Pressure and
temperature are two fundamental controls on diagenesis and formation of
economic minerals. To understand the possible influence of fault-related
hydrothermal fluids on uranium mineral deposits of Carboniferous to Jurassic
siliciclastic deposits in the Tim Mersoï Basin in Niger, Mamane Mamadou and others examine the
P-T conditions of diagenesis using petrography SEM observations and chemical
analyses, supplemented thermometric approaches of chlorite compositions and
fluid inclusions in quartz overgrowths. Chlorite thermometry indicates
that all Carboniferous to Jurassic section was subjected to elevated
temperatures of around 125°C (Carboniferous) and 115°C (Jurassic). These
temperatures suggest a strong thermal disequilibrium between incoming fluids
and reservoirs, reflecting burial temperatures in excess of those expected at
maximum burial depth. The fault-influenced fluid circulation affecting
these strata are interpreted to to be linked to major geodynamic events related
to the opening of the Atlantic Ocean, and likely have analogs elsewhere.
Forearc basins are depositionally important tectonic elements of
ocean-continent collision zones, and commonly contain strata influenced by
subduction and magmatic arc evolution. However, detailed spatial and temporal
understanding of the timing and mechanisms of forearc development is limited,
in part because these basins are rarely exhumed and easily accessible on
land. To better understand tectonics and sedimentation of forarc basins, Orme and Laskowski
examine Albian–Santonian strata of southern Tibetan Xigaze forearc basin, a
large and well-preserved forearc basins, and which records upper-plate
processes active prior to and following the inter-continental collision between
India and Asia. Using regional geologic mapping, detailed sedimentologic
and facies analysis, modal petrographic characteristics, and U-Pb
detrital-zircon geochronologic data, the study documents the relationship between
the Yarlung-Tsangpo ophiolite and Xigaze forearc basin and reconstructs the
sedimentary environments of the southern margin of Asia during the initial
stages of forearc deposition. Results reveal details of the evolution of
the southern margin of Asia prior to its collision with India, including
diachronous development of the forearc basin prior to India–Asia collision.
Mixed siliciclastic-carbonate depositional systems occur throughout
the geologic record. Although their general sedimentologic aspects are
well-constrained, substrate-controlled trace fossil assemblages that can
represent discontinuities in these successions are less well understood. By
integrating ichnological descriptions and facies analyses of the Baldonnel
Formation at Williston Lake, B.C., Canada, Schultz and others
explore the ichnological composition of paired Trypanites- and Glossifungites-demarcated
discontinuity surfaces, the significance of these surfaces in regional
stratigraphy, and the relationship of these two assemblages to each other in
individual parasequences sets. Recognition of these surfaces, and an
understanding of their character, is paramount to the stratigraphic evolution
and architecture of this mixed carbonate–siliciclastic depositional system, and
may have analogs in other mixed systems.
Although fine sediment represents an important proportion of
sediment transferred by rivers, many studies of basin margin progradation
emphasize supply and accretion of sand at and beyond the shelf edge. To
understand how and when basin margin clinothems prograde in the absence of coarse-grained
sediment supply to the shelf edge, Poyatos-Moré and others examine clinforms of the
Permian Waterford Formation, in the Karoo Basin (South Africa). In these
strata, normal and inverse grading, erosion surfaces, and moderate to low
intensity of bioturbation are common in low-gradient (< 0.7°) clinoforms of
thick, outer-shelf to upper-slope mudstone overlain by thin sandy shelf topset
strata. The study provides a depositional model for shelf-to-slope
transitions in fine grained successions, illustrating how progradational
mud-rich, shelf-confined deltaic clinothems with muddy shelf-edge rollovers
develop.
Clastic shallow marine systems can be grouped into wave- and
tide-dominated systems, but carbonate successions are interpreted less commonly
in this manner. In this study, Matheson and others document
Mississippian-age carbonates with abundant physical sedimentary structures from
the Canadian Rocky Mountain Front Ranges. The observations lead to
development of a more comprehensive model of sedimentation across a
swell-dominated, open-marine carbonate ramp. Comparing and contrasting shelf
carbonate and siliciclastic sands suggest two primary differences: 1) in
carbonate settings, there may be an increased preservation of deposits created
by swell waves, and 2) that swaly cross-stratification in carbonate settings is
not limited to the shoreface as it is in siliciclastic settings. These
differences are due to the variability in spatial grain-size distributions and
aggradation rates between siliciclastic and carbonate environments due in
situ grain production in carbonate settings.
