Non-condensed shell beds in hiatal successions: instantaneous cementation associated with nutrient-rich bottom currents and high bivalve production

Palaeogeographic reconstruction and sequence architecture of the middle‐upper Jurassic successions of Hawraman Basin (NW Iran): Implications for tectono‐depositional processes of the northeastern passive margin of the Arabian Plate

“…Typically, the assemblage of Posidonia ( Bositra ) and related genera have been reported as representing pelagic to hemipelagic conditions under low‐oxygen conditions (Abdula et al, 2015; Molina, Reolid, & Mattioli, 2018). In addition, this study as well as some other studies (Molina et al, 2018; Navarro et al, 2009; Negra et al, 2011; Rivas, Aguirre, & Braga, 1997; Tomašových, Schlögl, Michalík, & Donovalová, 2020) suggests the presence of Bositra shells is concentrated in storm‐dominated inner and outer shelf strata or slope/basin strata deposited under the influence of turbidity currents. In the Sargelu Formation, Bositra shells are found in both coarse‐ (calciturbidite facies; F2a and F2b or storm‐generated beds; F3a–F3b, and F4a) and fine‐grained (mud‐rich, low‐energy facies; F1b–F1c) substrates of the middle shelf to the basin with oxic to dysoxic conditions.…”

“…Typically, the assemblage of Posidonia (Bositra) and related genera have been reported as representing pelagic to hemipelagic conditions under low-oxygen conditions (Abdula et al, 2015;Molina, Reolid, & Mattioli, 2018). In addition, this study as well as some other studies (Molina et al, 2018;Navarro et al, 2009;Negra et al, 2011;Rivas, Aguirre, & Braga, 1997;Tomašových, Schlögl, Michalík, & Donovalová, 2020) ). The accumulation rate of the pelagic to hemipelagic deposits was uniform, and thus, Bositra limestones (F1-F3) formed that do not show significant facies changes.…”

“…The predominant impact of currents and high‐energy waves can be deduced from the presence of mud‐winnowing bottom current criteria, rapid cementation, coarse size of shells (thick fragments more resistant to wave‐induced transportation), and high degrees of fragmentation and abrasion of Bositra valves. Moreover, the co‐occurrence of thick and thin (or small patches) shell fragments with different degrees of packing density (dense to sparse) suggests that many of the Bositra shells were either influenced by multiple events of shell accumulation (not far from their life habitat; Tomašových et al, 2020) or subjected to bioturbation before cementation.…”

“…The predominant impact of currents and high-energy waves can be deduced from the presence of mudwinnowing bottom current criteria, rapid cementation, coarse size of shells (thick fragments more resistant to wave-induced transportation), and high degrees of fragmentation and abrasion of Bositra valves. Moreover, the co-occurrence of thick and thin (or small patches) shell fragments with different degrees of packing density (dense to sparse) suggests that many of the Bositra shells were either influenced by multiple events of shell accumulation (not far from their life habitat;Tomašových et al, 2020) or subjected to bioturbation to the Oxfordian boundary (Middle to Late Jurassic boundary), a sharp shift occurred from the Sargelu deep marine deposits with high-density oligotypical fossils (Bositra limestones) to shallow marine deposits of the Najmah Formation (represented mainly by stromatolitic-thrombolitic boundstones) (Figures12d and 13c).Facies evolution, internal architecture, and stacking pattern of the short-term T/R facies cycle (TR4-TR6) in the Najmah Formation show that the deposition was characterized by relatively low accommodation space. The evidence of subaerial exposure at the top of the cycles, the absence of flooding and onlapping on the sequence boundaries, and lateral thickness variations indicate that tectonic instability was still heavily involved during the deposition of this formation (see below).Despite the occurrence of deep subtidal facies, the absence of gravity flow deposits advocates that the gradient along the newly established platform was not steep enough to create allochthonous sediments as before.…”

Bottom currents on a pelagic carbonate platform: Mounds and sediment drifts in the Jurassic succession of the Sciacca Plateau, Western Sicily

“…They have been interpreted as nektoplanktonic organisms (Jefferies & Minton, 1965), as benthic organisms (Kauffman, 1978, 1981; Pompecky, 1901) or as pseudoplanktonic living in “pendent” position attached to floating wood or seaweed (Duff, 1978; Stanley, 1972), or to living or dead floating ammonite shells (Seilacher & Westphal, 1971). Investigations in the Umbria‐Marche region, based on functional morphology studies, facies analysis, and stratigraphical distribution of two different species, Bositra buchii and Lentilla humilis (Conti & Monari, 1992), seem to exclude a planktonic mode of life for Jurassic Tethyan thin‐shelled bivalves (see also Molina et al, 2018, and references therein, and Tomašových et al, 2020). Having said that, posidoniid bivalves could hardly have acted as sediments bafflers or trappers or binders, nor do we see at San Vincenzo any evidence for a constructional strategy, which should be revealed by a range of morphotypes or species of the bivalve itself across the mound (see Liassic bivalve biostromes from the Moroccan High Atlas as described by Wilmsen & Neuweiler, 2008, for contrast).…”