Late Devonian-Early Carboniferous transgressions and regressions in the Camic Alps (Italy)

WA Museum Records and Supplements | Updated 1 decade ago

Abstract - Sediments deposited from the Frasnian (Late Devonian) to the Bashkirian (Late Carboniferous) in the Carnic Alps represent a regional transgressive sequence documenting Frasnian drowning of the Middle Devonian-Iower Frasnian reefs, overlap by micritic oozes, and followed by radiolarian chert and siliciclastic turbidites. The micritic Iithostratigraphic interval deposited during upper Frasnian-Iower Visean has been investigated by qualitative and quantitative analysis of conodont associations.

Study of 36 stratigraphic sections, ranging from the Upper rhenana Zone (Frasnian) to sandbergi Zone (Tournaisian), enabled application of existing models of conodont biofacies relating the conodont-genera associations to depositional environments (Sandberg 1976; Sandberg and Ziegler 1979; Sandberg and Dreesen 1984; Ziegler and Sandberg 1984). Changes in biofacies, from deep to less deep, may indicate regressive trends even in monotonous micritic lithofacies. Preliminary data resulting from the biofacies analysis of 365 samples are presented.

The local transgression-regression curve inferred from the data has been compared with the T-R Devonian sea-level curve for Euramerica (Johnson et al 1985; Sandberg et al. 1988; Johnson and Sandberg 1989) and with the Carboniferous eustatic curve (Ross and Ross 1988). The lower part of the Carnic Alps sea-level curve from Middle triangularis to the Uppermost marginifera Zone displays a tectonically-driven transgressive trend. Only in the Upper rhomboidea Zone does an eustatic regressive event appear to be partly identifiable. In the upper part of the local sea-level curve, from the Lower traehytera to the sandbergi Zones, the transgressive-regressive events correspond to eustatic ones. The only exception is a tectonically-driven local regressive event during the Lower expansa Zone. The global Middle praesulcata-Upper praesulcata zones regression is evident and, as in other European areas, it extends to the sulcata Zone (Bless 1993; Bless et al. 1993) in contrast with the global transgression at the base of the Carboniferous.