In the Alpine Realm the Early Jurassic is characterized by the disintegration and partial drowning of vast platform areas. In the eastern part of the Southern Alps (present-day NW Slovenia), the ...Julian Carbonate Platform and the adjacent, E-W extending Slovenian Basin underwent partial disintegration, drowning and deepening from the Pliensbachian on, whereas only nominal environmental changes developed on the large Dinaric (Friuli, Adriatic) Carbonate Platform to the south (structurally part of the Dinarides). These events, however, were preceded by an earlier - and as yet undocumented extensional event - that took place near the Triassic/Jurassic boundary. This paper provides evidence of an accelerated subsidence from four selected areas within the Slovenian Basin, which show a trend of eastwardly-decreasing deformation. In the westernmost (Mrzli vrh) section - the Upper Triassic platform-margin - massive dolomite is overlain by the earliest Jurassic toe-of-slope carbonate resediments and further, by basin-plain micritic limestone. Further east (Perbla and Liščak sections) the Triassic-Jurassic transition interval is marked by an increase in resedimented carbonates. We relate this to the increasing inclination and segmentation of the slope and adjacent basin floor. The easternmost (Mt. Porezen) area shows a rather monotonous, latest Triassic-Early Jurassic basinal sedimentation. However, changes in the thickness of the Hettangian-Pliensbachian Krikov Formation point to a tilting of tectonic blocks within the basin area. Lateral facies changes at the base of the formation indicate that the tilting occurred at and/or shortly after the Triassic/Jurassic boundary
The Middle and Upper Jurassic succession of the Slovenian Basin is characterized by pelagic sedimentation of siliceous limestones and radiolarian cherts. In the southern and central part of the basin ...two packages of resedimented limestones are interbedded within pelagic sediments. The Lower resedimented limestones are lower-middle Bajocian to lower Callovian in age. In the southern part of the basin they form laterally discontinuous sequences composed of limestone breccias, calcarenites and micritic limestone and in the central part of the basin calcarenite intercalations within pelagic beds. They were transported by turbidity currents from highly productive ooidal shoals of the Dinaric Carbonate Platform. The Lower resedimented carbonates correlate with the lower three members of the Travnik Formation in the Bovec Trough and similarly developed but much thicker Vajont Formation in the Belluno Basin. The difference in thickness is interpreted as a consequence of shallow-water and longshore currents on the Dinaric Carbonate Platform that transported platform material towards southwest in the direction of the Belluno Basin. The Upper resedimented limestones are upper Kimmeridgian to lower Tithonian and occur within radiolarian cherts in the upper part of the succession as calcarenite beds that originated by turbidity currents. Onset of resedimentation coincides with the emersion-related demise of barrier reef and following deposition of micritic and rare oolitic limestones on the Dinaric Carbonate Platform. Approximatelly coeval resedimented limestones occur in the fourth member of the Travnik Formation in the Bovec Trough, but are not reported from the Ammonitico Rosso Superiore Formation in the Belluno Basin.
The Middle and Upper Jurassic succession of the Slovenian Basin is characterized by pelagic sedimentation of siliceous limestones and radiolarian cherts. In the southern and central part of the basin ...two packages of resedimented limestones are interbedded within pelagic sediments. The Lower resedimented limestones are lower-middle Bajocian to lower Callovian in age. In the southern part of the basin they form laterally discontinuous sequences composed of limestone breccias, calcarenites and micritic limestone and in the central part of the basin calcarenite intercalations within pelagic beds. They were transported by turbidity currents from highly productive ooidal shoals of the Dinaric Carbonate Platform. The Lower resedimented carbonates correlate with the lower three members of the Travnik Formation in the Bovec Trough and similarly developed but much thicker Vajont Formation in the Belluno Basin. The difference in thickness is interpreted as a consequence of shallow-water and longshore currents on the Dinaric Carbonate Platform that transported platform material towards southwest in the direction of the Belluno Basin. The Upper resedimented limestones are upper Kimmeridgian to lower Tithonian and occur within radiolarian cherts in the upper part of the succession as calcarenite beds that originated by turbidity currents. Onset of resedimentation coincides with the emersion-related demise of barrier reef and following deposition of micritic and rare oolitic limestones on the Dinaric Carbonate Platform. Approximatelly coeval resedimented limestones occur in the fourth member of the Travnik Formation in the Bovec Trough, but are not reported from the Ammonitico Rosso Superiore Formation in the Belluno Basin.
The Slovenian Basin is a Mesozoic deep-water paleogeographic unit, located along the border between the eastern Southern Alps and the Dinarides, that records geodynamic signals from the opening of ...both the Piedmont-Liguria and the Neotethys oceanic domains. In the Middle Jurassic, it was bordered by the Dinaric (Adriatic) Carbonate Platform to the south and the Julian High submarine plateau to the north. The southern margin of the basin is characterized by a several tens of meters thick sedimentary sequence of Bajocian-Bathonian (Callovian?) age that is dominated by limestone megabreccia shed from the Dinaric Carbonate Platform, sedimented by debris-flows in a toe-of-slope sedimentary environment. It is accompanied by rud/grain/packstone beds sedimented via (high-density) turbidity-flows. This megabreccia unit represents the proximal equivalent of the lower resedimented limestones of the Tolmin Formation. The matrix within lithoclasts indicates resedimentation from ooidal shoals and the erosion of basinal and slope sediments. Lithoclasts are of Norian to Lower Jurassic age, and originated from (A) platform margin carbonates, i.e. Triassic marginal reef and Lower Jurassic sand-shoal limestones, (B) deep open-shelf or slope coarse bioclastic limestones, and (C) older basinal rocks. The lithoclast analysis enables the reconstruction of the platform-basin transitional zone that is not preserved (exposed) due to overthrusting. The limestone megabreccia indicates enhanced tectonic activity causing repeated collapse of the platform margin, probably connected to the initiation of intraoceanic subduction within Neotethys followed by ophiolite obduction onto the eastern distal margin of the Adria.
For better understanding of long-term environmental changes in a mid-oceanic atoll-type buildup (Akiyoshi Limestone Group), we examined the atoll-derived carbonate sediments that were ...contemporaneously shed into the spicular chert of slope-to-basin facies (Ota Group) in the Akiyoshi area, southwest Japan. Both of these two units are dated as upper Viséan to Capitanian, and interpreted as having laterally passed into each other on and around a mid-oceanic seamount.
The resedimented limestone occurs as slab-like bodies, and isolated and laterally discrete smaller pods, and thin beds and laminae in the spicular chert of the Ota Group. The composite succession (ca. 105 m thick) measured in slab-like bodies is divided into the lower and upper units, each of which consists of the sharp-based calcarenite, siliceous lime–mud/siltstone, and spicular chert with blocks, beds, and laminae of calcarenite and siliceous limestone in ascending order. The calcarenite forming slabs and smaller blocks are massive and dominantly comprises densely packed, coarse crinoid debris and subordinate bryozoan fragments with no or sparse matrix. The siliceous lime–mud/siltstone consists of siliceous sponges spicules, minute calcitic skeletal debris, and peloidal particles, with the matrix of a mixture of lime–mud and microcrystalline quartz. The calcarenite and siliceous lime–mud/siltstone beds and laminae in the spicular chert occasionally contain graded and parallel laminae. The massive calcarenite, and graded calcarenite and siliceous limestone were formed by redeposition mainly of skeletal sands presumably by grain flows with minor debris flows, and by mixing of sponge spicule-rich siliceous sediments and redeposited lime–mud and skeletal debris by muddy turbidites of distal facies, respectively. These redeposited carbonate sediments were exported from the Akiyoshi Limestone Group onto the spicular sediments of slope-to-basin facies. Conodonts, fusulinids, and smaller foraminifers from the resedimented limestone indicate that the redeposition of the carbonate sediments took place dominantly in late Viséan to late Bashkirian, and subordinately in Sakmarian to Capitanian time. The absence of the Moscovian to Asselian resedimented limestone implies much-reduced carbonate shedding during this period.
The correlation with the interpreted palaeoenvironmental episodes in the Akiyoshi buildup shows that late Viséan to late Bashkirian and Sakmarian to Capitanian carbonate-shedding events correspond to the times of generally warm climate and high sea-levels in the Akiyoshi buildup. Moscovian to Asselian time, which lacks records of carbonate shedding, nearly corresponds to the Moscovian to Gzhelian cooling and sea-level lowering in the Akiyoshi buildup. We consider that the shedding of carbonate sediments from the Akiyoshi atoll is its depositional response to the warm climate and elevated sea-levels and is compared with a sedimentary system of the highstand shedding of carbonates. Atoll-type carbonates equivalent to the Akiyoshi Limestone Group in the Akiyoshi terrane also show comparable responses of carbonate shedding into slope and slope-to-basin facies.
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