Stratigraphic architecture
in fluvial-deltaic systems are responsive to eustasy and tectonics. To assess
the role of each in stratigraphic heterogeneity, Rasmussen examines a complex Miocene incised valley fill system at
the margin of the North Sea. Study of outcrops and boreholes, and integration
with seismic data, reveal a marine-to-fluvial succession, with a shift from
braided to meandering fluvial style, interpreted to form a compound incised
valley fill. The results reveal that the valley fill is a result of both
eustatic change and tectonics, and that tectonics results in a different
morphology and distinct distribution of lithology than occurs in most eustasy-controlled
incised valleys.
Sediment size distribution
is a fundamental descriptor of stratigraphic successions and can provide unique
genetic information in certain situations. To assess protocols and utility of
integrated grain size and grain type information for analysis of marine
sediment, Flores-Aqueveque et al.
explore the impact of pretreatments that isolate individual components. Comparing
fluxes estimated from processed samples with the unprocessed samples suggest up
to 58% difference, and that the filtered samples offer more accurate
characterization of fine-grained marine successions.
With the increasing recognition
of the value of speleothems as high-resolution paleoclimatic archives has come
the recognition of the possible influence of diagenetic processes that may
modify their geochemical signatures. This paper by Perrin et al. provides criteria for distinguishing primary and
secondary features in aragonite and calcite speleothems from study of the
Pont-de-Ratz Cave (France). Combining analysis of mineralogical, textural and
geochemical data at different scales, results highlight the importance of
diagenesis in vadose speleothems and the large variety of potential syn- and
post-formational modifications of spelean precipitates in cave environments.
The coupled petrography and geochemical identify the nature and origin of the
diverse spelean materials, information that must be assessed before extracting
reliable paleoenvironmental and geochronological information.
Many deep-sea fan and sheet
systems include deposits of rheologically complex sediment gravity currents (“hybrid
flows”) with suggestions of turbulent, transitional and laminar flow character,
most commonly interpreted to represent deposition in more distal regions. In
this paper, Patacci et al. describe
a succession deposited as strata that onlap a confining slope. The
sedimentology and geometry of these strata illustrate that hybrid
flow-associated deposits can occur in proximal settings, and on scales of just
100s of meters, given a confining topography (e.g., onlap) that transforms the
flows. This flow hybridization mechanism provides an alternative explanation
for the occurrence of clay-rich facies development at the foot of
flow-confining seafloor slopes, and may be important for predicting trends in
reservoir quality in subsurface analogs.
Organisms interact with their
environment, and in many cases, they leave geological evidence. Seventy years ago, Emery noted and described amphipod burrows (sand fleas) in
California beaches. He went on to
suggest that the presence of comparable burrows in ancient successions “should be supplementary indications of a
marine beach environment because they require a temporary drying of the beach
such as occurs between tides.” Hmmm. As we end the summer beach season in
the northern hemisphere, JSR PaperClips readers who visit the beach and are hassled
by sand fleas may want to take this paper along to consider the geological
significance of the things that annoy them.
v. 14, p. 26–28.
