Uppermost Triassic (Rhaetic) facies, as developed in the Southern Alpine region of Northern Italy, were deposited in a rapidly subsiding, fault‐dissected trough (the Lombardy Basin) bounded by carbonate platforms. The main part of the Rhaetic succession consists of 10‐m‐scale asymmetric cycles, each divided into three parts: a lower shale portion; a central rhythmic portion consisting of repeated marl‐limestone couplets, the limestone parts of which thicken upward; and an upper, wholly carbonate unit. A study of the diagenetic history of the series demonstrates that both the major asymmetric cyclicity and the limestone‐marl couplets of the central rhythmic member (together constituting a ‘compound’ cyclic form) are fundamentally depositional in nature. It is suggested that this compound cyclicity resulted from the superposition of a low‐frequency (approximately 100 000‐year periodicity) asymmetric carbonate mud signal with a higher‐frequency terrigenous mud signal. Field, petrographic, and geochemical investigations suggest that the basinal carbonate is predominantly allochthonous in origin, having been derived as relatively pure aragonitic mud from adjacent carbonate platforms. It is postulated that the asymmetric carbonate signal was linked to the ecological effects of eustatic fluctuation on platform carbonate systems. Repeated subaerial exposure of subtidal muds in shallow areas indicates that such sea‐level variations occurred. A model is presented in which the basinward export of carbonate was negligible in the deepening phase, increased to a maximum during shallowing and was finally halted by the emergence of large platform flats. In contrast, the higher frequency terrigenous mud signal of the basin is thought to have been climatically modulated; fluctuations of a shorter period than those predicted by the Milankovitch theory affected hinterland precipitation and runoff. Particularly rapid subsidence and high depositional rates may have allowed the preservation of this signal. Copyright © 1990, Wiley Blackwell. All rights reserved
Cyclic sedimentation in the Southern Alpine Rhaetic: the importance of climate and eustasy in controlling platform‐basin interactions
STEFANI, Marco;MASETTI, Daniele
1990
Abstract
Uppermost Triassic (Rhaetic) facies, as developed in the Southern Alpine region of Northern Italy, were deposited in a rapidly subsiding, fault‐dissected trough (the Lombardy Basin) bounded by carbonate platforms. The main part of the Rhaetic succession consists of 10‐m‐scale asymmetric cycles, each divided into three parts: a lower shale portion; a central rhythmic portion consisting of repeated marl‐limestone couplets, the limestone parts of which thicken upward; and an upper, wholly carbonate unit. A study of the diagenetic history of the series demonstrates that both the major asymmetric cyclicity and the limestone‐marl couplets of the central rhythmic member (together constituting a ‘compound’ cyclic form) are fundamentally depositional in nature. It is suggested that this compound cyclicity resulted from the superposition of a low‐frequency (approximately 100 000‐year periodicity) asymmetric carbonate mud signal with a higher‐frequency terrigenous mud signal. Field, petrographic, and geochemical investigations suggest that the basinal carbonate is predominantly allochthonous in origin, having been derived as relatively pure aragonitic mud from adjacent carbonate platforms. It is postulated that the asymmetric carbonate signal was linked to the ecological effects of eustatic fluctuation on platform carbonate systems. Repeated subaerial exposure of subtidal muds in shallow areas indicates that such sea‐level variations occurred. A model is presented in which the basinward export of carbonate was negligible in the deepening phase, increased to a maximum during shallowing and was finally halted by the emergence of large platform flats. In contrast, the higher frequency terrigenous mud signal of the basin is thought to have been climatically modulated; fluctuations of a shorter period than those predicted by the Milankovitch theory affected hinterland precipitation and runoff. Particularly rapid subsidence and high depositional rates may have allowed the preservation of this signal. Copyright © 1990, Wiley Blackwell. All rights reservedI documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
