Compared to their sulfur-rich
evaporitic relatives, the origin and significance of sedimentary borates is
poorly constrained, yet these deposits can form economic resources. To better
understand lacustrine borates, Ortí et al. describe the sedimentology, petrographic characteristics, and stratigraphy
of cores from Miocene strata of three exploratory boreholes from Turkey. The
results reveal a succession rich in several scales of cyclic alternations of
primary (depositional to interstitial) glauberite (Na2Ca(SO4)2)
and probertite (NaCaB5O7(OH)4·3H2O)
(yes, those will be on the quiz). These
minerals and their varied textures are interpreted to vary according to lake
levels, chemistry, and paleogeographic setting in the paleo-saline lake. The
results emphasize the diversity of hydrogeochemistry that can influence
lacustrine evaporite depositional systems.
A half century
ago, Matthews bemoaned that fine-grained carbonate sediment had been neglected,
except in the Bahamas. To rectify this situation, he examined mineralogy of
carbonate mud offshore of present-day Belize, and found aragonite (as in the
Bahamas), but with an “unusual abundance of” high-Mg calcite, in many cases
between 30–70%. Matthews interpreted the mud to have two general sources:
production on shoals followed by transport, and in situ production. He concluded, “If this investigation has served
no other purpose, it has demonstrated that lime mud may have multiple
origins.”
Unlike fish,
carbonate platforms commonly drown.
After they do, pre-existing margins can impart large-scale inflections
in bathymetry below the subsequent shelf break that influence sediment
dispersal on the slope. Here, Hurd etal. explore the relation between a pre-existing, relict break in slope and
subsequent accumulations in Permian strata of West Texas. The data reveal that
increased slopes favor bypass and channels, which in turn favor accumulation of
downdip, onlapping organic-rich shale and carbonate mud-dominated mass
transport complexes. These results are interpreted to have general
applicability to drowned carbonate platforms. And, no fish were harmed during
this study either.
High-resolution
sequence stratigraphy can be challenging, but mixed carbonate-siliciclastic
systems offer unique challenges, but at times unique insights as well. In this paper, Schwartz et al. document
outcrop and subsurface data from a Cretaceous proximal to distal transect of
the Neuquén Basin of western Argentina, exploring controls on high-resolution
stratal patterns. The results suggest transgressive carbonate hemicycles,
overlain by storm- and wave-influenced siliciclastic shoreface deposits during
regressions. The data suggest that autogenic or eustatic controls did not
control facies patterns; rather changes in sediment supply related to more
arid–more humid shifts are interpreted to control the patterns in these
strata. These results emphasize the
potentially important role of climate shifts on stratigraphy, and highlights
its potential use for correlation.
Following up on his recent magnum opus that considered universal sequence stratigraphy, Burgess
explores the depth of “what we really know” about the fundamental controls on
sequence stratigraphy in his most recent contribution. Wasting no words,
with nary a flaw in style or content, he lucidly and simply describes his
perspective on this question, and in doing so, illuminates gaps in
understanding. His unusually succinct style carefully avoids the jargon common
in sequence stratigraphy, so even beginners can grasp his points. [Ed. Note: We
appreciate Dr. Burgess’ brief abstract, shorter than even many physics
articles, which are notoriously curt. Even though it is a
review paper, it is the early favorite to win the Outstanding Paper Award.]
Mud-dominated coastal systems play by their own rules. Many of
the generalizations derived from years of study of sandy shoreline systems
appear to not apply to shaley successions, or at least apply in unique ways. To
better understand spatial changes and proximal-to-distal facies relationships
of muddy systems, Harazim and McIlroy describe sedimentological, ichnological, and
geochemical characteristics of the Lower Ordovician (Tremadocian) Beach
Formation, Bell Island Group, Newfoundland. The results demonstrate that
ancient fine-grained coastal systems are incompletely incorporated into
sequence stratigraphic models owing to their atypical proximal-to-distal facies
relationships, unique physical sedimentologic properties, and burial
efficiency. The interpretations highlight the importance of 1) frequency
of sediment supply events, 2) direction of mud transport, 3) diagenetic
reactivity of minerals and bioavailable organic carbon, and (4) residence time
of mineral grains and organic matter close to the sediment–water interface on
facies architecture of this non-uniformitarian paleoenvironment, deposited
prior to the evolution of soils. Face
it, mud is complicated.
