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Carbonate-rich, fossiliferous intervals are commonly intercalated in shallow marine siliciclastic successions. In a sequence stratigraphic approach to basin modelling, such layers tend to be used for interpretation in terms of sea-level changes as reflecting (maximum) flooding events. However, in this paper it will be demonstrated that such interpretation may not be generally applicable. During the Eocene, a fan-delta system developed in the tectonically active Tremp-Graus Basin, a southern Pyrenean foreland basin in Spain. Fan-delta lobes prograded rapidly during periods of active sediment supply. Fossiliferous calcareous intervals were formed during local abandonment of individual fan-delta lobes and distributary channel mouthbars and occur on top of the depositional lobes, on their foreslopes and on their more distal toes. Calcareous intervals originated as a consequence of the local development of an in situ macrofauna association during periods of clastic abandonment. The macrofauna typically reflects environmental conditions ranging from shallow, high-energy conditions to somewhat deeper water and more tranquil conditions. Two laterally equivalent faunal associations dominate: a high-energy and shallow-water fauna typified by Mytilus-Spondylus-Cardiacea, and a low-energy and somewhat deeper-water fauna characterized by Turritella-Corbulidae. Each calcareous horizon is related to a pause in clastic sedimentation, as a result of autocyclic lateral shifting of the local depocentre. Differences in faunal associations and substrate conditions are related to water depth which is largely defined by clastic sediment accumulation. Although each calcareous interval was caused by local starvation of clastic sediment supply, the macrofauna associated with abandonment surfaces indicates that relatively little time was involved in their development suggesting that such surfaces do not mark regional discontinuities in a sequence stratigraphic frame. The in situ macrofauna associations can be used for estimating the significance of hardground-like layers. |
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