7. Controls on basin stratigraphy Driving mechanisms for basin stratigraphy Tectonic mechanism: (a) flexure under Steer’s-head applied loads (rift basin and foreland geometry basin); (b) fault array evolution; (c) in- Elastic plane stress Eustatic mechanism: (a) change in volume of the ocean basin; (b) changes of ice volume on polar regions Climate change: Influence on sediment discharge to basins Viscoelastic 7.1 Tectonic mechanisms: flexure under applied loads 7.1.1 Effects of flexure on stratigraphy in basins due to stretching 1 Basin Analysis Dept. Earth Sci., Nat. Central U. Prepared by Dr. Andrew T. Lin The mechanisms of subsidence in stretched basins comprise: (i) fault-controlled initial subsidence caused by mechanical stretching of the upper brittle layer of the lithosphere, (ii) a thermal subsidence caused by the cooling and contraction of the upwelled asthenosphere, and (iii) sediment and water loading. In rift basins: •During the stage of rifting: fault-controlled Airy-type subsidence. •During post-rift stage: flexural-controlled subsidence. •The increase of Te with increasing plate thermal age leads to stratigraphic onlap pattern for post-rift strata. 2 Basin Analysis Dept. Earth Sci., Nat. Central U. Prepared by Dr. Andrew T. Lin • Initial strong onlap onto the basement at the transition from fault-controlled Airy-type subsidence to flexural-controlled subsidence. • Lateral heatflow causes thermal uplift on the coastal plain, abruptly terminating onlap. • By about 16 Myr after rifting, flexural subsidence outstrips thermal uplift and the sediments again progressively onlap basement. 3 Basin Analysis Dept. Earth Sci., Nat. Central U. Prepared by Dr. Andrew T. Lin 7.1.2 Role of flexure in generating foreland basin stratigraphy Transition from passive margin to foreland basins 1. Early stage: thermal age of passive continental margin is an important control on foreland basin development. 2. Later stage: the thickness of the overthrust load is more important. Fig. 4.31 4 Basin Analysis Dept. Earth Sci., Nat. Central U. Prepared by Dr. Andrew T. Lin Wedge-shaped basin geometry and progressive stratigraphic onlap of a foreland basin 5 Basin Analysis Dept. Earth Sci., Nat. Central U. Prepared by Dr. Andrew T. Lin 6 Basin Analysis Dept. Earth Sci., Nat. Central U. Prepared by Dr. Andrew T. Lin 7.1.3 The flexural forebulge unconformity 7 Basin Analysis Dept. Earth Sci., Nat. Central U. Prepared by Dr. Andrew T. Lin Forebulge unconformity in eastern Switzerland Calculated erosion Inherited deeper bathymetry: Bathymetry before orogenic loading no erosion Observed erosion Hiatus of the unconformity 8 Basin Analysis Dept. Earth Sci., Nat. Central U. Prepared by Dr. Andrew T. Lin 7.1.4 Foreland basin isopachs and pinch-outs A A’ Progressive eastward shift of depocenters during Sevier orogeny A A’ 9 Basin Analysis Dept. Earth Sci., Nat. Central U. Prepared by Dr. Andrew T. Lin 10 Basin Analysis Dept. Earth Sci., Nat. Central U. Prepared by Dr. Andrew T. Lin 11 Basin Analysis Dept. Earth Sci., Nat. Central U. Prepared by Dr. Andrew T. Lin 7.2 Tectonic mechanisms: fault array evolution 12 Basin Analysis Dept. Earth Sci., Nat. Central U. Prepared by Dr. Andrew T. Lin 13 Basin Analysis Dept. Earth Sci., Nat. Central U. Prepared by Dr. Andrew T. Lin 14 Basin Analysis Dept. Earth Sci., Nat. Central U. Prepared by Dr. Andrew T. Lin 15 Basin Analysis Dept. Earth Sci., Nat. Central U. Prepared by Dr. Andrew T. Lin 16 Basin Analysis Dept. Earth Sci., Nat. Central U. Prepared by Dr. Andrew T. Lin 17 Basin Analysis Dept. Earth Sci., Nat. Central U. Prepared by Dr. Andrew T. Lin 7.3 Changes of in-plane stress In-plane stresses acting on a deflected plate may enhance or reduce the curvature of the deflection. Compressive in-plane stress causes basin margin to uplift and basin center to subside. Tensile in-plane stress caused basin margin to subside and basin center to uplift. In-plane stress may have buckled layered lithosphere and produced long wavelength lithospheric folds. 18 Basin Analysis Dept. Earth Sci., Nat. Central U. Prepared by Dr. Andrew T. Lin 7.4 Eustatic mechanisms Eustasy, relative sea-level, water depth Relative sea-level: the distance between a local datum (e.g. top of the basement of a basin) and sea-surface. Relative sea- level change is influenced by: (1) eustasy, (2) basin uplift/subsidence. Water depth: the distance between the sea-bed and the sea-surface or water level. Eustatic sea-level (or eustasy): This is global sea-level and is a measure of the distance between a fixed datum, usually taken as the centre of the Earth, and the sea-surface. Figure 3.6 Cartoon showing the relationship between relative sea- level, water depth, eustatic sea-level, tectonics (uplift and subsidence), and accumulated sediment. Note that relative sea-level incorporates subsidence and/or uplift by referring to the position of sea-level with respect to the position of a datum at or near the sea-floor (e.g. basement rocks, top of previous sediment package) as well as eustasy. Eustasy (i.e. global sea-level) is the variation of sea-level with reference to a fixed datum, for example the centre of the Earth. Coe et al. (2003) 19 Basin Analysis Dept. Earth Sci., Nat. Central U. Prepared by Dr. Andrew T. Lin Five possible causes that may cause global sea-level changes 1. Continuing differentiation of lithospheric material as a result of plate tectonic processes. (not important) 2. Changes in the volumetric capacity of the ocean basins caused by sediment influx or removal. (not important) 3. Changes in the volumetric capacity of the ocean basins caused by volume changes in the mid-ocean ridge system. (important for first-order eustasy) 4. Thermal expansion and contraction of the oceanic water reservoirs. (not important, 1°K increases, 0.45 m rise in sea level) 5. Changes of available water by abstraction in and melting of polar ice caps and glaciers (glacial eustatic). 20 Basin Analysis Dept. Earth Sci., Nat. Central U. Prepared by Dr. Andrew T. Lin Considering isotasy in eustatic sea level changes ⎛ ρ − ρ ⎞ ⎜ m w ⎟ If ρ = 3.3 g/cm3; ρ = 1.0 g/cm3 ΔSL = S⎜ ⎟ m w The isostatic subsidence of the ⎝ ρw ⎠ ocean floor is approximately 0.4 of the sea level change ( Δ SL ). Or the seal level change is 0.7 of the increase in the water depth of the ocean (h2-h1). 21 Basin Analysis Dept. Earth Sci., Nat. Central U. Prepared by Dr. Andrew T. Lin However, sediment in the ocean is removed by tectonic accretion and subduction at active margins and continued spreading creates new ocean floor. It is likely that the balance between influx and removal of sediment, when averaged over long periods of time, is insufficient to cause rates of sea-level change of more than ~ 1 mm per 1000 yr. 22 Basin Analysis Dept. Earth Sci., Nat. Central U. Prepared by Dr. Andrew T. Lin Volume of present world ridge system is about 10% of the volume of the ocean water. Change in spreading rate and change in the length of ridge systems influence the ridge volume. 23 Basin Analysis Dept. Earth Sci., Nat. Central U. Prepared by Dr. Andrew T. Lin First-order (225- 300 Myr) eustatic curve of Vail: due to ridge volume fluctuation. ~ <100 to 150 m above present Maximum sea level: latest Cretaceous (Maastrichtian) 24 Basin Analysis Dept. Earth Sci., Nat. Central U. Prepared by Dr. Andrew T. Lin Melting of Present Ice Cap Total melting of Antarctic land ice would result in an increase in water depth, ranging from 60~75 m. Melting Greenland ice cap > 5 m rise, Taking into account the isostatic effect, if all the land-locked ice melted, it would cause a 50 m rise in sea level (ΔSL). In geological term, rate of melting ice caps is a rapid process (~10 mmyr-1) Melting of Pleistocene Ice Cap May cause about 150 m sea level rise. 25 Basin Analysis Dept. Earth Sci., Nat. Central U. Prepared by Dr. Andrew T. Lin For the last 120,000 yrs, 8 sea-level cycles. Magnitudes: 20~180 m, periods: 100,000 yrs (primary), 40,000 yrs, and 20,000 yrs. 26 Basin Analysis Dept. Earth Sci., Nat. Central U. Prepared by Dr. Andrew T. Lin 現在溫度 地質 百萬 年代 年前 全球溫度隨地質時間的變化第四紀 上新世 新生代 北半球開始發育冰川 中新世 大片冰帽覆蓋南極洲 第三紀 漸新世 南極洲冰帽開使成形 始新世 古新世 中生代白堊紀 南北極 沒有冰帽 侏羅紀 三疊紀 古生代二疊紀 二疊紀~石炭紀冰河期 石炭紀 泥盆紀 27 志留紀 Basin Analysis Dept. Earth Sci., Nat. Central U. Prepared by Dr. Andrew T. Lin 日照量改變:米蘭科維奇循環 (Milankovitch cycles) (a) 地球公轉軌道的偏心率 (eccentricity)改變造成41萬年與10萬6千年的氣候循環 (b) 地球自轉軸傾角 (obliquity)的改變造成4萬1千年的氣候循環 (c) 地球自轉軸繞著垂直軸運轉一圈的週期(即歲差或進動, precession) ， 28 造成1萬9千年至2萬3千年的氣候循環 Basin Analysis Dept. Earth Sci., Nat. Central U. Prepared by Dr. Andrew T. Lin 氧同位素,冰帽與海水面變化 冰河期 間冰期 海水中的O18/O16比值增加 海水中的O18/O16比值減小 29 Basin Analysis Dept. Earth Sci., Nat. Central U. Prepared by Dr. Andrew T. Lin 改變地球日照量與古代海水溫度測量 地球公轉與自轉的 軌道與參數變化 改變地球 日照量 造成氣候 若地球表面平均溫度低， 改變 南北兩極形成冰川； 平均溫度較高， 南北極沒有冰川 測量有孔蟲殼體 的氧同位素得知 以前海水的溫度 30 Basin Analysis Dept. Earth Sci., Nat. Central U. Prepared by Dr. Andrew T. Lin 7.5 A primer on process stratigraphy Process stratigraphy (as defined in Allen and Allen (2005), p.268) is the science of the recognition and interpretation of the genetic structures of stratigraphy. The fundamental aim of process stratigraphy is to understand the driving mechanisms for the range of stratigraphic architectures found in sedimentary basins. The key concept of process stratigraphy is the generation/destroy of accommodation space and the amount of sediments supplied. The stratigraphy in a sedimentary basin is the result of the interplay of the generation of space or accommodation and the influx of sediments (sediment supply).