Paleogene deposits are scattered in S Sardinia. According to previous reconstructions, during the Paleogene an E-W transition between open to confined environments occurred, giving place to a restricted gulf in the W. In SW Sardinia the uppermost Paleocene – Lower Eocene 200 m thick Produttivo group rests unconformably over older rocks. Its base is formed by continental siliciclastics (Arenarie di Monte Margiani Fm, 0–25 thick). These siliciclastics are followed by the marine Miliolitico limestones (30–50 m), and by the Lignitifero siliciclastic to carbonate succession with lignite layers at the top (70–120 m), of coastal-deltaic (tidal?) environments. This succession represents the tectono-sedimentary transgressive – regressive Pyrenean cycle. The Middle Eocene – Lower Oligocene continental Cixerri Fm follows unconformably. The link between the SW and the SE Sardinia successions is represented by the central Sardinia 35 m thick Nuraghe Sioco Lower Eocene siliciclastic to minor carbonate fossiliferous succession. It represents an emerged(?) mudflat featured by frequent marine ingression. The exposed sections of this unit are too restricted to allow any reconstruction by now. Limited tidal influences are possible. In SE Sardinia crops out a 250 m thick Early-Middle? Eocene transgressive-regressive sedimentary cycle. It rests unconformably over all the previous units and it is represented by the 10 m thick Santa Caterina Fm, followed by the unconformable 240 m thick Monte Cardiga Fm. The Santa Caterina Fm is a palustrine-lacustrine-alluvial unit reworking the Triassic carbonate substrate and followed by thin siliciclastic deposits. The upper Monte Cardiga Fm is featured by siliciclastic deposits with carbonate-siliciclastic intercalations. Five lithofacies delimited by gradual boundaries are present. They are, from the bottom 1) continental, siliciclastic coarse lithofacies; 2) transitional sandstone-carbonate lithofacies; 3) laterally discontinuous, lens-shaped marine calcarenitic lithofacies; 4) marine to transitional carbonate-sandstone-pelitic lithofacies; 5) continental sandstone-conglomerate lithofacies. Every lithofacies represents a specific depositional environment. Following the continental environment of the lithofacies 1, the transitional lithofacies 2 is represented by sandstones whose bedforms are arranged in meanly EW fashion as meter-sized dunes with 35°-dipping foresets. The lithofacies 3 carbonate nummulite-bearing intercalations stick out as EW-elongated humpback from a plateau: they are organized as lens-shaped, slim bodies featured by fossiliferous grainstonespackstones with cross-bedding EW-oriented. The lithofacies 4 show possible tidal bundles with slack-water laminae. The lithofacies 5 marks a return to the original continental conditions. Sandstones are litharenites rich in rock fragments from the Variscan basement passing to lithic arkoses with k-feldspars deriving from the lower granitoid and so implying the unroofing of the basement itself. The features of the Monte Cardiga Fm marine part suggest deposition in tide-influenced environments with elongated tidal bars. The outcrops distribution and the former palaeogeographic reconstructions indicate that a tidal mechanism forced by a tectonics-related funneling was in effect. Ephemeral communications between the Sardinia SW and SE sectors were assured by the shallow, choked Tanca Aru strait where a thin Produttivo Group succession 7–8 m thick featured by lagoonal to palustrine-lacustrine deposits. This way the S Sardinia Eocene outcrops have a possible paleoenvironmental framework.
The Eocene Monte Cardiga Fm (Sardinia, Italy): a tidal strait towards a westward coal basin?
Costamagna Luca Giacomo
2021-01-01
Abstract
Paleogene deposits are scattered in S Sardinia. According to previous reconstructions, during the Paleogene an E-W transition between open to confined environments occurred, giving place to a restricted gulf in the W. In SW Sardinia the uppermost Paleocene – Lower Eocene 200 m thick Produttivo group rests unconformably over older rocks. Its base is formed by continental siliciclastics (Arenarie di Monte Margiani Fm, 0–25 thick). These siliciclastics are followed by the marine Miliolitico limestones (30–50 m), and by the Lignitifero siliciclastic to carbonate succession with lignite layers at the top (70–120 m), of coastal-deltaic (tidal?) environments. This succession represents the tectono-sedimentary transgressive – regressive Pyrenean cycle. The Middle Eocene – Lower Oligocene continental Cixerri Fm follows unconformably. The link between the SW and the SE Sardinia successions is represented by the central Sardinia 35 m thick Nuraghe Sioco Lower Eocene siliciclastic to minor carbonate fossiliferous succession. It represents an emerged(?) mudflat featured by frequent marine ingression. The exposed sections of this unit are too restricted to allow any reconstruction by now. Limited tidal influences are possible. In SE Sardinia crops out a 250 m thick Early-Middle? Eocene transgressive-regressive sedimentary cycle. It rests unconformably over all the previous units and it is represented by the 10 m thick Santa Caterina Fm, followed by the unconformable 240 m thick Monte Cardiga Fm. The Santa Caterina Fm is a palustrine-lacustrine-alluvial unit reworking the Triassic carbonate substrate and followed by thin siliciclastic deposits. The upper Monte Cardiga Fm is featured by siliciclastic deposits with carbonate-siliciclastic intercalations. Five lithofacies delimited by gradual boundaries are present. They are, from the bottom 1) continental, siliciclastic coarse lithofacies; 2) transitional sandstone-carbonate lithofacies; 3) laterally discontinuous, lens-shaped marine calcarenitic lithofacies; 4) marine to transitional carbonate-sandstone-pelitic lithofacies; 5) continental sandstone-conglomerate lithofacies. Every lithofacies represents a specific depositional environment. Following the continental environment of the lithofacies 1, the transitional lithofacies 2 is represented by sandstones whose bedforms are arranged in meanly EW fashion as meter-sized dunes with 35°-dipping foresets. The lithofacies 3 carbonate nummulite-bearing intercalations stick out as EW-elongated humpback from a plateau: they are organized as lens-shaped, slim bodies featured by fossiliferous grainstonespackstones with cross-bedding EW-oriented. The lithofacies 4 show possible tidal bundles with slack-water laminae. The lithofacies 5 marks a return to the original continental conditions. Sandstones are litharenites rich in rock fragments from the Variscan basement passing to lithic arkoses with k-feldspars deriving from the lower granitoid and so implying the unroofing of the basement itself. The features of the Monte Cardiga Fm marine part suggest deposition in tide-influenced environments with elongated tidal bars. The outcrops distribution and the former palaeogeographic reconstructions indicate that a tidal mechanism forced by a tectonics-related funneling was in effect. Ephemeral communications between the Sardinia SW and SE sectors were assured by the shallow, choked Tanca Aru strait where a thin Produttivo Group succession 7–8 m thick featured by lagoonal to palustrine-lacustrine deposits. This way the S Sardinia Eocene outcrops have a possible paleoenvironmental framework.
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