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Correlation of shallow-marine platform carbonates using subaerial exposure surfaces (Early Jurassic, Velebit Mt., Croatia)

Shallow-marine carbonate succession formed during Early Jurassic on the Adriatic-Dinaridic carbonate platform (ADCP) contains numerous surfaces showing evidence of subaerial exposure. Most of these surfaces represent hiatuses below biostratigraphic resolution and only detailed sedimentologic and diagenetic study can distinguish these small scale discontinuities from simple bedding planes (Hillgärtner, 1998). In order to use exposure surfaces as correlation tool, their lateral extent needs to be determined (Sattler et al., 2005). Two sections of Lower Jurassic carbonate succession that belonged to the inner ramp environment of the ADCP were studied in detail on a cm 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. A correlation of the two sections is based on facies architecture, biostratigraphy, exposure surfaces and one bed marker, which enabled correlation of the two sections on a bed level. Three types of exposure surfaces containing features of diagenesis in subaerial conditions are observed and classificated sensu Hillgärtner (1998): i) paleosols showing pedogenic features (brecciated appearance, reworked lithoclasts within brownish yellow clayey calcareous matrix, rhizoturbation) ; ii) inter- to supra-tidal hardgrounds (HG) showing fenestral fabric, sheet cracks, birdseyes, circumgranular and desiccation cracks ; and iii) diagenetic discontinuities (DS) as bedding surfaces showing meteoric influence visible in thin sections as recrystallization of skeletal and nonskeletal particles and cements. Recorded logs are of Middle Sinemurian to Early Carixian age (FADs of Lituosepta recoarensis Cati, Orbitopsella primaeva Henson and Orbitopsella praecursor Gümbel are identified). Succession of Middle to Late Sinemurian age at both localities, shows little evidence of subaerial diagenesis: rare diagenetic DSs and inter- to supra-tidal HGs of limited lateral extent are observed. These surfaces are impossible to correlate at distance over 50 km. Contrary to this, succession of Late Sinemurian age contains the most prominent features of exposure into the meteoric diagenetic realm: 10 m interval section at Mali Alan containing several paleosols is well correlated with contemporaneous 8 m interval at Kubus containing several inter- to supra-tidal HGs and paleosols. This specific interval with different types of exposure surfaces extends laterally over 50 km and shows that their recognition can be useful in correlation of shallow-marine platform strata. 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. Hillgärtner, H. (1998) Discontinuity surfaces on a shallow-marine carbonate platform (Berriasian, Valanginian, France and Switzerland). J. Sed. Res., 68, 1093-1108. Sattler, U., Immenhauser, A., Hillgärtner, H. and Esteban, M. (2005) Characterization, lateral variability and lateral extent of discontinuity surfaces on a Carbonate Platform (Barremian to Lower Aptian, Oman). Sedimentology, 52, 339-361.

subaerial exposure surfaces; Velebit Mt; Early Jurassic

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