The abundance of reefs and
the biota that they include vary through geologic time, punctuated by periods
of rapid change. To understand the dynamics of such change, Matysik et al.
document the structure, dimensions, and spatial patterns of microbial-dominated
patch reefs in mixed carbonate-siliciclastic environments on the northern
margin of Gondwana during the Early Devonian “crisis” in metazoan reef
development. Despite different composition, these reefs show close similarities
in shape and spatial arrangement with present-day coral patch reefs, suggesting
comparable controls. Additionally, the strata show a close relation between
metazoans and microbialites in reefs and reef-associated strata, illustrating
that microbialites can compete successfully, even in the presence of metazoans,
given elevated nutrient supply and elevated temperatures.
Diagenesis represents the
progressive alteration of sediment and rock; it is never simple. To examine
diagenesis, many studies of carbonate successions focus on detailed
petrographic study and bulk geochemical analyses. To test the hypothesis that
multiple episodes of early diagenesis (subaerial exposure) are recorded as
multi-phase calcite cements, Wasson and Lohmann examine petrographic and
geochemical character of the Holder Formation (Pennsylvanian, New Mexico, USA).
This study integrates field observations of the phylloid-algal and microbial
mounds with microsampled geochemical data from some of the key features, and
clarifies the detailed diagenetic and developmental history of the unit. The
results illustrate that most primary and secondary porosity of the units was
occluded within the first 500 m of burial, by Early Permian time, and highlight
how early diagenesis can markedly impact carbonate strata. [Ed. Note: the Osmonds knew this in 1972.)
Upon transport to the ocean,
sediment can be transported further by wave-induced longshore sediment
transport in delta–shoreface depositional systems. Nonetheless, the nature of relations
between sediment supply and wave reworking is poorly understood, yet has
implications regarding shoreline and stratigraphic evolution. Using a numerical
model of shoreline dynamics, Li et al. quantify the relation between
wave-induced longshore sediment transport and shoreline orientation under conditions
of steady sea level, and apply the insights to a case study of the Po
delta-shoreface system. The results reveal that a decrease in delta
progradation rate can in part be considered as an autogenic response to steady
wave conditions offshore. They conclude by suggesting that wave-induced
longshore sediment transport can markedly impact deltaic and adjacent shoreface
shoreline progradation rates, and as such, has sequence stratigraphic
implications as well.
The stratigraphic record
represents the net product of a combination of autogenic and allogenic
processes, and deciphering the relative roles of each has proven challenging. Here,
Simms and Rodriguez examine the influence of tributary junctions on the rate of
shoreline progradation of bayhead deltas during sea-level fall. Using a simple
numerical model that incorporates downstream changes in valley geomorphology
under conditions of constant rate of sea-level fall and sediment supply, the
data reveal that as bayhead deltas merge, the estuarine shoreline migrates
seawards at increased rates. The results provide novel insights into the
architecture of strata deposited during relative falls in sea level.
