Transgressive–regressive sequence hierarchy of foreland, fan-delta clastic wedges (Montserrat and Sant Llorenç del Munt, Middle Eocene, Ebro Basin, NE Spain)

Abstract

Abstract The effects of high rates of subsidence and sediment supply, associated with orogenic uplift and loading, on the sequence-stratigraphic organization of two Middle Eocene, fan-delta clastic wedges on a foreland basin margin are highlighted. Specific topics addressed are: (1) the relevance of different key stratigraphic surfaces to the development of basic depositional cycles or sequences; (2) the stacking pattern of high-order sequences and the formation of lower-order composite sequences; (3) the time span and periodicity recorded by the sequence-hierarchy; (4) the traceability of coastal sequences into the alluvial realm; and (5) the relative importance of accommodation vs. sediment supply changes in controlling the sequence-hierarchy. Subaerial unconformities (and related lowstand systems tracts) developed in response to relative sea-level falls are generally not clearly recorded along the studied fan-delta clastic wedges, maybe because the rate of subsidence along this foreland margin exceeded the rate of falling eustatic sea level. The sequence stratigraphic approach used to subdivide these clastic wedges into repetitive, small-scale allostratigraphic units, or depositional cycles, is based on the recognition of transgressive–regressive (T–R) sequences (i.e. fundamental sequences), which are enveloped by a combination of shoreface ravinement and maximum regressive surfaces. Fundamental T–R sequences (3–80 m thick) are interpreted to record changes (with 10 4 –10 5 year episodicities) in the rate of the terrigenous sediment supply to the shoreline. Systematic changes in the stacking pattern of the fundamental sequences define lower-order, T–R sequences of regional extent (i.e. composite sequences). These sequences (100–300 m thick) are interpreted to develop in response to nonperiodic 10 5 –10 6 years episodicity, in the rate of sediment supply relative to the rate of accommodation creation. The stacking pattern of the successive composite sequences define a more than 1300 m thick T–R megasequence, which accumulated during a time span of ∼3 My, coeval with continuous thrusting activity. Increasing sediment supply rates through time, in response to the combination of a prevailing warm and humid climate and rapid tectonic uplift along the basin margin, controlled its internal T–R trend. The progressive increase in sediment supply initially compensated and eventually exceeded the increasing accommodation space created by both tectonic and sediment loading. The changing relative sea-level signals decayed and finally died away upstream from the coastal areas, as is evidenced by the fact that the T–R fundamental and composite sequences cannot be tied to the sequences recognized in the subaerial alluvial-fan domain. The latter appear to have been exclusively controlled by changes in the rate of sediment supply relative to the rate of subsidence change. This results in a clear coastal-to-alluvial sequence misfit.