Have you ever come home and
found a blonde stranger sleeping in your just-right sized bed? The three bears
did just that, and their intruder now has been immortalized in a recent paper
by Ashley et al., who explore the conundrum of the occurrence freshwater
limestones in arid rift basins. Documenting and interpreting the carbonate
petrology in the context of well-documented geochronology and climate history
of the East African Rift System reveals groundwater-fed limestone in wetlands,
driven by precession-moderated cycles of precipitation, geologic structural
controls (i.e., faults), playa flooding frequency and the general hydrology of
the basin. These results implicate the multiple spatial and temporal controls
on freshwater limestone, and illustrate how freshwater limestones require conditions
have to be just right, i.e. the Goldilocks effect. Likewise,
the interpretation of non-lacustrine carbonate suggests a new source of potable
water for the early hominins of Olduvai Gorge.
Orogenies can have a
regional or even global influence on climate, but means to assess these
influences can be masked in the stratigraphic record. Extracting oxygen
isotopes from a diverse set of vertebrate phosphate materials, Suarez et al. explore
the effect of the Sevier Orogeny on regional climate and paleohydrology, as
recorded in the Cretaceous Cedar Mountain Formation. The results within the
Cedar Mountain Formation suggest a regional rainshadow effect from the Sevier
Orogeny. Compared to the average meteoric water values for this paleolatitude
as determined by pedogenic carbonates, meteoric water values determined from
crocodiles and turtles are within the range of values for 34°N paleo-latitude. The
Sevier Orogeny had a major effect on the isotopic composition of river water, the
result of significant depletion in runoff of high altitude water or snow. These
sorts of analyses that distinguish between regional paleoclimatic effects of
orogeny and those of global climatic factors are essential for evaluating
perturbations to the global carbon cycle and the related changes in temperature
(e.g. mid-Cretaceous global warming).
In assessment of
fine-grained unconventional reservoir rocks, discrimination of kerogen
(primary, detrital organic matter (OM)) from bitumen (a diagenetic product of
kerogen maturation) can be difficult, even at the SEM scale. To explore the
nature of these systems, Milliken et al. describe observations of low-maturity,
and presumably bitumen-free, samples of Pliocene-Pleistocene marine sapropels. The
data illustrate the diversity of organic-matter-hosted pore systems, providing
a useful baseline for identification of these fundamentally different OM types
in mature OM-rich mudrocks, and highlighting some of the challenges. Knowing
the proportion of detrital versus diagenetic organic matter in these types of
mudrocks has significant implications for understanding the nature of petroleum
systems in unconventional reservoirs.
The facies architecture of
siliciclastic sedimentary systems is shaped by a range of controls. To assess
the source-to-sink dynamics and controls on Cretaceous strata of the Western
Interior Seaway, Hampson et al. examine the influence of sediment supply on
stratigraphic architecture. Applying a new methodology of mass-balance analysis
to a succession of linked alluvial-coastal-shelfal deposits, the paper
quantifies downsystem-fining, sediment-partitioning and sediment-budget
characteristics within the sediment routing systems of several sequences. As
this work illustrates a novel approach to quantifying the relations between
accommodation and sediment supply, parameters that are rarely constrained or
quantified in conventional sequence stratigraphic interpretations, it holds the
potential for new insights into stratigraphic forcing mechanisms.
