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Stratigraphic Concepts

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Stratigraphic Concepts Demonstrating equivalency (genetic/time) between strata (rock units) • lithostratigraphy - organization of strata on the basis of their lithologic characteristics • biostratigraphy - organization of strata on the basis of the fossils they contain • magnetostratigraphy - organization of strata on the basis of their magnetic characteristics • chemostratigraphy - organization of strata on the basis of their isotopic characteristics • seismic stratigraphy - organization of strata on the basis of their seismic characteristics • chronostratigraphy - time relationships • sequence stratigraphy - depositional sequences, packages of strata bounded by unconformities San Benito Gravels • Age? • Sedimentation Rate? S.R.=∆d/∆t • Episodic deposit.? – pause Unconformity? – erosion ∆d=? ∆t=? 1 Absolute vs. Relative Time • Absolute time - set within the framework of geologic time – essential for reconstructing tectonic history, paleoclimates, etc. • Relative time - time represented by the outcrop – essential for computing accumulation rates, etc. time time ∆t depth depth ∆d 2 “Timing is everything” • Chronology (stratigraphy) is essential for understanding all earth history & hence earth system processes, – tectonic – climatic – biologic (evolution). Lithostratigraphy the study & organization of strata on the basis of lithologic characteristics • lithology - type, color, mineral composition, and grain size LITHOSTRATIGRAPHIC UNITS - bodies of rocks distinguished on the basis of observable lithologic characteristics – no connotation of age other than the law of superposition – separated by contacts – Stratotype - type section (most complete) 3 Fundamental Units*: n o i t • Group - collection of formations a m r o • Formation- lithologically distinctive unit f that is large enough in scale to be mapable (single lithology or regular alternation of lithologies) C – e.g. Monterey Formation • Member - (subdivision of a formation) characteristics that distinguish it from other parts of the formation B members? , – e.g. series of phosphatic-rich layers interfingering with dolomites/cherts Group I • Beds - subdivision of a member, smallest unit Lithology *Only applied to land-based sedimentary C sections Paleocene-Eocene Strata: Gulf & Atlantic Coast 4 Paleocene-Eocene Strata: Gulf & Atlantic Coast Clayton Wilson Lake 5 Paleocene-Eocene Strata: Gulf & Atlantic Coast Clayton Wilson Lake Gl-91 Purisma Santa Cruz MS Monterey Vaqueros San Lorenzo Butano SS Locatelli 6 Contacts boundaries between units 1. plane or irregular surfaces 2. Conformable or unconformable • Conformity (Conformable) - no physical evidence of non- deposition – Abrupt – Gradational • progressive - gradation Site 1263A - 13H S6 50.5 cm • intercalated - gradation is an Eocene inter-bedded interval Paleocene 7 Contacts • Unconformity (unconformable) - break or hiatus in deposition (erosional, non-deposition) current scour surface, or sub-aerial weathering surface, slump or slide surfaces – Angular - younger sediments atop eroded surface of tilted or folded rocks – Disconformity - parallel bedding planes, but erosional surface (channeled, paleosols, lag-gravel deposit) - uplift, sea-level regression • lithology may change – Paraconformity - same lithology above and below, non deposition or dissolution. • Can only be recognized by other stratigraphic techniques • Nonconformity - sedimentary / igneous or metamorphic rock Angul?ar UC Discon?formity Nonco?nformity 8 Recognition of Unconformities • basal conglomerates • deeply weathered soils horizons • truncated bedding • clasts • burrowing or hardgrounds • channel deposits • truncated fossil ranges (several lineages) 9 Lateral Contacts • pinch-out - progressive thinning of a bed • intertonguing - lateral splitting of units that pinch out independently – shoreline migrating back and forth • progressive lateral gradation Stratigraphic completeness: hiatuses (diastems) - abundant in the rock record - – more frequent in high energy environments • Sedimentation rates high, but episodic (erosion common) • continental – non-deposition and erosion - uplift – episodic deposition - flooding • shallow marine depth time – Erosion - regressions (local, global) gap – non-deposition, wave erosion – deposition during storms • hemi-pelagic – slumps, turbidites represent episodic deposition • pelagic gap – sedimentation tends to be lower, but more continuous 10 ODP Site 1207, northwest Pacific • Drilling Objectives: – P/E Boundary – K/T Boundary – Aptian/Albian ODP Site 1207, northwest Pacific Age/Depth 11 Facies Relationships in Space and Time Prograding Delta 12 Vertical and Lateral Successions of Strata: • conformable and unconformable contacts divide sedimentary rocks into vertical successions. Walther’s law - to be conformable, vertically adjacent facies must reflect those facies which occur side by side • facies - a body of rock with some consistent characteristic • lithofacies - a consistent lithologic characteristic within a formation (shale facies, evaporite facies) Walther’s Law Deepening upward Shallowing upward 13 Cyclic successions rhythmic sedimentation (repetitions of strata) • Temporal scales annual to m.y. • All environments pelagic - limestone/marl delta - repeated coarsening upward cycles Paleocene Scalia Rosa, Dolomites, Italy 14 Cycle Order (Scales) Cycle Order Major sedimentary cycle durations as influenced by eustatic sea level changes Type Duration (m.y.) Cause 1st 200-400 Tectonic: formation and breakup of supercontinents 2nd 10-100 Mid-ocean ridge spreading changes - volume 3rd 1-10 Mid-ocean ridge spreading changes - volume 4th 0.2-0.5 Milankovitch glacioeustatic cycles 5th 0.01-0.2 Milankovitch glacioeustatic cycles 15 Cyclic successions Causal Mechanisms: • Autocyclic - internal to the basin switching delta lobe, storm beds, floodplain – beds show limited lateral continuity Flood Plain Deposits, • Allocyclic - mainly external Paleocene-Eocene Bighorn Basin, WY to the basin climate, sealevel, tectonic movements in source area – beds may show extensive lateral continuity U. Carboniferous, S Wales Allocyclic Mechanisms Milankovitch cycles - oscillations in earth's orbit primary periods: • 19, 23 ky - precession of the pole (wobble) • 41ky - obliquity (tilt) • 100, 410 ky - eccentricity Perihelion -147x106km Aphelion - 152x106km 16 Orbital cycles:Effect on Insolation • Eccentricity 95, 100, 120, 413 ky (2.3 m.y.) – Earth - sun distance 0.0 to 0.06 Effect on insolation: ca. 0.7 W/m2 uniform across latitudes • Tilt 41 ky (29, 54 ky,1.25 m.y.) – angle ~ 22.0-24.3° hotter summers / colder winters in both hemispheres Effect on insolation: up to 17 W/m2 at high latitudes • Precession 19, 23 ky – wobble - gravitational pull of sun on earths’ equatorial bulge – elliptical precession hot summers/cold winters in one hemisphere, and cold summers/hot winters on the other. Effect on insolation: up to 40 W/m2 Ice-sheets (18 kya, present day) 17 Sea-level Change, 150 kya to present Sea Level 120 meters 18 90 m 100 m ~10 km Transgressions/Regressions Transgressions - shoreline moves landward Regressions - shoreline moves seaward 3 Causes: 1. Sea level - rise and fall 2. Tectonic - uplift /subsidence 3. Sediment Supply • eustatic changes – ice-volume (glacioeustatic), basin geometry Global signal • relative changes – Local subsidence/uplift or sediment supply Regional signal 19 Sea level Rise sand silt mud transgression coastal onlap time lines fining upward sequence • Transgression – Sea level rise w/ no change in sediment supply • Stationary – Sea level rise w/ balanced by sediment supply • Regression – Sea level rise w/ large increase in sediment supply • all three produce coastal onlap because sea level is rising Sea level rising (high sediment supply) regression erosional surface coastal onlap • Rising sealevel - coastal onlap, – regression Top lap Regression no onlap Sea level stationary • Standstill of sealevel - no coastal onlap, but top-lap – regression 20 Coast Basin Section C Section A Section B Lithofacies are time transgressive Transgression (Deepnening) Regression (Shallowing) Sea level Rise 21 Asymmetry of Transgressive and Regressive Cycles: • Transgressive - classic fining non-marine marine non-marine marine upward? regression – Rare- fining upward less common than coarsening upward transgression • Rapid rise in SL - erosion/non- Delta Progradation deposit during transgressive phase regression limestone (rapid) limestone – coastal and shallow marine deposits - shaly shale thin or non-existent (marine) transgression coal Rapid transgression – deposition mainly during regressive shale (sandy) phases channel sand disconfomity – e.g. delta progradation - Carboniferous Cyclothems Carboniferous (299 to 359 Mya) 22 Carboniferous (299 to 359 Mya) Cyclothems: Allocyclic vs. Autocyclic Carboniferous Coals, West Virginia Correlation of Lithostratigraphic Units • Lateral Tracing • most direct method • only where strata are continuously lithologic exposed similarity Key bed • Lithologic Similarity and Stratigraphic (ash) position • indirect method • correlation based on facies sequence • difficult to apply to cyclic successions • Event Stratigraphy • Marker beds • ash (bentonites) (e.g. Bishop Tuff, Long Valley Caldera; 740 ky) • lava flows 23 Correlation of Lithostratigraphic Units Bishop Tuff • Long Valley Caldera • 740 ky Lithologic Similarity and Stratigraphic position 24.
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