Geosciences 528 Sedimentary Basin Analysis Spring, 2011 G528 – Sedimentary Basins • Prof. M.S. Hendrix – Office SC359 – Office Phone: 243-5278 – Cell Phone: 544-0780 – [email protected] • Textbook = Principles of Sedimentary Basin Analysis - Andrew Miall • Lab 320 • Syllabus Introduction to sedimentary basin analysis What is a sedimentary basin? • thick accumulation (>2-3 km) of sediment • physical setting allowing for sed accumulation e.g. Mississippi Delta up to 18 km of sediment accumulated • significant element of vertical tectonics which cause formation of sed basins, uplift of sed source areas, and reorganization of sediment dispersal systems • Study of history of sedimentary basins and processes that influence nature of basin fill Vertical tectonics caused primarily by: • plate tectonic setting and proximity of basin to plate margin • type of nearest plate boundary(s • nature of basement rock • nature of sedimentary rock Requires working or expert knowledge on wide variety of geologic subdisciplines • sedimentology (basis of interpretation of depositional systems • depositional systems analysis • paleocurrent analysis • provenance analysis • floral/ faunal analysis • geochronology • crustal scale tectonic processes, geophysical methods • thermochronology (Ar/Ar, apatite F-T, etc.) • special techniques - organic geochemical analysis - paleosol analysis - tree ring analysis • involves both surface and subsurface data • involves large changes in scale and may involve long temporal histories Location/ exposure quality Stratigraphic measurements, sedimentology, paleoflow data Clast Composition Analysis Paleogeographic/ paleoenvironmental interpretation Regional tectonic picture Basin models: 1) a norm, for purposes of comparison 2) a framework and guide for future observation 3) a predictor 4) an integrated basis for interpretation of the class of basins it represents Francis Bacon: ‘Truth emerges more readily from error than from confusion.’ S.J. Gould: ‘Classifications are theories about the basis of natural order, not dull catalogues compiled only to avoid chaos.’ Vertical crustal controls on sed basins (Subsidence mechanisms) • Crustal thinning: extensional stretching, erosion during uplift, magmatic withdrawal • Mantle-lithospheric thickening: cooling of mantle following cessation of stretching or heating due to asthenospheric melts • Sedimentary or Volcanic loading • Tectonic loading • Subcrustal loading (underthrusting of dense lithosphere) • Asthenospheric flow, for example due to descent of subducted lithosphere; emplacement of high density melts into lower density crust • difference in thickness and density between oceanic and continental crust (isostacy) • thermal history of continental & oceanic crust • combinations of above More specific details of sedimentary basin fill patterns governed by: • geometric shape and size of basin and evolution of floor and flanks of basin • nature of stratigraphic fill • structures that develop within basin during its evolution (e.g. growth faults, salt diapirs Dickinson, 1976 Basin Analysis and Classification • Basins should be classified according to their tectonic setting at the time of deposition of given stratigraphic interval; basins may change their teconic seting rapidly and often. • A complete basin analysis must incorporate all phases of development of a basin and must consider both proximal and distal tectonic influences. • Sedimentary successions (basin fill) may accumulate due to subsidence of a shallow substrate (‘sinking substratum’) OR from filling of a space below base level (usually sea level; ‘filling hole’). Most basins are hybrid. • Preservation potential of a basin is an important factor in basin analysis.e.g. trench-slope basins have low preservation potential, whereas intracratonic basins are likely to be preserved • Fundamental difference between preservability of a sedimentary basin vs. tectonostratigraphic assemblages that make up the basin fill (e.g. Bay of Bengal vs. Bengal Fan turbidites). Convergent margin settings, cont. Brief Survey of Basin Models I. Extensional basins • Most modern passive continental margins resulted from breakup of supercontinent Pangea. Likewise, many Paleozoic plate margins (e.g. Cordillera, Appalachians) originated from breakup of Rodinian supercontinent. • Supercontinent cycle 350-400 Ma. Several different hypotheses to explain breakup of supercontinent • 1) random motions of continents around earth • 2) Supercontinent acts as a thermal blanket, inducing thermal upwelling of mantle to initiate rifting and eventual breakup. • 3) Subduction principally of old, cold crust during times of supercontinent formation.Slab roll-back (i.e. density driven ‘slab pull’) = important phenomena that may induce supercontinent breakup. Formation of ‘passive continental margins’ • Pre-Rift phase includes sedimentary and tectonic setting prior to initiation of rifting. Depends on pre-rift setting. Commonly continental sedimentation on craton. • Rift phase is tectonically active, with normal faulting, crustal thinning, volcanism, high heat flow and locally high rates of subsidence and sediment accumulation. • Drift phase (post-rift) = dominated by lithospheric cooling, thermal subsidence, and development of broad flexural basins dominated by sediment loading (e.g. continental embankments). Simple model for evolution of passive continental margins Dickinson, 1976 “Active” vs. “Passive” Rifting: • Active: Kinsman (1975): early, domal uplift preceeded crustal stretching; surficial and tectonic erosion of thermal dome thins upper crust and produces major subsidence once margin rafted away from heat source. – Test: Predicts >15 km subaerial erosion = amount cont. crust has been thinned by rifting. Also predicts centrifugal drainage patterns and sediment starvation (unless significant volcanism). Basaltic volcanism predominates in early rift stages. • Passive: early thermal doming in response to crustal thinning. Doming is caused by mantle upwelling. Active vs. Passive Rifting Models www.mantleplumes.org/WebDocuments/InfolioEngIvanov.pdf Baikal Rift regional tectonic setting www.mantleplumes.org/WebDocuments/InfolioEngIvanov.pdf Baikal Rift Lithospheric structure www.mantleplumes.org/WebDocuments/InfolioEngIvanov.pdf Rift Geometries • simple shear (no mechanism for bringing mid-crustal rocks to shallow levels) • delamination between upper and lower crust • low angle detachment faults: either as through-going (cut entire lithosphere) or intracrustal Rift Geometries cont. Crustal Detachment Rift Model • Asymmetry of rift systems by presence of major detachments will produce ‘upper plate’ and ‘lower plate’ margins – Upper plate: originate in hanging walls of detachments; thick continental crust with narrow continental shelves and thin sedimentary cover; structurally simple with only weakly rotated normal faults – Lower plate margins: originate in footwalls of detachments; thin continental crust with broad shelves, thick sedimentary cover, and exhumed middle to lower crustal rocks and remnants of upper plate in strongly tilted blocks. Crustal scale detachment rift model, cont. Busby and Ingersoll, 1991 Detachment model, cont. Asymmetry of rift systems involving major detachments produces ‘upper plate’ and ‘lower plate’ margins Upper plate: originate in hanging walls of detachments; thick continental crust with narrow continental shelves and thin sedimentary cover; structurally simple with only weakly rotated normal faults. Lower plate margins: originate in footwalls of detachments; thin continental crust with broad shelves, thick sedimentary cover, and exhumed middle to lower crustal rocks and remnants of upper plate in strongly tilted blocks. Busby and Ingersoll, 1991 Crustal scale detachment rift model, cont. polarity reversals exert Major control on passive margin geometry Busby and Ingersoll, 1991 Rift basins Characteristics of Rift basins • very early stages of rift development • high heat flow • extensional • interstratified lavas and redbed sediments; evaporites common due to screening of rivers by rift shoulders - e.g. Red Sea, Rio Grande Rift Red Sea is only true proto-oceanic basin on earth, so our understanding of these basins is strongly weighted towards examination of that basin. Post-rift basins Post-rift basins (as opposed to syn-rift) are formed mainly from subsidence resulting from thermal relaxation Miogeoclinal Prisms • require one-sided open ocean setting • transition of sedimentary facies - nonmarine to shallow marine deltaic - shelf and paralic sediments of continental terrace - marine turbidites of slope and rise • may be characterized by salt diaparism and growth faulting • Heavily influenced by fluctuations in sea level Continental Embankments • advancement of shelf break to point over oceanic crust -Mississippi embankment is 1000 km wide from OK coastal plain to edge of Sigsbee escarpment • series of lensoidal sedimentary packages • immense sediment accumulation (16-18 km) which loads continental and oceanic margins • unstable: results in growth folding, gravitational failure, salt diaparism, etc. Sigsbee salt nappe is one of largest single structural features of the North American continent • usually result from drainage of large continents toward mouths of failed rifts and away from ‘normal rifted continental margins (e.g. Mississippi delta, Nile delta) • Rapid subsidence (10-100m/Ma) results from -sedimentary load which induces lithospheric flexure -listric normal faulting (growth faulting) -salt withdrawal