Rift-related differentiation of sedimentary environments: a case study from the Middle Triassic of NW Croatia

The opening of the Neotethys Ocean had a major influence on the sedimentary processes of the wider Greater Adria promontory. A stable Early Triassic shallow marine environment was disrupted by coeval tectonic and volcanic activity, peaking during the Late Anisian – Early Ladinian. Consequently, Middle Triassic volcano-sedimentary successions were deposited on top of Lower Triassic shallow-marine carbonates. These successions, recorded in the Southern Alps, the Dinarides, and the Transdanubian range, are interpreted as deposited in grabens/half grabens created by extensional tectonics and filled with products of volcanic activity intercalated with hemipelagic and pelagic sediments. Although the newly formed Middle Triassic rift-related basins were spatially limited, investigated successions from NW Croatia indicate their complex depositional environments with multiple factors controlling sedimentation.

The investigated Middle Triassic successions of NW Croatia are composed of both shallow-marine and pelagic carbonates and radiolarian chert intercalated with volcanic and several types of volcaniclastic lithologies. Shallow-marine carbonates are characterized by dolostone and limestone with algae, foraminifera, sponges, and in places reefal biota. In dolostone, a faint lamination resembling stromatolite can be observed.

Pelagic limestone contains abundant thin-shelled bivalves, calcified radiolarians, rare sponge spicules, and scarce ammonoids of Pelsonian to Illyrian age. In places, medium- to coarse-grained resedimented shallow-water material is present. Well-bedded, red, and often horizontally laminated radiolarian chert yielded radiolarians of late Illyrian-early Fassanian age. Volcanic rocks are geochemically determined as trachy-basalt and andesite-basalt, while several volcaniclastic lithologies are determined in the studied successions. Pietra verde deposits, composed of vitroclastic and crystalloclastic tuffs, represent the dominant volcaniclastic facies. These deposits show normal grading and horizontal lamination. Occasionally, volcaniclastic particles are mixed with pelagic deposits. Trachy-basaltic autoclastites, the second volcaniclastic facies, are found intercalated with pelagic biomicrite and pietra verde deposits. Volcanogenic sandstone and siltstone are found on top of other lithologies. These deposits exhibit a coarsening-upwards sequence and horizontal lamination. The mixing of volcanic and pelagic material is recorded in coarser intervals.

The described successions add to the existence of a coeval carbonate platform area and adjacent deeper basins. Resedimented carbonates indicate erosion, shedding, and gravitational redeposition of an active carbonate platform bordered by the steep normal faults. These same faults could have served as conduits for basaltic magma to reach the surface. Once in the cold marine environment, lava quenched and autofragmented serving as a source for autoclastic deposits. Newly formed clasts were subsequently reworked and redeposited into deeper parts of the basin. Pelagic limestone and radiolarian chert were deposited in a deeper environment by suspension settling in episodes of volcanic standstill. Pietra verde type tuffs were generated by explosive eruptions and deposited in the pelagic environment by gravitational mechanisms and syn-eruptive redeposition. Volcanogenic sandstone and siltstone are interpreted as deposited by turbiditic currents with the material sourced from the reworking of unconsolidated volcanic detritus.

The Middle Triassic differentiation of sedimentary environments in the limited area of the NW Croatia is inferred from specific sedimentary conditions controlled by multiple factors: basinal topography, gravitational processes, reworking and redeposition, availability of source material, and active tectonic/volcanic processes.