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Basin Modeling Week 2: Subsidence and Geohistory Modeling (part one) 1. Review Isostacy (Airy/Pratt). Local compensation Principle of isostatic compensation – applied to basins Subsidence mechanisms2. Geohistory Analysis: what is it and why is it useful? y g y y 3. Early approaches to geohistory analysis 4. How-To: Geohistory analysis made easy Readings – all are online: Van Hinte, 1978 Angevineet al., 1990. AAPG Continuing Ed. Course Notes, available online at: Allen and Allen textbook Ch. 2, 9 Homework (due next week): Create your own1 page ‘cheat sheet’ or personal reference guide for geohistory cheat sheet modeling.

1) Review Fundamental cause of subsidence = isostacy. y Isostatic compensation: below a certain depth,asthenosphere of equal density underlies both columns (amount displaced is related to load). Above this depth, both columns must have equal weights. weights Mass column 1 = Mass column 2 So, crustaltopography is controlled by density and thickness variations of the crust.

(Angevine et al., 1990)

Air (p=0 g/cc): Z=0.9 km Water (p=1 g/cc): Z 1.3 km (p 1 Z= Sediment (p=2.3 g/cc): Z=3 km

(Angevineet al., 1990)

Review cont’d

Isostatic models (local compensation): density vs thickness variations. Geohistory modeling assumes Airy isostatic balance.

Review cont’d

Regional isostacyand flexural loads (subsidence)

Review: subsidence mechanisms

2) Intro to Geohistory Analysis What is a geohistory diagram? A graphical representation of the vertical movement of astratigraphic horizon in a sedimentary basin as an indicator of subsidence and uplift history in the basin since the horizon was deposited ( g y p (Angevine et al., 1990). , ) 3) Early attempts

Van Hintediagram

Van Hinte, 1978

Van Hinte diagram

Example: generating a geohistory diagram A) : What data do you need to collect?

• Formation Information: thickness, lith l thi k lithology, age...