中国地质大学（武汉）：《地史学》课程电子教案（PPT教学课件）第二章 沉积古地理学 Sedimentary paleogeography 2.3 sedimentary Paleogeography -4

Chapter 2 Sedimentary paleogeography- 4 2.1 concepts and law 2.2 sedimentary-facies indicators 2.3 sedimentary environments

Chapter 2 Sedimentary paleogeography-4 2.1 concepts and law 2.2 sedimentary-facies indicators 2.3 sedimentary environments

How to reconstruct paleoenvironments Fossils Lithofacies Geochemistry Taxonomic uniformitarianism: Actualism: comparison Interpretation of chemical comparison with nearest living with products of signatures: comparison with relatives and communities present-day processes present-day signatures and known controls Determination of physical Determination of physical Determination of chemical and biological aspects of aspects of environment spects of environment environment Palaeoenvironmental model Figure 16.1 The palaeoenvironmental model: determination of palaeoenvironments fron fossils, lithofacies and geochemistry

How to reconstruct paleoenvironments

Fluvial Braided stream Meandering stream Sand dune Desert Alluvial f: Continental Playa lakes environments Ice deposition moraines) Glacial Fluvial deposition Outwash) Glacial lakes Stream dominated Delta Wave dominated Tide dominated Transitional environments Beach Beach Barrier island Sand dune Lag detrital Continental deposition shelf deposition Carbonate Marine platform environments Continental Submarine fansⅵ slope and rise (turbidities) Deep-ocean Oozes basi Pelagic clay Evaporite nvironme

Classifications of Sed. Environments - I

sediments

Fluvial sediments

Active only at Braided and meandering stream flood stage heart bar In-channe a Mud deposited in abandoned channels (b) Point bar N Point bar belt Channel deposits Floodplain deposits (d) FIGURE 4.13 (a)A braided stream near Chester, California. (b)Facies model for a braided stream system. Braided streams deposit mostly sand and gravel with subordinate mud.(c)A meandering stream, otter Creek in Yellowstone National Park, Wyoming. Point bars are well developed on the inside banks of each meander. (d) Facies model for a meandering stream system. Meandering stream deposits are shoestring sands sur- rounded by mudstones

(b) (a) Point bar (d) Point bar heart bar (C) Braided and meandering stream

Facies model of flood basin meandering stream former channel filled with sediments point bar deposits natural levee ,∷ Crevasse clay ::,gravelly sand natural levee (fining upwards sequence) sandy clay/fine sand gravel (channel lag) crevasse-deposits sand with some lateral accretion surfaces older deposits gravel Block diagram of a meandering river

Facies model of meandering stream

Secondary flow refers to small component of flow velocity directed perpendicular to flow A Top diagram shows near-surface near bed velocity vectors A Near-surface secondary flow 4 Near-bed secondary flow A This diagram shows only the secondary components of velocity vectors

Velocity vectors

Vertical variations in point bar deposits Disrupted by burrow/s, = rooting, dessication cracks Small-scale cross-stratificatlon Large-scale cross-stratification Mud Sand

Fining upward Vertical variations in point bar deposits

Lateral variations in point bar deposits Cross-section perpendicular to flow: BIotcTbateu, dessicalIcnFsrackec mud Seals:alc crsc sfialiRed sand Chia'IIEI il dEposi.s ) Gare- scale cross stialifitsa2so-s Note basal erosion surface and lag deposit. Also note bedding geometry in bar deposit: lateral accretion beds dipping tonight, formed from lateral accretion on point bar ithin lateral accretion beds, paleocurrents are directed out of the outcrop, if the outcrop is oriented perp endicular to flow Channel fill deposits consist of fining upward sediments in beds conformable with channel, formed by vertical accretion

Lateral variations in point bar deposits

Braided alluvial sediments: basic vertical sequence with binary 2 m structures of coarse-and fine- grain units but the fine is less than the coarse in thickness Abandoned channel Sand deposition Mud deposition sequence In active in abandoned braided channels channel Active channel sequence 2.3 Physiography and sediments of a braided alluvial channel system. After Selley( 1978)

Braided alluvial sediments: basic vertical sequence with binary structures of coarse- and fine￾grain units, but the fine is less than the coarse in thickness