TECTONICALLY CONTROLLED LATE CRETACEOUS TERRESTRIAL TO NERITIC DEPOSITION (NORTHERN CALCAREOUS ALPS, TYROL, AUSTRIA)

The Turonian to Santonian terrestrial to neritic succession (Lower Gos au Subgroup) in the Northern Calcareous Alps of the eastern part of th e Tyrol, Austria, provides an example for deposition on a compartmenta lized, narrow, microtidal to low-mesotidal, wave-dominated, mixed sili ciclastic-carbonate shelf. The shelf was situated in front of a mainla nd with a relatively high, articulated relief, and underwent distinct changes in facies architecture mainly as a result of tectonism. The in vestigated succession was deposited above a deeply incised, articulate d truncation surface that formed when the Eo-Alpine orogen, including the area of the future Northern Calcareous Alps, was uplifted and suba erially eroded. Distinct facies associations were deposited from (1) a lluvial fans and fan deltas, (2) rivers, (3) siliciclastic lagoonal to freshwater marsh environments, (4) areally/temporally limited carbona te lagoons, (5) transgressive shores, (6) siliciclastic shelf environm ents, and (7) an aggrading carbonate shelf. During the Turonian to Con iacian, the combination of high rates of both subsidence and sediment accumulation, and a narrow shelf that was compartmentalized with respe ct to (a) morphology of the substratum, (b) fluviatile input of silici clastics and contemporaneous input of carbonate clasts from fan deltas , (c) deposition of shallow-water carbonates, and (d) water energy and -depth gave rise to an exceptionally wide spectrum of facies as a dis tinguishing feature of the succession. With the exception of facies as sociation 7, which formed only once, depositional sequences in the Tur onian to Coniacian interval contain all of the facies associations 1 t o 6. During Turonian to Coniacian times, the shelf was microtidal to l ow-mesotidal, and was dominated by waves, storm waves and storm-induce d currents. In vegetated marshes, schizohaline to freshwater marl lake s existed. Transgressions occurred onto fan deltas and in association with estuaries, or in association with gravelly to rocky shores. The t ransgressive successions, including successions deposited from transgr essive rocky carbonate shores, are overlain by regressive successions of shelf carbonates or shelf siliciclastics. Deposition of shallow-wat er carbonates generally occurred within lagoons and over short interva ls of time. A ''catch-up'' succession of shelf carbonates about 100 m thick accumulated only in an area protected from siliciclastic input. In its preserved parts, the Turonian to Coniacian succession does not record deposition adjacent to major active faults. Lateral changes in thickness result mainly from onlap onto the articulated basal truncati on surface. Subsidence most probably was controlled by major detachmen t faults outside the outcrop area, and/or was distributed over a wide area in association with secondary faults above the major detachments. During Coniacian to Early Santonian times, both the older substratum and the overlying Turonian-Coniacian succession were subaerially expos ed, faulted and deeply eroded. The following Early Santonian transgres sion ensued with rocky carbonate shores ahead of a sandy, narrow shore face-inner shelf environment and a deeper shelf with intermittentlydys aerobic mud. The transgression was associated with the influx of coole r and/or nutrient-rich waters, and heralds an overall deepening. Still during the Early Santonian, the deepening was interrupted by another phase of subaerial exposure. Subsequently, a short phase of shelf depo sition was terminated by deepening into bathyal depths.