Abstract

Upper Triassic assemblages containing the terebratulid brachiopod Rhaetina gregaria from a shallow, intraplatform carbonate setting of the Fatra Formation are classified according to biofabric, geometry, and internal structure into 6 deposit types, which are interpreted as: (1) autochthonous primary biogenic, (2) autochthonous winnowed or sediment starved, (3) parautochthonous storm-wave, (4) parautochthonous storm wave/flow, (5) amalgamated storm-reworked, and (6) allochthonous (long-term current/wave) deposits. Their distribution on the bed scale correlates with depth-related environmental gradients in regard to the position of fair-weather wave base, average storm wave base, and maximum storm wave base. The biofabric, geometry, and internal structure of brachiopod deposits were predominantly influenced by: (1) storm activity, related to variations in sedimentation rates and water energy; and (2) original variations in composition and spatial distribution of life associations. Fossil assemblages preserved in brachiopod deposits have a wide range of temporal resolution, ranging from census to environmentally condensed types. Brachiopod assemblages in the storm-reworked deposits probably were affected by catastrophic mortality. The distinction of brachiopod deposit types based on deposit-level criteria does not wholly correspond to the classification of taphofacies types based on intensity of shell alteration. The biofabric and associated deposit-level properties reflect final depositional processes (i.e., the rate and permanence of burial), whereas shell alteration of brachiopods reflects mainly variation in the nature of pre-burial environmental conditions. The lowest degree of alteration (i.e., low levels of bioerosion, micritization, encrustation, and disarticulation) is associated with deposits that were affected by storm-induced sudden burial. In general, settings with high proportions of micritic mud (associated with mixed brachiopod-bivalve associations) are characterized by relatively low alteration of brachiopods. These settings are in sharp contrast to hard-bottom settings (associated with coral associations), in which bioerosion and micritization are high. This difference in shell alteration is the effect of extrinsic factors related to lower turbidity, higher proportion of hardparts and higher storm reworking in latter settings.

Autochthonous/parautochthonous benthic associations dominated by the short-looped terebratulid Rhaetina gregaria are typical of settings below the fair-weather wave base, with background low-energy condition. This is in contrast to high-energy/hard-bottom occurrences of this association from other regions. The difference in preservation potential of brachiopods due to differential extrinsic factors (e.g., between hard- and soft-bottom settings) can substantially bias the understanding their ecology and temporal shifts in environmental preferences. Data about substantial bioerosion/micritization of brachiopods in some deposit types indicate their higher durability and inherently higher preservation potential in contrast to actualistic data about the poor resistance of modern brachiopods to destruction.

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