engleski

Discontinuity surfaces in Upper Cretaceous carbonate deposits from central Dalmatia, Croatia

Discontinuity surfaces, including subaerial exposure, condensation and composite surfaces, are commonly present in carbonate rock successions and may be of great significance in stratigraphic determinations and correlations. We conducted field and petrographic studies of features associated with several such surfaces in Upper Cretaceous shallow-marine platform carbonate strata from the Šibenik and Hvar localities in central Dalmatia, Croatia. These strata were deposited in the southern part of the longlasting (Late Triassic to Eocene) Adriatic-Dinaridic Carbonate Platform (ADCP) domain, also named the Adriatic Carbonate Platform (AdCP). At both study sites, paleosols and paleokarst are developed beneath a regional subaerial unconformity within Upper Cretaceous carbonates overlain by Paleogene deposits. This unconformity is commonly represented elsewhere on the ADCP domain by bauxitic deposits. The Šibenik locality is characterized by calcretes with dense or laminated micrite and sparite groundmass with various biogenic features (beta microfabric) including alveolar septal structures and micritic cylinders, rhizoliths represented by root moulds and tubules (extracellular calcification of roots), as well as multi-, single-layered and isolated isodiametric calcified root cells (intracellular calcification of roots), and controversial typical single-layered Microcodium aggregates (corn-cob and sphaeroidal). Typical Microcodium aggregates (predominantly laminar, but also corn-cob and transitional forms) are also commonly associated with isodiametric calcified root cells (arranged in multilayers), with some of the observed structures possibly representing intermediate morphologies between these two types of structures. Complex rhizocretions and root mats are also present. Some of these features are found in nodules, coated grains and crusts associated with peloids and indicating rhizobrecciation. Calcretes from the Hvar locality are represented by laminar, commonly dome-shaped and multi-component fabrics that coat cavity walls, form rims on spar-filled cavities (of possible root related origin), and are associated with coated and compound grains. Additional subaerial exposure features observed on Hvar include in situ and resedimented speleothems forming clast supported breccias, as well as calcretized non-carbonate paleosols. Several condensed surfaces are also present within Upper Cretaceous carbonate deposits at Šibenik. They are represented by firmgrounds characterized by well-developed Thalassinoides burrows of the Glossifungites ichnofacies. Also present is one composite surface with a well-developed firmground Thalassinoides boxwork burrow system filled with laminated calcretes interlayered with marine sediments. Calcretes show well developed beta microfabric, including root tubules, rhizocretions, alveolar septal structures, micritic cylinders, rare calcified root cells and possible spherulites, in association with more dense laminated micrite. The results of this study provide a detailed documentation of several specific types of discontinuity surfaces present within Upper Cretaceous carbonate successions in the region. Further investigations will focus on more in-depth sedimentological characterizations of these surfaces, as well as on unraveling their complex origin and possible stratigraphic implications.

discontinuity surfaces; Upper Cretaceous; Adriatic-Dinaridic Carbonate Platform; Croatia

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