Development of transient carbonate ramps in an evolving foreland basin

Abstract

This study of Eocene carbonate succession in the Dinaric Foreland Basin of northern Dalmatia, Croatia, integrates palaeontological and sedimentological data to document a range of carbonate ramps formed intermittently during the basin tectonic development. The end‐Cretaceous basal erosional unconformity records the coupling of Adria and Eurasia crustal plates, with an antiformal uplift along their suture zone. The overlying late Ypresian carbonate ramp, spanning biozones SBZ 11–12, developed on the forebulge flank of a shallow‐marine early synclinal basin. Basal grainstone/packstone facies, dominated by encrusting foraminifers with alveolinids and miliolids, pass upwards into packstones dominated by miliolids and rotaliids with bryozoan and echinoid fragments, indicating an increased bathymetry of the retreating forebulge flank. Deposition of grainstone facies preceded an end‐Ypresian (SBZ 12/13 transition) subaerial exposure due to post‐subductional isostatic uplift. The younger, middle to late Eocene carbonate ramps (SBZ 13–19) formed episodically as perched isolated features on blind‐thrust anticlines in a bathymetrically diversified wedge‐top basin, where phases of clastic and skeletal biogenic sedimentation alternated due to disharmonic thrusting and relative sea‐level changes. Clastic sedimentation reflects anticline crest erosion and a forced‐regressive progradation of gravelly foreshore and sandy shoreface facies over heterolithic offshore‐transition and muddy offshore facies on the anticline flank. Biogenic sedimentation represents inner‐ to middle‐ramp environments, with the latter terminating bluntly in muddy offshore environment. An outer‐ramp environment, known from classic ramp models, was lacking due to bathymetric threshold. Analysis of larger benthic foraminifers (LBF), as biostratigraphic age indicators and palaeobathymetric proxies, helped distinguish systems tracts and determine their time span. A comparison of local and global sea‐level changes allowed the interplay of tectonic and eustatic forcing to be deciphered for the study area.

Geological and structural map of the southeastern Pag Island, Croatia: field constraints on the Cretaceous - Eocene evolution of the Dinarides foreland

“…In the Ravni-Kotari peninsula, some tens of kilometres south-east of the study region, a Middle Eocene to Oligocene succession of neritic to terrestrial deposits ('Promina beds') was accumulated in a piggyback basin (Korbar, 2009) with a ridge-and-swale bathymetry controlled by blind-thrusts anticlines. Growth strata and migrating unconformable onlaps with the bedrock indicate that the Eocene Promina Beds were deposed during fold growth, and include clastic carbonates derived from the erosion of the anticline crests Ćosović et al, 2018). The Oligocene upper part of the Promina beds is a 'molasse' sequence of orogeny-derived clastic deposits within fluvio-deltaic complexes progradating on the Dalmatian flysch (Korbar, 2009;Zupanič and Babić, 2011;Vlahović et al, 2012).…”

“…Sedimentation resumed in Early Eocene times, when the Northern Dalmatia constituted the foreland basin of the Dinarides. The foraminiferal limestone (Tišljar et al, 2002;Ćosović et al, 2004) was deposited on a carbonate ramp, interpreted as a retreating forebulge flank (Korbar, 2009;Babić and Zupanič, 2016;Ćosović et al, 2018). Shallow water carbonates were replaced during middle Eocene times by clastic hemipelagic sediments known as Dalmatian flysch (e.g., Babić and Zupanič, 2008), and by the upper Eocene-lower Oligocene regressive molasse sequence of the Promina Beds (Mrinjek, 1993;Tari Kovačić and Mrinjek, 1994;Vlahović et al, 2012;Zupanič and Babić, 2011), exposed some tens of kilometres to the southeast of the Pag Island.…”

“…Shallow water carbonates were replaced during middle Eocene times by clastic hemipelagic sediments known as Dalmatian flysch (e.g., Babić and Zupanič, 2008), and by the upper Eocene-lower Oligocene regressive molasse sequence of the Promina Beds (Mrinjek, 1993;Tari Kovačić and Mrinjek, 1994;Vlahović et al, 2012;Zupanič and Babić, 2011), exposed some tens of kilometres to the southeast of the Pag Island. Sedimentological studies demonstrated that the Promina Beds were deposed during fold growth, thus constraining imbrication and folding in the southeastern part of the External Dinarides to Late Eocene-Oligocene times (Ćosović et al, 2018;Vlahović et al, 2012). Another clastic body which characterizes the External Dinarides is the so-called Jelar breccia, a chaotic polymictic breccia of prevailing Jurassic to Cretaceous carbonate clasts and including also clasts of middle Eocene age (Pamić et al, 1998;Korbar, 2009;Vlahović et al, 2012).…”

Larger foraminiferal biostratigraphy and microfacies analysis from the Ypresian (Ilerdian-Cuisian) limestones in the Sistan Suture Zone (eastern Iran)

“…inner platform/ramp/shelf) and then were replaced by hyaline-lamellar foraminifera assemblages such as nummulitids (i.e. Nummulites, Assilina, and Operculina) and orthophragminids with increasing water depth in a seaward direction (Luterbacher, 1998;Höntzsch et al, 2010;Ćosović et al, 2017). In the present study, the middle Ilerdian-middle Cuisian (SBZ8 and SBZ10-SBZ11) are represented by two different microfacies types comprising Alveolina-Opertorbitolites packstone (locally grainstone) (MFT1) ( Figures 11a-11d and 12a) and nummulitids-Alveolina rudstone (MFT2) ( Figures 11e, 11f, and 12b).…”

“…inner platform/ramp/shelf) and then were replaced by hyaline-lamellar foraminifera assemblages such as nummulitids (i.e. Nummulites, Assilina, and Operculina) and orthophragminids with increasing water depth in a seaward direction (Luterbacher, 1998;Höntzsch et al, 2010;Ćosović et al, 2017 MFT5 was deposited in the proximal middle ramp settings, below the FWWB and above the storm wave base, where orthophragminids thrived in deeper parts during SBZ12. In the above-mentioned area, morphological characteristics of orthophragminid tests (i.e.…”