SEDIMENTOLOGYSTRATIGRPHY GEOL 103A
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Date Created: 10/05/15
Geology 103 Lecture 14 Deltaic Sedimentation Reading Assignment Boggs 4th edition pp 289 306 I Deltaic systems A Introduction and importance Today s topic deltas deltaic sedimentation Deltas are part of a marginal marine spectrum that includes deltas beach and barrier island systems estuaries and lagoons Deltas occur where a river or stream enters a standing body of water Usually occur on prograding coastlines Deltas can occur in lakes too but we will focus on marine marginal environments Deltas form on the continental shelf water depths less than 120 m Deltas are the interface between continental and marine sedimentation Deltas are a major repository for continental sediment 10000 s of feet of sediment in the Gulf of Mississippi crust is subsiding under the weight Deltas are very common in modem environments Due to high stand of sea level May explain corresponding lack of deep sea fansturbidity deposits Usually occur on passive margins heavy suspended sediment load Deltas are very important economically Major petroleum reservoirs are found in deltas oil natural gas Some coal uranium Gulf Coast of US offshore oil drilling is in deltaic deposits from the Mississippi River Exploration is difficult producing intervals are not continuous shoestring sands B Deltaic facies and features Facies present will vary by delta type Here are some common deltaic facies See Figure 912 from Boggs 4th edition p 301 1 Prodelta delta slope Beds are called quotBottomset bedsquot gently inclined fmegrained clays Open marine fauna Water depth of 1039s to 100 m 2 Delta front Beds are called quotForeset bedsquot sand steeper 100 25 Slump features slumps mud ows GROWTH FAULTS Slump features take a variety of forms Water depth of 10 m or less 3 Lower Delta plain Shallow water intertidal zone Still subaqueous transitional t0 subaerial May be well sorted wave and tide action OR poorly sorted river processes dominate 4 Upper delta plain This is the subaerial delta Lies largely above sea water level The largest part of the delta Beds are called quotTopset bedsquot sand and gravel stream deposits Little effect by marine or deeper water sedimentation Have stream features Channels marshes swamps oodplain lakes may even include coal in ancient deposits Channels are called distributary channels C Classi cation of deltas Deltas formation is a balance between climate water and slope of the shelf area of drainage basin sediment input winds sediment supply wave energy tidal energy There are 3 types of deltas Galloway s classification system Fluvialdominated wavedominated and tidedominated Within each type grain size mudsandgravel content further distinguishes deltas See Figure 93 from Boggs 4th edition p 292 1 Fluvial dominated deltas River is sedimentladen River overpowers the effects of tides waves A jet plume of sedimentladen water ows into the basin Sediment size results in different delta morphologies homopycnal hyperpycnal hypopycnal ows See Figure 95 from Boggs 4th edition p 294 2 a Homopycnal ow River water and basin water have equal density Mixing is rapid Sediment is deposited quickly at a high angle 10200 A Gilberttype delta forms Named after GK Gilbert described a delta in 1885 See Figure 94 from Boggs 4th edition p 293 See Figure 914 from Boggs 4th edition p 303 Delta has topset foreset and bottomset beds topset beds sandy and gravelly prograde over top foreset beds sandy steep angle bottomset beds ner grained basin plain deposits deeper water These deltas are relatively rare b Hyperpycnal ow River water has higher density than basin water Common during oods Deposits have a gentle slope Turbidites may form Friction is a big factor See Figure 95 C from Boggs 4th edition p 294 c Hypopycnal ow Fine sediment is transported by hypopycnal buoyant ow See Figure 95 A from Boggs 4th edition p 294 Produces a large delta front Shape is elongate to lobate hypopycnal ow quotpunchesquot out into the ocean water Low gradient lt l0 Coarse sediment bedload is dropped quickly forms a bar crest Natural levees form subaerial and subaqueous Flocculation is a factor fine sediment clumps together when it interacts with sea water This is the most common type of uvialdominated delta Often called a quotbirdsfootquot delta 3 See Figure 96 from Boggs 4th edition p 295 Ex Mississippi River Delta Interdistributary muds encase delta lobes oil traps Bar nger sands a common oil term develop at the mouth of distributaries 2 Tide dominated deltas Form when tidal currents are stronger than river ow Usually occur where tidal range is high See Figure 97 From Boggs 4th edition p 297 Ex Ganges Bramaputra delta Currents are bidirectional tides go in and out Creates a network of linear longitudinal bars at channel mouth tidal ridges Occurs with high sand input more bedload Fluvial facies delta plain may look braided Bars are oriented parallel to direction of tidal ow See Figure 916 from Boggs 4th edition p 305 An example from the rock record 3 Wave dominated delta River ow is decelerated rapidly by strong waves A high energy environment Sediment is dumped in a narrow mound near river mouth doesn t build out arcuate delta form See Figure 98 from Boggs 4th edition p 298 Delta sediment is moved downshore by longshore drift Ex Brazos Texas Sao Francisco Brazil Bar forms are parallel to shore perpendicular to river ow Forms a series of parallel beach ridges as delta builds outward Interdistributary muds are rare See Figure 915 from Boggs 4th edition p 304 An example from the rock record 4 4 Mixed process deltas Many deltas are not limited to one type of sedimentation a miX of processes occurs EX Copper River Alaska tide and wave processes are both active See Figure 99 from Boggs 4th edition p 299 D Fan deltas A combination of alluvial fan and deltaic processes Occurs along an active margin where sediment is delivered by an alluvial fan system Sediment enters a nearby standing body of water deltaic deposits form Relatively rare need high relief close source See Figure 910 911 from Boggs 4 h edition p 300 A variety of alluvial fan and deltaic types may coexist wave uvial or tide dominated with humid or arid alluvial fans E Recognizing a deltaic sequence Several features or relationships will help identify a deltaic sequence in the rock record 1 Coarsens upward See Figure 917 from Boggs 4th edition p 306 As basin lls uvial facies prograde outward Note from the perspective of the ocean system Boggs This is also called a regressive sequence Could also be called progradational from the perspective of the delta Note quotclinoformquot beds from progradation of prodelta and delta front Coarsening upward may repeat for several cycles cycles are common See Figure 913 from Boggs 4th edition p 302 2 May have a ne cap The uvial subaerial facies swampy finegrained sediment and even coal 3 Geometry Deltas are often delta shaped triangular in map view Are wedgeshaped in cross section seismic etc Named by Herodotus in 490 BC to describe the Nile delta Marine deltas range from 1000 s ofkm2 to 125000 km2 4 Facies relationships Vertical changes Vertical gradation from marine to transitional to terrestrial Lateral changes High variability eX channel to overbank on delta plain 5 Sedimentary structures and fossils Aren39t diagnostic by themselves Ripples marks cross beds slump structures bioturbation Fossils are the one chance may grade from marine to nonmarine brackish Fossils are euryhaline tolerant of a wide variety of salinity Fossils include gastropods ostracods bivalves Summary Delta morphology varies depending on waves tides river input Shape facies relations possibly fossils help to identify a deltaic sequence