Continental Environments Chapter 9 Continental Environments Deserts Alluvial fans Rivers (fluvial) and floodplains Lakes (lacustrine) Glacial
Desert Biome http://www.blueplanetbiomes.org/climate.htm low-latitude deserts approximately between 18 to 28, edge of the equatorial subtropical high pressure belt and trade winds Descending air masses therefore rarely hold much moisture for precipitation covers 12 % of the Earth's land surface. Temperature Range: 16 C Annual Precipitation: 0.25 cm (0.1 in). All months less than 0.25 cm (0.1 in). World Climate Zones http://www.blueplanetbiomes.org/climate.htm Eolian Cross Sets
Eolian (Desert) Deposits Most between 20-30 latitude Coarsest clasts remain as deflation lag (desert pavement) Sands are well sorted & rounded; surface pitting and frosting Large-scale planar-tabular or trough- and wedge-shaped cross beds Diagnostic Features TECTONIC SETTING Mountain rain shadows, associated with desert deposits GEOMETRY Dune fields can cover 100's km2; tabular bodies up to 35 m thick SEQUENCE Large-scale x-beds, foresets dip 25-30; deflation gravel or pebble lags SEDIMENTOLOGY Well-sorted, well rounded quartz-rich sand; Large scale x-beds comprised of smaller scale low-amplitude wind ripples Alluvial fans common in modern and ancient deposits; most common in semi-arid region where pronounced gradient or abrupt relief exists (highland, hills, mountains, faulting scarps); preservation potential high when building into lakes, rivers, playas, flood plains; May be divided into upper, mid, and lower fans; commonly associated with intermittent current dunes and salt lake (playa) or salt flat (sabkha) deposits. prograding alluvial fans: overall coarsening-upward succession.
Boggs 2001 Alluvial Fans, Baffin Island http://www.physicalgeography.net/fundamentals/10z.html Different types of fans Debris-Flow Fans Braided Fluvial Fans Lowsinuosity/meandering Fluvial Fans Boggs 2001
Sediment Gravity Flows Boggs 2001 Proximal Fan Poorly sorted, large, angular clasts coarse, thick-bedded, poorly stratified alluvial fan deposits. Note lack of channels
Fluvial Environments Deposits of rivers and associated environments Widespread in sedimentary rock record Fluvial Environments Morphological components: Valleys Channels Interfluves (floodplains) Drainage basins Headlands erosional (generally ) Coastal plain depositional (generally ) Channel: Trough through which water runs. Press and Siever, 2001
Valley: Area between tops of slopes on both sides of the river Press and Siever, 2001 Floodplain: Flat area, about level with the top of the channel. Inundated during high discharges. Press and Siever, 2001 Channel Types Four principal types: Meandering winding, single channels Braided multiple channels that change location Anastamosing multiple channels that maintain their locations Straight (rare) Continuous gradation from one type to another
Fluvial Systems Straight (rare) Meandering Braided Anastamosed Boggs 2001 Components Bedforms (dunes, ripples, flat bed) Bars larger depositional units (transverse, linguoid, point) Channels different types Levees Overbank deposits floodplain, crevasse splays, paleosols, etc. Cutbank Point Bar Coarse-grained meandering Sandy bedforms Overbank Fines Walker and Cant 1984
Cutbank Point Bar Press and Siever, 2001 one km Timeslice 3-D seismic data Offshore, S.E. Asia Courtesy Henry Posamentier
Press and Siever, 2001 Modern Mississippi Floodplain Google Earth Walker and Cant 1984 Thalweg: deepest part of the channel, strongest currents
Lateral Accretion Surfaces Cretaceous (?) Colorado Lateral Accretion Surfaces Cretaceous Utah Fluvial Environments Floods: Streams overspill their banks, innundating floodplain Short-term increase in discharge Frequency variable, depends on location
Press and Siever, 2001 Press and Siever, 2001
Fluvial Environments Floodplain deposits Mud (shale) deposited during floods Crevasse splays breaches in levees Sandy, rippled, fan shape Rooted horizons Coal, carbonaceous shale Paleosols Floodplain (channels and paleosols) Cretaceous, New Mexico
Channel sand surrounded by finegrained floodplain deposits Classic finingupward point bar succession Thickness will be proportional to channel depth Walker and Cant 1984 Sandy Braided System Sand flats Downstream accretion Sandy bedforms Walker and Cant 1984
Gravel-bed braided system Gravel bars & bedforms Walker and Cant 1984 Sand Flat (Compound bar) 3-D Dunes (Linguoid Bars) Walker and Cant 1984 Brahmaputra River Bangladesh
braid plain defined by edge of vegetation Stacked?braided fluvial sandstones Cretaceous, New Mexico Braided stream facies broad and shallow channel bars (longitudinal, transverse) are main sites of sediment accumulation gravel sediment dominant vs. sandgrade sediment dominant 2-D dunes most common: planar (tabular) x-beds dominant vertical succession: stacked dunes with planar x-stratification
Geometry of Braided River Deposits Walker and Cant 1984 Anastamosing Overbank Fines Swamps (peat/coal) and small floodplain lakes Sandy bedforms Channel Walker and Cant 1984 Channel Types Morphology of fluvial system controls sandbody geometry Braided: sheet sandstones, high sand/shale ratio Meandering: sandstones lenticular in crosssection, moderate/low sand/shale ratio, finingupward successions Anastomosing: shoestring sandstones in shale, low sand/shale ratio
Lakes Open lakes have an outflow (i.e. a river) sediments dominated by terriginous particles and organic matter Closed lakes have no outflow solutes are not carried out of the basin alkalinity can build up such that carbonates and evaporites can precipitate. A lake can alternate between open and closed with tectonic or hydrologic changes Lakes Deposits in open lakes come mainly from rivers but may also be deposited by wind, ice-rafting, and other processes. Sedimentation in closed lake systems consists of evaporite minerals, carbonate muds, sands, and silts. Lacustrine deposits are often rich in organic matter Distribution of Open Lakes
Distribution of Closed Lakes Transport mechanisms and kinds of siliciclastic sediments in lakes with annual thermal layering Boggs, 2001 Varves-Ellismere Island coarser lighter spring runoff, then finer darker fall dieoff (http://www.geo.umass.edu/climate/tilphtml/lakesedssummary.html
Open Lakes Boggs 2001 Alkaline Lakes Closed lakes subject to limited replenishment from rainfall Buildup of salinity as there is no outlet Evaporites such as Sodium sulfate deposits of Saskatchewan The main uses of sodium sulphate are in the pulp and paper, powder detergent, glass and dyeing industries.
Boggs 2001 Diagnostic Features of LACUSTRINE DEPOSITS TECTONIC SETTING Fault grabens or downwarped basins; associated with other nonmarine settings GEOMETRY Circular or elongate; lenticular in cross section SEQUENCE Coarsening upwards from laminated shale, marl to ripple & cross- bedded sand. Cyclicity possible. SEDIMENTOLOGY Mudstones, shales, sandstones; carbonate, gypsum, halite; dessication features. Nonmarine fossils Glacial Deposits Glacier: Large masses of ice on land that show evidence of being in motion or of once having moved Two types: valley glaciers, continental glaciers
Press and Siever, 2001 Glacial Deposits Glacier formation Low Temperature (high latitude or high altitude) Snow accumulation > ablation Snow compacts, turns to ice Movement Gravity as driving force Basal slip and/or plastic flow Rates: m/yr (km/yr in surges) Press and Siever, 2001
Glacial Deposits Flowing ice erodes rock & regolith Erosional forms: Striations, U-shaped valleys, Fjords, Cirques, Roche moutonée Flowing ice transports sediment Sediment deposited where ice melts Press and Siever, 2001 Press and Siever, 2001
Glacial Deposits Deposition: Drift material of glacial origin Till: deposited directly by ice Outwash: deposited by glacial meltwater Loess: deposited by wind Glaciolacustrine: deposited in glacial lakes Glaciomarine: deposited in the sea by/close to ice Erratic: large boulder in till Press and Siever, 2001 Boggs, 2001
Glacial Deposits Composite that includes eolian, fluvial, lacustrine, and even marine environments Ice-contact deposits characterized by extremely poor sorting and lack of stratification Lakes and marine deposits may have dropstones Glacial Episodes Glacial sediments in N.A.
Summary