Sonoma State University
Geology
Well-preserved siliciclastic domal stromatolites, up to 2 m wide and 1AE5 m high, are found in a 10 to 15 m thick interval within the Late Ordovician Eureka Quartzite of Southern Nevada and Eastern California, USA. These stromatolites... more
Well-preserved siliciclastic domal stromatolites, up to 2 m wide and 1AE5 m high, are found in a 10 to 15 m thick interval within the Late Ordovician Eureka Quartzite of Southern Nevada and Eastern California, USA. These stromatolites appear as either isolated features or patchy clusters that contain more than 70% by volume quartz grains; their association with planar, trough and herringbone cross-bedding suggests that they were formed in an upper shoreface environment with high hydraulic energy. In this environment, sand bars or dunes may have provided localized shelter for initial microbial mat colonization. Biostabilization and early lithification of microbial mats effectively prevented erosion during tidal flushing and storm surges, and the prevalence of translucent quartz sand grains permitted light penetration into the sediment, leading to thick microbial mat accretion and the formation of domal stromatolites. Decimetre-scale to metre-scale stromatolite domes may have served as localized shelter and nucleation sites for further microbial mat colonization, forming patchy stromatolite clusters. Enrichment of iron minerals, including pyrite and hematite, within dark internal laminae of the stromatolites indicates anaerobic mineralization of microbial mats. The occurrence of stromatolites in the Eureka Quartzite provides an example of microbial growth in highly stressed, siliciclastic sedimentary environments, in which microbial communities may have been able to create microenvironments promoting early cementation/lithification essential for the growth and preservation of siliciclastic stromatolites.
Fault rocks associated with the Pelling thrust (PT) in the Sikkim Himalayan fold thrust belt (FTB) change from SL tectonites to local, transport-parallel L-tectonites that are exposed in discontinuous klippen south of the PT zone. By... more
Fault rocks associated with the Pelling thrust (PT) in the Sikkim Himalayan fold thrust belt (FTB) change from SL tectonites to local, transport-parallel L-tectonites that are exposed in discontinuous klippen south of the PT zone. By estimating the incremental kinematic vorticity number (W k) from quartz c-axes fabric, oblique fabric, and subgrains, we reconstruct a first-order, kinematic path of these L-tectonites. Quartz c-axes fabric suggests that the deformation initiated as pure-shear dominated (~56e96%) that progressively became simple-shear dominated (~29e54%), as recorded by the oblique fabric and sub-grains in the L-tectonites. These rocks record a non-steady deformation where the kinematic vorticity varied spatially and temporally within the klippen. The L-tectonites record ~30% greater pure-shear than the PT fault rocks outside the klippen, and the greatest pure-shear dominated flow among the published vorticity data from major fault rocks of the Himalayan FTB. The relative decrease in the transport-parallel simple-shear component within the klippen, and associated relative increase of transport-perpendicular, pure-shear component, support the presence of a sub-PT lateral ramp in the Sikkim Himalayan FTB. This study demonstrates the influence of structural architecture for fault systems for controlling spatial and temporal variations of deformation fabrics and kinematic path of deforming thrust wedges.
- by Matty Mookerjee and +1
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- Earth Sciences, Structural Geology, Vorticity, Kinematics
Fault rocks associated with the Pelling thrust (PT) in the Sikkim Himalayan fold thrust belt (FTB) change from SL tectonites to local, transport-parallel L-tectonites that are exposed in discontinuous klippen south of the PT zone. By... more
Fault rocks associated with the Pelling thrust (PT) in the Sikkim Himalayan fold thrust belt (FTB) change from SL tectonites to local, transport-parallel L-tectonites that are exposed in discontinuous klippen south of the PT zone. By estimating the incremental kinematic vorticity number (Wk) from quartz c-axes fabric, oblique fabric, and subgrains, we reconstruct a first-order, kinematic path of these L-tectonites. Quartz c-axes fabric suggests that the deformation initiated as pure-shear dominated (~56-96%) that progressively became simple-shear dominated (~29-54%), as recorded by the oblique fabric and sub-grains in the L-tectonites. These rocks record a non-steady deformation where the kinematic vorticity varied spatially and temporally within the klippen. The L-tectonites record ~30% greater pure-shear than the PT fault rocks outside the klippen, and the greatest pure-shear dominated flow among the published vorticity data from major fault rocks of the Himalayan FTB. The relative decrease in the transport-parallel simple-shear component within the klippen, and associated relative increase of transport-perpendicular, pure-shear component, support the presence of a sub-PT lateral ramp in the Sikkim Himalayan FTB. This study demonstrates the influence of structural architecture for fault systems for controlling spatial and temporal variations of deformation fabrics and kinematic path of deforming thrust wedges.
We report quartz c-axis patterns, grain-shape fabrics, and microstructures for 11 mylonitic quartzites and quartz-phyllosilicate schists from a transect across the Moine thrust at Loch Srath nan Aisinnin, North-West Scotland. In the... more
We report quartz c-axis patterns, grain-shape fabrics, and microstructures for 11 mylonitic quartzites and quartz-phyllosilicate schists from a transect across the Moine thrust at Loch Srath nan Aisinnin, North-West Scotland. In the footwall samples collected more than 42 m normal distance from the thrust surface, quartz c-axis textures indicate a general flattening strain (i.e. 0 , k , 1). Samples within 19 m normal distance of the thrust are completely recrystallized and exhibit asymmetric c-axis patterns. Recrystallized hanging wall fault rocks exhibit random c-axis patterns on the scale of a standard thin section.
- by Matthew Strine and +2
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- Geology, Structural Geology, Kinematics, Shape Analysis
We report quartz c-axis patterns, grain-shape fabrics, and microstructures for 11 mylonitic quartzites and quartz-phyllosilicate schists from a transect across the Moine thrust at Loch Srath nan Aisinnin, North-West Scotland. In the... more
We report quartz c-axis patterns, grain-shape fabrics, and microstructures for 11 mylonitic quartzites and quartz-phyllosilicate schists from a transect across the Moine thrust at Loch Srath nan Aisinnin, North-West Scotland. In the footwall samples collected more than 42 m normal distance from the thrust surface, quartz c-axis textures indicate a general flattening strain (i.e. 0<k<1). Samples within 19 m normal distance of the thrust are completely recrystallized and exhibit asymmetric c-axis patterns. Recrystallized hanging wall fault rocks exhibit random c-axis patterns on the scale of a standard thin section.Relict footwall grains provide the closest approximation of finite strain; they have octahedral shear strains (ϵs) between 1.10 and 1.47 and exhibit general flattening k-values (0.0524–0.659). The long axis of the mean relict grain shape trends parallel to the regional transport direction and plunges gently to the ESE. In contrast, recrystallized footwall grains have ...