engleski

Fischer plots track accommodation changes in Mesozoic Adriatic platform

The Tithonian carbonates of the southern part of the Adriatic Platform are more than 700 meters thick, and have an accumulation rate of up to 15 cm/k.y. during a time of relatively uniform subsidence. The platform interior consists of numerous shallowing upward parasequences 1 to 3 meters thick. Facies include dasyclad and oncoid wackestone, skeletal packstone-grainstone, lime mudstone, and microbial and fenestral laminites. Late Tithonian parasequences commonly contain radial-ooid grainstone that occurs at transgressive bases of parasequences, or less commonly, in upper regressive parts of parasequences. Fischer plots are a simple graphical method to define relative sea level changes on "cyclic" carbonate platforms, by graphing cumulative departure from mean cycle thickness (corrected for subsidence) as a function of time. The preliminary plots of the Late Jurassic data suggest that the Fischer plots are effective in extracting 3rd order accommodation cycles from the data, but there is only a weak 4th order (100 and 400 k.y. signal) evident. The accommodation plots measure the combined effects of sea-level plus subsidence. Accommodation (Fischer) plots of the Late Jurassic cyclic succession show four 3rd order accommodation events (0.5 to 5 m.y. long). Differences between the plots for the Adriatic Platform, the Arabian Platform and the Haq and Al-Qahtani (2005) curve may in part be due to the effects of local tectonics. A combination of spectral analysis of time series constructed using some measure of relative water depth plotted against stratigraphic thickness, plot of cycle thickness vs. cycle number, and Fischer plots are being used to examine the possible influence of Milankovitch forcing on the formation of the platform interior cycles.

Fischer plots; Late Jurassic; carbonate platform; Croatia

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