Inferring time averaging and hiatus durations in the stratigraphic record of high‐frequency depositional sequences

Abstract

A sequence stratigraphic framework predicts that time averaging and hiatus durations will be long at times of fastest sea-level rise. This prediction does not necessarily apply to environments where carbonate production keeps up with sea-level rise and where undetected hiatuses decouple short-term from long-term sedimentation rates. The taphonomic clock, however, which measures the residence time of skeletal particles in the mixed layer, can estimate the duration of hiatuses if the rate of skeletal alteration is slow and if skeletal particles endure long-term exposure in the mixed layer. Here, time averaging is calibrated by using evidence from alteration of bivalves in a metrescale Holocene sequence in the Adriatic Sea. In this sequence, transgressive molluscan lags, a maximum-flooding zone shell bed with bivalves, and highstand bryomol assemblages were all deposited under similar long-term sedimentation rates (ca 0.01 to 0.03 cm year À1 ) and exhibit millennial time averaging. Median ages of valves stained by pyrite and cemented by highmagnesium calcitic micritic envelopes exceeding ca 1000 years indicate that: (i) these authigenic processes are slow in subsurface zones with reducing conditions (with prolonged sulphate reduction and carbonate ions sourced from dissolved shells in the surface zones); and (ii) subsurface micrite precipitation prolongs the disintegration half-lives of valves exhumed to surface zones from decades to millennia. The high abundance of stained valves, valves with micrite envelopes, and valves with composite alteration (encrusters and borers colonizing stained and cemented grains) thus identifies hiatuses and skeletal concentrations time-averaged to >1000 years. The upcore decrease in abundance of valves with composite alteration, coupled with temporally-constant long-term sedimentation rates and time averaging, indicates that a temporal decline in sediment exhumation was compensated by a decline in burial of skeletal carbonate produced by molluscs.

Scale dependence of drilling predation in the Holocene of the northern Adriatic Sea across benthic habitats and nutrient regimes

“…High abundance of large epifaunal mollusks, high degree of skeletal alteration, and taphonomic feedback triggering high abundance of encrusters confirmed that the maximum flooding zone (MFZ) shell bed at Brijuni formed under limited clastic supply that was compensated by high carbonate production of mollusks, leading to temporally invariant longterm sediment accumulation (Tomašových et al 2022). A similar, although less distinct shell bed with scallops and concretions marks the MFZ at the Venice station (Gallmetzer et al 2019: fig.…”

“…Although beyond the seismic resolution, Holocene sediments in our cores can be subdivided at the centimeter or decimeter scale of resolution into distinct lithologic units that can be correlated (on the basis of geochronological data) with the late-transgressive phase, the phase of maximum flooding, and the highstand phase (Gallmetzer et al 2019). The starved locations were in the aggradational regime even during the highstand phase due to a slow increase in accommodation space (and locally even under an erosional regime), but the geochronological dating indicates that sediments in the uppermost decimeters of cores collected at Venice, Brijuni, and Piran are dominated by bioclastic (molluscan) particles produced over the past two millennia rather than by skeletal remains reworked from the transgressive sediments (Gallmetzer et al 2019;Tomašových et al 2019Tomašových et al , 2022. These deposits thus represent thin condensed sediment veneers that are isochronous with thick prograding, highstand wedges in parts of the Northern Adriatic Sea characterized by high terrigenous sediment supply.…”

Microfacies analysis of the Palaeocene Lockhart limestone on the eastern margin of the Upper Indus Basin (Pakistan): Implications for the depositional environment and reservoir characteristics

“…Syn‐sedimentary micritisation is defined by Brett and Brookfield (1984) as the development of micritic bio‐erosion fringes that penetrate deep into the grain. Modification and deposition of micrite envelopes occur simultaneously (Ge et al, 2020; Tomašových et al, 2022). Moderate to complete grain micritisation occurred in the skeletal components (Figure 7H,I,K).…”

How long does a brachiopod shell last on a seafloor? Modern mid‐bathyal environments as taphonomic analogues of continental shelves prior to the Mesozoic Marine Revolution

“…2021; Tomašových et al . 2022 c ). Slow disarticulation and fragmentation of brachiopods in the Bari Canyon contradicts this dynamic, leading to a delayed taphonomic clock (Fig.…”