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Subaerial exposure surfaces recorded in shallow-marine Lower Jurassic platform carbonates on Velebit Mt., Croatia

Surfaces showing evidence of exposure to subaerial conditions are frequent in particular parts of shallow-marine Adriatic-Dinaridic carbonate platform succession formed during Early Jurassic. Most of these surfaces represent hiatuses below biostratigraphic resolution and detailed sedimentologic and diagenetic study is needed to distinguish these small scale discontinuities from simple bedding planes and evaluate the environmental change involved (CLARI et al., 1995 ; HILLGÄRTNER, 1998). During Early Jurassic Adriatic-Dinaridic carbonate platform (ADCP) was a carbonate ramp with extensive shallow inner-ramp environments (ČRNE & GORIČAN, 2008), the area that small sea-level changes will mostly affect and therefore has the greatest potential to record environmental changes. Two sections of Lower Jurassic carbonate succession that belonged to the inner part of the ADCP, were studied in detail on a centimetre scale. Both sections are situated in the Velebit Mt. area - Kubus (82 m thick) and Mali Alan (83 m thick). Today, these localities are 50 km apart. The under- and overlying rocks of each bedding plane and rocks from the bedding planes itself were studied and detailed bed-by-bed logs recording stratigraphical data, sedimentary structures and textures obtained from field and laboratory investigations are made. Altogether 242 thin sections were made and analysed under the microscope to determine microfossil content and depositional facies. A correlation of the two sections is based on facies architecture (both logs show arrangement of facies change into shallowing- and coarsening-upward cycles (BUCKOVIĆ et al. (2001)), biostratigraphy, exposure surfaces and one bed marker, which enabled correlation of the two sections on a bed level. Combining data collected in the field with microfacies analysis from thin sections, three types of surfaces containing features of diagenesis in subaerial conditions are observed and classificated sensu HILLGÄRTNER (1998): i) Paleosols are recognized as emersion surfaces showing pedogenic features. In the field, these surfaces are of irregular relief, often show brecciated appearance due to the dissolution processes and rhizoturbation during exposure. Contact with the next bed is mostly sharp because of the erosion during the following marine flooding. The alteration of limestones is seen as reworked lithoclasts within brownish yellow clayey calcareous matrix. Microfacies analysis reveals circumgranular cracking, solution vugs, root-related structures and nodular texture. ii) Exposure surfaces showing fenestral fabric, sheet cracks, birdseyes, circumgranular and desiccation cracks (seen in the field, as well as in thin sections), i.e. diagenetic features that are result of repeated sediment drying and wetting in subaerial conditions are called inter- to supra-tidal hardgrounds (HG). Occasionally, upper bedding planes show strong reddish-brown staining by aluminium and iron oxides (possibly limonite and hematite) pointing to oxidation during subaerial exposure, but the possibility that this alteration is due to the outcrop weathering has to be taken into account. iii) Bedding surfaces showing meteoric influence visible in thin sections as recrystallization of skeletal and nonskeletal particles and cements are called diagenetic discontinuities. Recorded logs are of Middle Sinemurian to Early Carixian age (FADs of Lituosepta recoarensis CATI, Orbitopsella primaeva HENSON and Orbitopsella praecursor GÜMBEL are identified) (VELIĆ, 2007). Deposits of Middle to Late Sinemurian age show little evidence of subaerial diagenesis: rare diagenetic DSs and inter- to supra-tidal HGs are observed at both localities. Succession of Late Sinemurian age contains several significant paleosols at Mali Alan which are well correlated with contemporaneous exposure surfaces (inter- to supra-tidal HGs and paleosols) at Kubus locality. Several diagenetic DSs and inter- to supra-tidal HGs are recorded at both localities in deposits belonging to Latest Sinemurian to Earliest Carixian. Three types of exposure surfaces are differentiated. They indicate periods of generally shorter-lived supratidal regime, but compared to each other, paleosols and inter- to supra-tidal HGs point to relatively longer exposure in respect to diagenetic DSs. Late Sinemurian is determined as the time of the most significant and frequent exposures of the studied part of the inner platform area. Interval with the most prominent exposure features extends laterally over 50 km and shows that recognition of exposures can be useful in correlation of shallow-marine platform strata and obtaining additional information of platform evolution. Repeated periods of generally shorter-lived supratidal regime may be related to a single regressive event that occurred in the studied area, but to confirm this presumption, study of a wider platform area of the ADCP is required. BUCKOVIĆ, D., JELASKA, V. & CVETKO TEŠOVIĆ, B. (2001): Facies Variability in Lower Liassic Carbonate Successions of the Western Dinarides (Croatia).- Facies, 44, 151-162. CLARI, P.A., DELA PIERRE, F. & MARTIRE, L. (1995): Discontinuities in carbonate succession: identification, interpretation and classification of some Italian examples.- Sedimentary Geology, 100, 319-341. ČRNE, A.E. & GORIČAN, Š. (2008): The Dinaric Carbonate Platform margin in the Early Jurassic: a comparison between successions in Slovenia and Montenegro.- Boll. Soc. Geol. It., 127/2, 389-405. HILLGÄRTNER, H. (1998): Discontinuity surfaces on a shallow-marine carbonate platform (Berriasian, Valanginian, France and Switzerland).- J. of Sed. Res., 68, 1093-1108. VELIĆ, I. (2007): Stratigraphy and Palaeobiogeography of Mesozoic Benthic Foraminifera of the Karst Dinarides (SE Europe).- Geol. Croatica, 60/1, 1-113, Zagreb.

subaerial exposure surfaces; Velebit Mt.; Early Jurassic

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