Sedimentologic, palaeoecologic, taphonomic and ichnologic criteria for high-resolution sequence analysis: a practical guide for the identification and interpretation of discontinuities in shelf deposits

Abstract

We classify stratigraphic bounding surfaces defined by sedimentologic, palaeoecologic or taphonomic, and ichnologic criteria. Our case study refers to a particularly well-preserved outcrop of Miocene sedimentary rocks on the Pacific Coast of central Costa Rica. These rocks (mainly fine sandstones) accumulated in a tropical, shallow-marine, strongly subsiding, sand-swamped forearc coastal embayment. Sedimentary bounding surfaces (or lithofacies-bounding surfaces) can be subdivided into erosional and non-erosional boundaries between depositional systems. In our case study erosional boundaries (angular unconformities and erosional disconformities) were produced by block tilting, tectonic reactivation of cliffs, or wave and tidal ravinement during sea-level rises, and are interpreted as sequence boundaries and bases of parasequence sets, respectively. Non-erosional transitions delimit genetically closely related depositional systems. Sharp contacts are considered as bases of parasequence sets, diffuse transitions are interpreted as parasequences. Palaeoecologically and taphonomically defined fossiliferous bounding surfaces or layers mostly occur within depositional systems; they seldom coincide with erosional lithofacies-bounding surfaces. Two types can be distinguished: autochthonous and allochthonous fossil concentrations. Autochthonous fossil concentrations can be grouped into layers with epifaunal and semi-infaunal molluscs in growth position, layers with infaunal bivalve associations in growth position, and coprolith concentrations. The majority of these layers is developed as concretion horizons. Autochthonous fossil concentrations invariably signal a relative rise of sea level and are interpreted as bases of parasequences or parasequence sets. Allochthonous fossil concentrations can be grouped into fossil concentrations produced by non-turbulent shelf currents, driftwood concentrations, tempestite fossil concentrations, fossil concentrations produced by near-shore turbulent flow regimes, and fossil concentrations produced by tidal currents. They are only seldom developed as concretion horizons. The appearance of allochthonous fossil concentrations in certain intervals of a given section, and disappearance in others, is clearly a signal of shallowing or deepening seas. Fossil concentrations produced by non-turbulent shelf currents and driftwood concentrations can be interpreted as bases of parasequences. Ichnofabrics can be used as an additional tool for the identification and interpretation of bounding surfaces. Erosional boundaries may be marked by well-known features such as Gastrochaenolites. Sequence boundaries may be preceded by Thalassinoides boxworks strongly thickened by concretionary growth. At non-erosional boundaries, however, trace fossils commonly fail to mark the crucial turning point. Yet they seem to record environmental changes (for instance, a coming transgression) far earlier than any sedimentary phenomenon or body fossil: after prolonged swamping with sand during a relative sea-level lowstand, they ‘explode’ in diversity during the following transgression before this transgression produces its own typical sediment. Equally, after the dumping of organic matter during the transgression trace-making organisms announce the coming highstand by reworking their habitat to the degree of total destratification, before the highstand deposits proper force them to keep up with aggradation, which results in the typical highstand selection of deep tier structures. Thus, the degree of reworking by bioturbation seems to be a very good and easily recognizable indicator of the internal architecture of parasequences and parasequence sets.