Reading: Boggs Chapter 9.1-9.3
Today we introduced the idea of studying modern systems plus using lithofacies
analysis (process and product) to interpret ancient depositional environments
in the stratigraphic record. Then we discussed three kinds of continental environments:
(1) alluvial fans; (2) braided streams; and (3) meandering streams. I gave a
series of handouts that help to illustrate many of the key concepts, and a short
list which is reproduced below.
Below is a summary of some of the main features and distinguishing characteristics of alluvial fans, braided streams, and meandering streams. Alluvial fans are best thought of as unique and different than braided streams, although many published papers have blurred this distinction, producing some debate over this question. Be sure to look at the handout I gave you along with this list.
1. Alluvial Fans:
A. Setting: Base of mountain front where confined stream valley emerges onto
unconfined valley floor or basin plain. Geographically restricted, close to
steep mountain front.
B. Depositional Processes: Debris flows, sheet floods (upper flow regime), rock
avalanches (may be absent or minor), and reworking of fan by rainfall and surface
run-off.
C. Common Deposits: Poorly sorted conglomerate (prod. by debris flows); planar
bedded, moderately sorted cgl and sandstone, often in couplets (sheet flood
deposition); possibly also angular breccia with boulder- to house-size clasts
(prod. by rock avalanches). No X-bedding.
2. Braided Streams:
A. Setting: Can form long distances from steep topography in relatively low-gradient
plains. Transport large load of coarse sand and gravel (bedload), clay and silt
in susp. load. Rapid deposition of bedload (fines carried away downstream),
creates unstable, shifting bars and bedforms, easily erodible banks. Multiple
active channels in characteristic braided geometry.
B. Depositional Processes: Mostly by lateral accretion = deposition from laterally
migrating bars and bedforms (mostly lower flow regime), relatively minor vertical
accretion of silt and clay.
C. Common Deposits: Mainly conglomerate and/or sandstone (one or the other may
dominate), abundant trough and planar cross-bedding. Often form fining-up intervals
(lateral migration of bars and channels), with only minor thin overbank mudstone
(can find paleosols in these).
3. Meandering Streams:
A. Setting: Low-gradient, low-energy systems, may form very far from
steep topography, occupy broad flat flood plain (meander belt). Transport sand
in bedload, silt and clay in suspended load. Creates stable cohesive banks,
single active channel, meandering geometry.
B. Depositional Processes: Two modes: (1) lateral accretion of point bars (mostly
sand) in channel; and (2) vertical accretion of fines (silt and clay) in overbank
setting. Much greater vertical accretion than in braided streams. Overbank environment
collects alot of fines.
C. Common Deposits: Sandstone and mudstone, often in subequal amounts. Fining-up
intervals record (1) lateral migration of point bar (lateral accretion), followed
by (2) overbank deposition (vertical accretion). Mud cracks and paleosols (root
traces, oxides, ped structures) are common in overbank fines.
Reading: Boggs Chapter 10 (parts, as clarified below)
Marginal-marine depositional environments occupy coastal areas, and include:
Boggs Figure 10.1 sums these up pretty well. Important controls on type of depositional system include: (1) sediment supply (high or low); (2) strength of currents in receiving basin relative to rate of sediment input, determines ability of basinal currents to rework and remold incoming sediment; (3) type of currents, if any, in receiving basin (tidal, wave and current); (4) long-term behavior of shoreline (transgressive or regressive, that is, relative sea level rising or falling). Note that #4 varies around the world because of variations in tectonic regime (coastal region rising or subsiding).
Today's discussion focused on deltas. Deltas are areas where the coastline is deflected outward into the ocean (or any standing body of water) due to high sediment input. Deltas are very diverse and complex systems that contain various types of sub-environments characterized by different depositional processes and deposits. The main realms of delta sedimentation are the delta plain, delta front, and pro-delta (deeper water, offshore).
Main Types of Deltas:
(1) Fine-Grained Deltas: (a) fluvial-dominated; (b) tide-dominated; (c) wave-dominated. Unique characteristics of each type, and the processes that create those characteristics.
(2) Coarse-Grained Deltas: are formed by rapid supply of coarse sediment to a narrow coastal region with steep topographic gradients, typical of tectonically active settings. Also mentioned braid deltas.
Figures 6.3 and 6.4 (in handout) and Figure 10.4 (text) summarize main delta types, overall depositional geometries, and some modern examples.
What Deltas Do: (1) They Prograde through time due to high sediment input, and this produces a coarsening- and shallowing-up section 10's of meters thick (e.g. Figure 10 in handout); and (2) Delta-lobe Abandonment, which occurs when the main channel avulses and switches to a new location, ending input of sediment to the area of former progradation and causing it to gradually subside.
The Mississippi River Delta is a classic fluvial-dominated delta. Figure 11 (in handout) shows shifting delta lobes that have formed over the past 5,000 years. Note the Achafalaya River problem, which we discussed in some detail.