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Refresher Lecture 1 A Sedimentary Rocks, Facies, and Stacking Patterns ESS 400 A Summer 2017 Note: The Sedimentary refresher comes in two parts. Part 1 A covers many of the basics. Part 1 B, a more comprehensive lecture from sepmstrata. org has many more details on things like sedimentary structures, environments and stratal packaging. Please study both references as you see fit.
Outline • Basic Classification Schemes • Texture and Composition • Clastic vs carbonate vs chemical • Grain size, sorting, rounding, matrix • Tectonic signatures • Depositional Environments and Facies • • Sedimentary structures Fossils and trace fossils Clastic environment examples Carbonate environment examples • Vertical and Lateral “Stacking” • Transgressions, regressions • Sequence stratigraphy • Columns and written descriptions
Classification of Rocks
Clastic grain size subdivisions
Sedimentary Rock Descriptors
Sandstone texture – rounding, sorting
A Simple Evolutionary Sedimentary Rock Model
Clastic Sediment and Rock Naming Conventions
Sand Provenance (QFL Diagrams) “Mature” “Immature” (Granitic source) (Volcanic-sedimentary source)
Four fundamental components of sedimentary rocks
Arenites vs. Wackes
Sand Provenance (QFL Diagrams) The proportion of Qtz, Feldspar and Lithics in a sandstone will generally reflect the nature of the source area that flanked the sedimentary basin at the time of deposition.
Sedimentary structures • Sedimentary structures are features found within the sedimentary section, and/or between, bedding plane surfaces subdividing that section • Related to scale and hierarchy of features they occur in, whether in sediments that have confined (as in a channel) or unconfined settings (as on a shelf), & associated but similar sized structures • Sedimentary structures provide critical versus general clues to depositional setting • There are MANY kinds of sedimentary structures. See Refresher 1 B for examples of many of these.
Fossils and trace fossils • Fossils are found in many sedimentary rocks, and can be diagnostic of water depth, age of the strata and aerobic conditions in the water column. • Trace fossils are burrows, tracks, etc that were created by an organism which itself was not preserved. Ichnology is the study of trace fossils. • Fossils and trace fossils should be included in rock descriptions if they are present. • See Refresher 1 B for examples of some of these.
Clastic depositional environments Wave-dominated shoreline example* (*see Refresher 1 B for additional clastic environments)
Carbonate depositional environments Barrier reef example* (*see Refresher 1 B for additional examples)
Basic stacking, transgressions and regressions Transgression (sediments “retrograde”) Regression (sediments “prograde”)
Sea level, sediment supply, subsidence
Cyclicity and parasequence stacking
Sequence stratigraphy (Sepmstrata. org)
Real example of parasequence stacking
Sequence and chronostratigraphy Vertical axis is Time
Stratigraphic column examples
Stratigraphic column examples
Stratigraphic correlation between outcrop measured sections • Parasequence boundaries represent chronostratigraphic (i. e. time-synchronous) surfaces • Facies and environment interpretations are based on sedimentary structures, textures, and biota • Note that within any parasequence, more proximal facies grade to more distal facies from updip (west/left) to downdip (east/right) in the basin
Written descriptions of sedimentary rocks – an excellent example from a recent publication* The Palm Spring Formation consists of lower and upper members separated by a widespread angular unconformity (Figs. 4, 6 B, 8 E). The unconformity marks the boundary between regionally extensive and relatively uniform facies of the lower member, and highly variable localized facies of the upper member in the central Mecca Hills. The lower member of the Palm Spring Formation is 340 m thick in lower Painted Canyon where it conformably and gradationally overlies the Mecca Formation on the SW limb of Mecca Anticline (Figs. 3, 4, 6 B). It consists of tabular, uniformly bedded couplets of grus-rich, cross-bedded pebbly sandstone with plutonic and gneissic composition that fine up into biotite-rich ripple cross-laminated green siltstone and fine-grained sandstone to siltstone (Table 1, Fig. 7 B). Individual beds are laterally continuous with sharp boundaries that are traceable for several kilometers along strike, typically with little change in facies. Paleocurrent indicators suggest SE-directed transport (Fig. 6 B). The lithofacies assemblage and stratal architecture of the lower member in the central Mecca Hills is similar to that seen in the Indio Hills 25 km to the northwest, providing evidence for its regional extent. Exceptions are observed in Eagle Canyon where the lower member consists of poorly sorted cobble to boulder conglomerate (Table 1), and in the southeast part of the study area near Hidden Spring Wash (Fig. 2)where similar coarse facies were previously documented (Chang et al. , 1987; Boley et al. , 1994). The laterally extensive architecture and uniform facies of the lower member suggest deposition in a fluvial system composed of a migrating main channel or multiple channels (cross-bedded sandstone) and adjacent overbank floodplain (ripple cross-laminated green siltstone) that occupied a broad basin floor. The unconfined lateral migration of river channels created tabular-bedded fluvial architecture (e. g. , Miall, 1985; Hampton and Horton, 2007). Felsic-plutonic and gneissic clast compositions, and SE-directed paleocurrents suggest that the lower member was deposited in a large river system that flowed SE down the paleo-Coachella Valley into the Salton Trough, with sediment sources mainly in * Mc. Nabb, J. C. , et al. , Stratigraphic record of Pliocene-Pleistocene basin evolution and deformation within the Southern the Cottonwood and Little San Bernardino Mountains. Localized coarse facies of the San Andreas Zone, Mecca Hil. . . , Tectonophysics (2017), lower member likely Fault were deposited in proximal basin-margin alluvial fans. http: //dx. doi. org/10. 1016/j. tecto. 2017. 03. 021 Intro, brief generalization Detailed OBSERVATIONS. Places in regional context. Detailed INTERPRETATIONS.