GEOL 462: Principles of Sedimentology and Stratigraphy

Lecture 10: Tuesday, February 17, 2009

Topics
1. Characteristic processes of the marine environments
2. Characteristic sedimentary features of the marine environments
3. The delta environment: overview
4. Different types of deltas: sedimentary sequences
5. Ancient delta environments: identifying features
6. Estuaries: processes, microenvironments, and depositional sequences

Characteristic processes of the marine environments
In a marine environment, several processes may produce identical features:
1) Tides;
2) Waves and storms;
3) Thermo-haline currents;
4) Marine ecosystem: body fossils and trace fossils;
5) Chemical precipitation: glauconites, phosphorites, and organic-rich sediments

Characteristic sedimentary features of the marine environments
Characteristic sedimentary structures formed by tidal activities
a. Tidal cycles: diurnal cycles, neap-spring tidal cycles, annual tidal cycles
b. Bipolar (Herring-bone) cross stratification
c. Mud drapes
d. Reactivation surfaces
e. Tidal bundles
Characteristic sedimentary structures formed by Waves and Storms
a. Symmetric wave ripples and ripple cross laminations
b. Hummocky and swaley cross stratification
Characteristic sedimentary structures formed by Thermo-haline currents
a. influence waves, storms, and nutrient supply
b. forming reefs or bioherms at upwelling zones
c. relatively deep-water current ripples, etc.
Characteristic sedimentary features of marine ecosystems
a. Marine body fossils, reefs, carbonate complexes;
b. Trace fossils: borings, burrows, surface traces and trails; see Fig. 11.4, p. 147
Division of the marine environments
a. Supratidal (Supralittoral): above sea level;
b. Intertidal (Littoral): between low and high tides
c. Subtidal (Sublittoral)--Neritic zone: below low tides but above sotrm wave base (~200 m);
d. Bathyal zone: from 200 m to about 4000 m water depth;
e. Abyssal zone: larger than 4000 m water depth;
In a wave-dominated marine system, a, b, and c is commonly divided into the following environments: a. Foreshore: between low and high mean water level;
b. Upper shoreface: between mean low water level and Fair weather wave base;
c. Lower shoreface: between Fair weather wave base and Storm wave base;
d. Offshore: below storm wave base;

The delta environment: overview
1. Different types of deltas:
a. The Nile delta: the 'original' delta
b. The Mississippi delta: a river-dominated delta
c. The Rhone delta: a wave-dominated delta
d. The Ganges delta: a tide-dominated delta
2. Delta microenvironments:
a. Delta top environments: delta plain, delta distributary channels, swamps, crevasse splay, interdistributary bays;
b. Delta front environments: subaqueous mouth bars, delta slope, and prodelta
3. Progradation of deltas:
Formation of coarsing-upward sequences, the most identical sequence for delta deposits.
4. Controlling factors of delta environments:
a. Hinterland characteristics (tectonic and climate): slope, relief, discharge--determine grain size and sediment supply
b. Basin characteristics (tectonic, eustatic, and climate): subsidence -- water depth; waves and tides--reworking of sediments
c. Water level changes: controlling the progradation and retrogradation cycles
5. Processes and sedimentary features
Physical processes:
a. Currents (river channels): fluvial sequences we have talked before;
b. flooding in delta plains: ripples, mudcracks, coarser beds within fine-grained sediments (silt and mud)...
c. Wave and storms: Hummocky and Swaley cross stratification, symetric ripples and cross laminations, low angle cross stratification...
d. Tides: tidal channel deposit (trough cross stratification, herringbone cross stratification, parallel bedding, ...), tidal bundles,
tidal cycles, mud drapes, reactivitation surfaces...
e. slumping in delta front: slump deformation, graded bedding
f. gravity flows (prodelta): turbidites, graded beddings
Chemical processes:
a. Evaporation: carbonate cements or nodules, gypsum, etc.
b. carbonate precipitation: marine carbonate beds containing brackish or marine fossils
c. soil formation in delta plain: root casts, plant fossils, arbonate nodules, ...
Biological processes:
a. Plants (coal formation in delta plain): plants, roots, carbonate nodules, coal beds, etc....
b. brackish fossils;
c. marine body and trace fossils……More importantly, the mixing of marine and terrestrial organisms

Different types of deltas: sedimentary sequences
Sequences of river-dominated delta (Fig. 12.9, p. 154): the upper part is most fluvial
Sequences of wave-dominated delta (Fig. 12.12, p. 156): the upper part is mostly beach and delta plain
Sequences of tide-dominated delta (Fig. 12.14, p. 158): The upper part contains tidal channels
All these sequences show a coarsing-upward trend
The grain sizes and sequences of deltas may be controlled by tectonics, climates, and sea level changes
The thickness of deltaitic cycles may be controlled by the bathymetry of the basin

Ancient delta environments: identifying features
1. The association of marine and continental facies in the vertical sequences;
2. Sedimentary structures of wave, tidal, and river current;
3. Commonly coarsing-upward sequences;
4. Brackish and marine fauna, and land flora together

Estuaries: processes, microenvironments, and depositional sequences
Estuaries: A semi-enclosed coastal water body where there is a mixture of river and sea water and where there is a mixture of fluvial and marine processes;
AN estuary can be considered as a not well-developed delta system;
Tide-dominated Estuary: Tidal channels and tidal flats may be dominant;
Wave-dominated Estuary: Sand barrier, tidal delta, and lagoon;
Identifying features:
1. Mixing of river and marine processes such as waves and tides
2. Mixing of marine fauna and land plants
3. Mostly tide-dominated, containing tidal channels
4. Similar to delta environments but not have identical coarsing-upward sequences; vertical sequences are aggradational.