Compositional changes in sigmoidal carbonate clinoforms (Late Tithonian, eastern Sardinia, Italy): insights from quantitative microfacies analyses

Abstract

Quantitative analysis of sediment composition was performed on a kilometre wide section of Upper Tithonian low relief (up to 70 m), gently inclined (3° to 15°), sigmoidal carbonate clinoforms (eastern Sardinia) to identify changes in sediment composition along the slope and across the studied succession. These changes may reflect modifications of the carbonate factory and of processes responsible for sediment transport. Point‐count analysis of carbonate microfacies, Q‐mode/R‐mode cluster analysis and Spearman’s rank provided a composition‐based classification of microfacies and highlighted relationships among sediment components. The studied clinoforms are mainly composed of non‐skeletal grains (70%), such as peloids and lithoclasts, together with micrite and cements and only a limited contribution from coated grains (2%). Among skeletal grains (28%), the greatest contribution derives from a coral–stromatoporoid–encruster reef that provided 15% of the components. Crinoids, brachiopods and other along‐slope thriving biota provided nearly 5% of the allochems, whilst fragments of molluscs (gastropods, bivalves and diceratids) from the backreef sourced another 2%. The contribution of platform interior biota is negligible (1%). The association of composition‐based facies varies along the slope. The upper slope beds consist of coral‐stromatoporoid grainstone to rudstone; the middle slope deposits are dominated by encruster‐lithoclast grainstone and packstone. At the lower slope, peloidal lithoclastic packstone as well as brachiopod–crinoidal wackestone prevail. Also the association of skeletal grains changes along the slope. The encruster–frame builder association typifies the upper slope whilst encrusters characterize the middle slope sediments. In the lower slope encrusters are equally represented as the brachiopod–crinoid association. Along‐slope compositional changes evidence a scarce downslope transport of frame builders and a progressive enrichment in along‐slope thriving biota. Quantitative analysis of microfacies allowed the sigmoidal clinoforms to be grouped into six sets. Each set gathers sigmoids with a similar sediment composition. Coated grains are dominant in the first set whilst they are lacking in the overlying sets reflecting a change in the carbonate factory. Other major compositional changes among the sets concern the relative amounts of peloids, micrite, frame builders (corals and stromatoporoids) and encrusters. The contribution of peloids varies inversely to that of cements and micrite as evidenced in the third and fifth sets which, respectively, record the highest occurrence of peloids or cement and micrite. Variations in the amount of frame builders and encrusters are instead non‐linear. High percentages of both frame builders and encrusters, as recorded in the second and fifth sets, are related to low amounts of peloids and lithoclasts that probably reflect episodes of reduced background sedimentation. This study demonstrates that quantitative analysis of carbonate microfacies represen...

Comments on “The Cenozoic fold-and-thrust belt of Eastern Sardinia: Evidences from the integration of field data with numerically balanced geological cross section” by Arragoni et al., 2016

“…The rotation resulted in a series of strike‐slip faults controlling the present‐day distribution of Mesozic carbonate outcrops in Eastern Sardinia [ Carmignani et al, ; Pasci , ; Pasci et al, ; Dieni et al, ; Oggiano et al, ]. This Mesozoic succession consists of a thick Jurassic‐Cretaceous carbonate succession [ Dieni and Massari, ; Jadoul et al , , ; Lanfranchi et al, ] that records the depositional history of the northern margin of the Penninic Ocean, unconformably covering the Variscan Basement. Structural studies on Eastern Sardinia identify an Oligo‐Miocene strike‐slip tectonic phase affecting the Mesozoic carbonates, locally with transpressional or transtentional component [ Carmignani et al, ; Pasci , ; Pasci et al, ; Oggiano et al, ].…”

“…However, available geological literature indicates the presence of Oxfordian to Early Tithonian oolitic shoal and reefal facies of the Tului Fm. [ Lanfranchi et al , ; Jadoul et al, , ]. The authors' statement that the Pedra Longa Formation is lateral equivalent with the Mt.…”

“…[ Lanfranchi et al . , , ; Jadoul et al, , ]. The authors ascribe all the present‐day bedding attitude to compressional tectonism, without restoring this data set to remove the effects of the younger strike‐slip tectonism, which played a major role in defining the present‐day structural architecture of the study area [ Pasci , ; Pasci et al, ; Oggiano et al, ]. For example, increasing bed dips recorded along the western flank of the “Baunei Sincline” (their Figures 2 and 5) reflects the presence of prograding carbonate clinoforms and not only tectonic deformation. For example, Lanfranchi et al .…”

“…For example, Lanfranchi et al . [] clearly illustrate bedding changes related to clinostratification in a prograding platform above flat‐lying basinal sediments. The Late Tithonian carbonate clinoforms of Nuraghe Punnacci (shown by Arragoni et al [, Figure 6d]), display primary slope angles up to 15–20°, dipping to the east.…”

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

“…It is worth considering that Dieni et al (2013) interpreted the Genna Selole Fm upper Ussassai-Perdasdefogu lithofacies sediment as the lower member of the Dorgali Fm ("Perda Liana Mb"). Jadoul et al (2010), Lanfranchi et al (2011) andCasellato et al (2012) proposed a more detailed frame of the Middle to Late Jurassic carbonate stratigraphy of E Sardinia: that evidences the presence of ephemeral intraplatform basins during part of the Late Jurassic.…”