DESCRIPTIVE STRUCTURAL GEOLOGY I. DESCRIPTIVE STRUCTURAL GEOLOGY A. Mainly concerned with geological contacts 1. Infinitesimally thin 2. Separate Formations (basic mappable unit = Formation) 3. Most common datum for defining structures 4. Nature of contact - imperative for both geometric and historical interpretation II. MULTIPLE WORKING HYPOTHESIS - in the field devise a story about a contact/ Formation then test it A. Constantly changing due to "walking" a contact and mapping B. Devised by Chamberlain, T. C., 1897, The Method of the Multiple Working Hypothesis: Journal of Geology, v. 5, p. 837-848. C. In field methods book D. Always look for things that disprove your hypothesis - not just ones that support it III. TWO TYPES OF CONTACTS: A.Primary - formed at deposition or intrusion 1. Depositional (stratigraphic) a. Conformable - sequence of oldest to youngest rx b. Nonconformable - rx history missing in some part * Angular unconformity - rocks beneath unconformity are tilted * Nonconformity - forms above igneous or metamorphic rx * Disconformity (paraconformity) - unconformity between beds that are parallel --- these are hard to recognize in the field 2. Intrusive contacts - next to igneous intrusion a. Xenolith b. x-cutting c. Contact metamorphism e.g., sillimanite, kyanite, staurolite, garnet, biotite, chlorite d. Chilled margins e. Highly irregular oscillating boundaries form between intrusion and country rx B. Secondary - overprint primary events, formed after deposition or intrusion e.g., faults, folds, shear zones 1. Fault contacts a. Anomalous age relationships e.g., normal faults tend to excise units (leave some missing) e.g., reverse faults tend to repeat units (place older on younger) b. Structural discordance c. Fault rocks: * Mylonite - plastic flow (deep) * Breccia - brittle (shallow) * Striated fractures (slickensides, grooves, etc...) * Alterations d. Geomorphic expression - linear valleys or ridges, springs, radon leaks 2. Ductile deformation zones - ductile shear a. Deformation by plastic flow 3. Metamorphic or structural front a. Hard to recognize b. Gradational c. Changes in lithology or texture of the same rock units due to metamorphism and/or deformation IV. PRACTICAL FIELD PROBLEM A. How would you distinguish an unconformity from a fault? 1. Look for: a. Younger rocks to parallel the contact if it is an unconformity b. Conglomerate or breccia at the base of the younger sequence, determine whether clasts were being derived from the underlying "basement", indicating an unconformity V. PRIMARY STRUCTURES - formed when the rock formed (most of what we will talk about are formed in sedimentary rx) A. Cross stratification - forms due to the migration of bed forms in current 1. Stoss - gentle, erosion 2. Leeward - steep, deposition 3. Tops are truncated, bottoms are tangential B. Channel migration - younger channels cut older ones C. Ripple marks - periodic waves of sand 1. Asymmetric 2. Symmetric / Oscillatory D. Graded beds - fining upward sequences e.g., turbidite sequences 1. Sharp break between turbidites 2. Metamorphic rx - compositional gradation of turbidite Al2O3 - schist SiO2 - quartzite E. Sole marks and related structures - bottom features produced by erosion and/or loading of the bottom during the next sedimentation event 1. Load casts - form due to density contrast combined with a plastic substratum a. Ball and pillow structures 2. Tool marks - scour marks left by sticks, rocks, etc. that are dragged along the bottom of a channel a. Found as cast 3. Flute cast - an obstruction in the current causes an erosional trough down stream 4. Rip-up clasts and flame structures - current rips up substratum (plastic) a. Related to load casts F. Desiccation cracks - drying of mud causes shrinkage producing cracks that curl up G. Fossils - evidence of past life 1. Can be used for up and down indicators e.g., footprints, life habits, trees, stumps, corals, death assemblages, and ichnofossils (trace fossils) - burrows VI. PRIMARY STRUCTURES IN VOLCANIC ROCKS A. Volcano-clastic deposit -reworking of igneous rx by sedimentary processes 1. Inverted / reverse graded beds - fining downward 2. Rare - large pumice is on top 3. Not a clean gradation B. Lavas 1. Rubbly flow bottom - pieces of rock from the erosional surface picked up and incorporated into the basal parts of the flow 2. Auto-brecciation - chilling along bottom and top, adds to rubby flow bottom appearance but be careful because top may be auto-brecciation too 3. Oxidized or baked zone - due to heating of the erosional surface by the hot lava; usually reddish coloration due to oxidation of Fe-bearing materials 4. Vesiculation - due to gases escaping, move upward due to gravity 5. Pillow lavas - submarine volcanism produces slightly flattened (rounded) tops whereas bottoms conform to the shape of underlying flow structures C. How do we distinguish sills from flows in the field? 1. Vesticulation is less common in a sill but they may occur at the top of a shallow sill and be closed off giving an inverted teardrop shape (this tells which way is up) 2. Contact effect such as baked zones or fining toward margins is mirrored along the top and bottom of a sill 3. No auto-brecciation or rubbly flow bottom in a sill; no evidence for transport along an erosional surface 4. General intrusive features such as a. Wedges of intruded material into country rock b. Coarse grain size (phaneritic texture) c. Evidence for crystal settling or floating * Ferromags sink - g > 3 gm/ml * Plagioclase may float - g ~ 2.6 e.g., Palisades Sill, Hudson River, NY (Triasic)