Middle Jurassic continental to marine transition in an extensional tectonics context: the Genna Selole Fm depositional system in the Tacchi area (central Sardinia, Italy)

Abstract

In Eastern Sardinia during the early Middle Jurassic, Alpine Tethys opening triggered the rise of a temporary tectonic high. The high collapsed\ud rapidly, was fragmented into separate blocks, and subsequently covered by continental, transitional, and finally shallow marine deposits\ud forming a narrow depositional system comprising the Genna Selole Fm. Present-day exposures in the southern part of the palaeo-high allow\ud the sedimentological evolution of the transgressive cover sequence to be ascertained. Initial terrestrial deposits comprise alluvial fan deposits\ud located at the mouths of palaeovalleys. These pass into braid-deltas and in the coastal areas located between adjacent valleys mouths,\ud palustrine and coastal plain tidally-influenced environments developed. These environments interfingered laterally and passed seaward into\ud a transitional, siliciclastic to carbonate tidal environment. With the collapse of the tectonic high, the continental to transitional environments\ud were transgressed with deposition of marine carbonates. A comparison with similar coeval deposits of the W-Mediterranean domain has been\ud undertaken

The Middle Jurassic Alpine Tethyan Unconformity and the Eastern Sardinia - Corsica Jurassic High: A sedimentary and regional analysis

“…20e). The presence of paleokarstic features in the limestones of the topmost Genna Selole Fm lithofacies suggests ephemeral emersions punctuating the subsiding trend (Costamagna, 2015).…”

“…Finally, the topmost lithofacies is 3) the Usassai-Perdadefogu lithofacies, which alternates carbonates and fine-to middle-grained siliciclastics locally with coal seams. This is typical of a deltaic-mixed lagoon environment ("coastal wetlands" sensu Suárez-González et al, 2015), and suggests the gradual passage to the carbonate shelf of the upper Dorgali Fm (Costamagna and Barca, 2004;Costamagna, , 2015 (Fig. 2).…”

“…2) overlies a regional unconformity (see below) and starts with continental conglomerates and sandstones before passing into marine carbonates of the Genna Selole Fm (Bajocian-Bathonian) (Del Rio, 1976Dieni et al, 1983;Dieni and Massari, 1985a;Costamagna and Barca, 2004;Costamagna et al, 2007;Costamagna, 2015). This unit is up to 80 m thick in the SE of the island, and is composed of: 1) the Laconi-Gadoni lithofacies, which is a coarse, pebbly to sandy, deposit indicating fluvial high-to middle-energy braided alluvial fan environments, followed by 2) the Nurri-Escalaplano lithofacies, which is composed of finely-laminated sandstones, siltstones and clayey siltstones sedimented in low-energy coastal-palustrine-deltaic environments.…”

“…While in the SE area, the Genna Selole Fm typically has a significant thickness and lateral persistence ( Fig. 7), moving northwards it is usually characterized by a few metres of laterally discontinuous, quartz-rich impure carbonates that are locally affected by paleokarstic features (Costamagna, 2015) belonging to the topmost, mainly calcareous, facies. The lower, coarser siliciclastics are only found in small pockets and swales representing morphological irregularities of the transgressed basement surface.…”

“…Relying on previously revealed data, along with the irregular northwards thinning trend, the Genna Selole Fm shows a clear mean fining-upwards trend, as well as a transgressive trend. The related depositional environments are, from the base: continental braided fluvial fans interfingered in the upper part with transitional mixed coastal-palustrine-deltaic contexts; and carbonate lagoons / shoals (Costamagna, , 2015. The fining-upwards succession and the evolving sedimentary environments have been also interpreted as an evidence for the polyphase peneplanation of the mainland of Sardinia.…”