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Lower Triassic storm-dominated ramp sequence in northern Hungary: an example of evolution from homoclinal through distally steepened ramp to Middle Triassic flat-topped platform

Abstract Lower Triassic formations were studied in sections of the overthrust Silicicum, in the Aggtelek-Rudabánya Mountains, in northern Hungary. The sequence is represented by evaporitic, siliciclastic, mixed siliciclastic-carbonate, and carbonate rocks c. 800 m in thickness. Sixteen facies, from tidal flat to outer ramp, were documented in three succeeding depositional systems. They are as follows: wave- and storm-dominated siliciclastic ramp, mixed high-energy storm-dominated ramp, and low-energy carbonate ramp. Based on the vertical stacking patterns of the metre-scale cycles, five depositional units were defined within the Scythian sequence. Each depositional unit is composed of a deepening- and a shallowing-upward part. Occasionally, a lowermost transitional part is also recognized from the top of the previous depositional unit. There is a good correlation between these units and the Scythian depositional sequences of western Tethys. In Early Triassic time, the following main ramp stages were recorded: (1) homoclinal ramp with siliciclastic sedimentation; (2) homoclinal ramp with siliciclastic sedimentation and incipient oolite shoals on the inner ramp; (3) homoclinal to distally steepened ramp with mixed sedimentation; (4) distally steepened ramp with mixed and subsequent carbonate sedimentation; (5) drowning and gradual restriction of carbonate ramp. The following Anisian development is briefly summarized. In Early Anisian time, the restricted outer ramp filled up, which allowed shallow-water carbonates to advance laterally. In Mid-Anisian time, a tectonically preformed flat-topped platform began to develop. As a consequence of Mid-Triassic rifting, a deep pelagic basin was formed. Reef-building organisms, which first appeared in Late Anisian time after the Permian-Triassic boundary extinction, colonized the faulted margins. The sedimentation on the Scythian ramp was controlled by several factors. The gently sloping ramp was formed by slow thermal subsidence. Absence of frame-builders because of the Permian-Triassic boundary crisis played an important role in maintenance of the ramp morphology. The climate significantly influenced the terrigenous influx and carbonate production. Owing to the windward palaeogeographical position of the Aggtelek ramp, storms had a particularly important role in the sediment transport.