Sedimentology, Stratigraphy, and Paleontology
What to know
Siliciclastic sediment is described by grain size (Wentworth scale), sorting, roundness, and maturity. Transport mode (traction, saltation, suspension) leaves structures: cross-bedding, ripples, horizontal lamination, graded bedding, mudcracks, and bioturbation. Carbonates are classified by grain type and texture (Dunham: mudstone, wackestone, packstone, grainstone, boundstone).
Stratigraphy has parallel frameworks: lithostratigraphy (rock units), biostratigraphy (fossil zones), chronostratigraphy (time), magnetostratigraphy, and sequence stratigraphy (systems tracts bounded by unconformities and flooding surfaces). Unconformities are angular, disconformity, or nonconformity. Walther's Law states that vertical facies successions record laterally adjacent environments, provided the contact is conformable. Transgressions produce fining-upward, deepening successions; regressions produce coarsening-upward, shoaling successions.
Paleontology emphasizes index fossils (wide geographic range, short time range), biozones, and preservation mode (permineralization, molds/casts, carbonization, trace fossils).
Traps
Reading cross-bed foresets as dipping upcurrent (they dip downcurrent); confusing fining-upward with regression; assuming every fossil is a good index fossil; calling a disconformity an angular unconformity without evidence of tilted beds below; treating a biozone as an exactly isochronous surface.
Rule of thumb
Look at the base of the bed: rip-up clasts, sole marks, and coarse sand fining upward into mud = turbidite (Bouma sequence, deepening then abandonment), not a beach.
Worked example: a sandstone with tabular cross-beds dipping southwest and a southwest-thinning wedge indicates paleoflow to the southwest - use the dip direction of foresets, not the set boundaries.