsidence rates decreased and were more evenly distributed over the margins; during this stage, subsidence apparently followed a curve of exponential decay. Sedimentary facies of the distal continental margins were then determined by increasing water depth and basin-wide paleo-oceanographic events. This general paleotectonic reconstruction of Tethyan margins is confirmed by c mparable sedimentary facies in undeformed margins of the Mesozoic central Atlantic. In the Mid-Cretaceous, plate motions in the Atlantic-Tethyan system changed drastically, and sinistral and opening movements in the Mediterranean were replaced by dextral and compressive ones leading to the complete elimination of the oceanic Tethys between the Late Cretaceous and late Eocene. End_of_Article - Last_Page 793------------

Depositional and Diagenetic Relationships Between Gulf of Elat (Aqaba) and Mesozoic of United States East Coast Offshore

The tectonic setting and depositional environments along the ancient margin during the Mesozoic of the East Coast of the United States may have been similar to those in the present Gulf of Elat (Aqaba). The Gulf of Elat (Aqaba) is the northern continuation of the Red Sea rift zone, where carbonates are accumulating contemporaneously with clastics under arid conditions. Clastics are deposited primarily in alluvial-fan complexes which spill out onto a narrow shelf. Carbonate deposits, including reef complexes, occur along the shelf break. In comparison, along the United States East Coast margin, Triassic to Early Jurassic rifting coincided with carbonate and clastic deposition. The Middle Jurassic and Cretaceous strata reflect the transition from synrift to postrift stages f tectonic evolution. Seismic and well data of the Jurassic and Cretaceous sections show an increase in the carbonate content approaching the United States East Coast shelf break. A discontinuous belt of carbonate buildups is inferred along the United States East Coast shelf and slope. The Gulf of Elat (Aqaba) represents an analogous model for the Early Jurassic East Coast margin of the United States. Beginning in the Middle Jurassic, the East Coast shelf-edge carbonate sequences were being deposited on a passive margin. Carbonate growth may have outpaced subsidence and may be similar stratigraphically to the Gulf of Elat (Aqaba) where carbonate deposits are prograding seaward. In the Gulf of Elat (Aqaba), calcium carbonate cementation has significantly reduced the porosity (28%) and permeability (0.01 md) of still-submerged carbonate reefs. However, Pleistocene carbonates on uplifted blocks in the adjacent onshore have undergone leaching and development of secondary porosity as a result of dissolution by meteoric fresh water. The uplifted carbonates contain high secondary porosity (60%) and permeability (10,000 md). Emergent carbonate strata exposed to percolating meteoric waters generally develop such high porosities. In the subsurface, former emergent surfaces are recognized as unconformities; among examples of Mesozoic carbonate reservoirs formed during subaerial exposure and marked by unconformities are the Casablanca (Upper Jurassic, Spain) and Golden ane (Cretaceous, Mexico) oil fields. Unconformities in the Mesozoic carbonate rocks of the United States East Coast suggest that potential carbonate reservoirs (one Jurassic and one Cretaceous) formed during periods of subaerial exposure. Hydrocarbons generated during the Early or Late Cretaceous may have moved into the reservoirs prior to deep burial, possibly inhibiting further subsurface diagenesis, including porosity and permeability changes, and preserving the reservoirs.