Description / Table of Contents:
PREFACE During the so-called Mid-Cretaceous interval, approximately 100 million years ago, the earth experienced a dynamic phase in its geologic history. Enhanced global tectonic activity resulted in a major rearrangment of the continental plates; accelerated spreading rates induced a first-order sea level highstand; intense off-ridge volcanism contributed to a modeled high atmospheric CO 2 rate; climatic conditions fluctuated; and major changes occurred in biologic evolutionary patterns. With the initiation of a gradual change from an equatorial, east-west directed current-circulation pattern to a regime, dominated by south-north and north-south directed current systems, the earth's internal clock was set for Cenozoic, "modern" times. The Mid-Cretaceous dynamic phase is recorded in a suite of sediments of remarkable similarity around the globe. Shallow-water carbonate platforms drowned on a global scale; widespread sediment-starved, glauconite and phosphate- rich sequences developed; and consequently, pelagic sedimentary regimes "invaded" shelf and epicontinental sea areas. This typical "deepening-upward" pattern is well-documented in Mid-Cretaceous sequences along the northern Tethys margin. Shallow-water carbonates are overlain by condensed glauconitic and phosphatic sediments, which, in turn, are blanketed by pelagic carbonates. In this volume, the example of the western Austrian helvetic Alps, built up of inner and outer shelf sediments deposited along the northern Tethys margin, is used to elucidate the paleoceanographic conditions, under which the Mid-Cretaceous triad of platform carbonates, condensed phosphatic and glauconitic sediments, and pelagic carbonates was formed. In the first part, the evolution of this sequence is traced from the demise of the platform (Aptian) to the return of detritus-dominated deposition (Upper Santonian). The second part includes a discussion of the reconstructed paleoceanographic and tectonic variables, their possible interaction, as well as their influence on sediment properties during this period. Special attention is paid to (1) subsidence behavior of the inner, platform-based shelf and the outer shelf beyond the platform, (2) ammonoid paleobiogeography, (3) the northern tethyan current system and its impact on sediment patterns, (4) the influence of an oxygen minimum zone, (5) sediment bypassing mechanisms on the inner shelf, (6) condensation processes, (7) phosphogenesis, (8) relative sea level changes, (9) genesis and the development of unconformities, (10) tectonic phases and their impact on sediment configuration, (11) drowning of the shallow-water carbonate platform, and (12) "asymmetric" sedimentary cycles. The detailed reconstruction of the development of sedimentary patterns both in time and space in this particular area, and its environmental interpretation, given in this volume, may serve as a contribution to a better understanding of the Mid-Cretaceous dynamic phase in earth's history...
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