Early Jurassic foraminiferal assemblages in platform carbonates of Mt. Krim, central Slovenia

Abstract

During the Early Jurassic, the subtropical carbonate platforms of the peri-Tethys Ocean experienced significant changes in their architectures, as well as in their biota compositions. Shallow-water carbonates from the northern part of the ancient Adriatic Carbonate Platform (External Dinarides) were investigated in six sections, which taken together cover the development of the platform from deposition of the uppermost Triassic Main Dolomite to the middle Lower Jurassic, lithiotid limestone. Our aim was to establish a detailed foraminiferal biostratigraphy and to observe the changes in size, abundance and diversity of foraminifera in different types of facies. As a result, the succession was successfully divided into stage or substage levels. Foraminiferal assemblages were shown to experience a gradual change in taxonomic composition (including an increase in the proportion of complex agglutinated forms), a general increase in abundance of specimens, and greater diversity in each facies type, except in bindstone and mudstone. Notable is the difference between Hettangian assemblages, which display fairly uniform compositions in all facies types and the predominance of opportunists, and the post-Hettangian assemblages, which become progressively more species-rich and where the differences in facies are perhaps more pronounced. Changes in the size of the species Meandrovoluta asiagoensis Fugagnoli & Rettori, and of the largest specimen in the assemblages, however, are less clear, but are arguably present. Faunal changes roughly correspond to the gradual change from the flat-top platform of the upper Triassic - Hettangian, where biota would be repeatedly subjected to stressed peritidal conditions, to a platform differentiated into lagoon, sand bars and ephemeral emergent areas, offering numerous habitats and perhaps more stable living conditions for organisms.