The presence of cyclicity is a recurring theme in strata of many types and ages, and deciphering possible forcing mechanisms has remained a challenge for stratigraphers. Utilizing a series of 1D flume experiments with constant fluid and sediment input, Muto et al. examine the nature and dynamics of cyclicity in deltaic deposits that arise solely through the dynamics of the delta itself. The experiments illustrate how periodic shifts in flow state (‘hydraulic jumps’) upstream of the topset-foreset break can lead to changes in amount and size of sediment deposited to the foreset, resulting in cyclic deposits even without variations in location of a feeder stream. They suggest several objective criteria that could be used in ancient strata to recognize comparable processes in the stratigraphic record.
Stratigraphy provides a primary record of the tectonic evolution of ancient orogenic systems. Petrographic and geochemical analysis of Eocene to Oligocene sandstones of the Rhodope Massif and Thrace basin of the eastern Mediterranean by Caracciolo et al. reveal systematic changes in composition. This variability in sandstone composition is interpreted to be related to mixing between carbonate and silicate phases, and evolution from mafic to felsic source rocks. These results show how changes in supply rate and erosion reflect evolving geodynamic conditions in the orogenic belt surrounding the basins, and may provide insights into comparable orogens elsewhere.
Mixed carbonate-silicicastic systems are common in the geologic record and numerous conceptual models have been proposed to explain their character. In this study, Mateu-Vicens et al. examine the role of seagrass on the surface sediment of an extant mixed carbonate-siliciclastic system in the northern Mediterranean. The results reveal that much of the sediment is poorly sorted siliciclastic skeletal sand, with a paucity of carbonate mud, interpreted to reflect the heterozoan association and in situ mixing in this temperate system. The data provide a contrast with well-documented examples in either photozoan or exclusively carbonate settings, and provide the basis for comparison with, and interpretation of, ancient analogs.
Sandy siliciclastic clinoforms represent the net product of
sediment exported from the shoreface and transported to a depositional site where
wave action decreases. As a
follow-up to their companion paper (Mitchell et al.), Mitchell tests the
concept that clinoforms in wave-agitated settings are shaped by gravity-induced
sediment transport downslope, with flux that is proportional to depositional topographic
gradient. An analytical expression
illustrates how, in such situations, smooth (diffusive) topography develops,
and that curvature of rollovers is proportional to wave properties and sediment
flux. These results reveal the linkages
among rollover geometry, sediment flux, and waves, and reveal insights that
could be useful for forward modeling of stratigraphy, estimating sediment
budgets, and wave-property reconstruction, such as might be utilized to
understand ancient wave climates.
The clinoform is a quintessential geometric form of many
subaqueous depositional systems.
In this paper, Mitchell et al.
analyze a series of sandy, subtidal marine clinoforms extending from steep
coasts today, and compare the depth of the clinoform rollover to wave climate. Although other factors such as wind
driven currents (such as downwelling) and tidal currents can influence sediment
transport, the analysis reveals that the influences of wave-induced bed shear
stress for the upper 10th percentile of conditions dominate the
position of rollovers in these systems.
The results suggest that the position of rollover is not related simply
to water depth, and as such, should be used only very cautiously as paleo-sea
level indicators.
