GEOL 462: Principles of Sedimentology and Stratigraphy

Lecture 11: Thursday, February 19, 2009

Topics
1. Overview of the costal and shallow marine environments
2. Terrigenous clastic coastal environments
3. Carbonate-rich coastal environments
4. Terrigenous clastic shallow seas
5. Carbonate platforms
6. Summary: identifying features

Overview of the costal and shallow marine environments
Characteristic features:
1. Tidal channels;
2. Large-scale sand dunes (wave/tides) forming dune ridges;
3. Marine organisms;
4. Bioturbation

Terrigenous clastic coastal environments
Wave-dominated coastal environments (No obvious dune-ridges as barrier, no back-barrier lagoon):
1. Foreshore (above low mean water level);
2. Upper shoreface (between low mean water level and fair weather wave base);
3. Lower shoreface (between fair weather wave base and storm wave base);
4. Offshore (below storm wave base);
Wave and tide-influenced coastal environments
1. Develope large dunes and dune ridges as a barrier;
2. back-barrier lagoon;
3. Tidal flat in the continental side of the lagoon
Important features of coastal environments:
1. Trace fossils;
2. Low-angle cross stratification;
3. Marine fossils (fragments) and brackish fossils (lagoon);
4. Herringbone cross stratification (tidal channels);
5. Vertically, change to either land (root casts, mud flat, etc.) or shallow marine facies

Carbonate-rich coastal environments
1. Physical processes are similar to terrigenous coastals;
2. Chemical processes: different, e.g., evaporation;
3. Biological processes: different, e.g., Stromatolites and Microbials;
Warm and wet climate:
1. Carbonate ooid shoals;
2. Mixed carbonate-siliciclastic lagoon;
3. Tidal flat with stromatolites and microbilites;
Arid climate:
1. Carbonate ooid shoals as barrier;
2. Back-barrier sabkha;
3. Gypsum and anhydrites are the two main evaporites in sabkha, sometimes salt;

Terrigenous clastic shallow seas
General features:
1. Storms, tides, waves, ocean currents;
2. bioturbation;
3. carbonate factory;
4. sand ridges, ribbons, and sand waves
Environments:
1. Upper shoreface: coarser grains (sand), low angle cross stratification; wave ripples, cross beds; burrows;
2. Lower shoreface: humoccky cross stratification, interbedded sandstone and mudstone; wave ripple cross lamination;
3. Offshore: siltstone and mudstone, parallel lamination, bioturbation;

Carbonate platforms
1. Carbonate ramps: low gradient, no steep shelf-slope transition;
2. Rimmed shelf: reefs are marginal barrier separating shelf lagoon from open ocean seawater;
3. Epeirie platform: very stable and flat platform extending hundrends to thousands of kilometers;
4. Isolated platform: Regions of shallow water carbonate sedimentation surrounded by deep water environments;
5. Drowned platform: Sudden sea level rise covering the shallow water carbonate platform with deeper water environments
Carbonate ramps:
No clear Reefs; similar to terrigenous clastic seas; easily to get interbedded siliciclastic-carbonate sediments
Rimmed carbonate shelf:
1. Biological framework – Reefs;
2. Reefs can be a complex-form (by organisms and clasts, forming a beach-like barrier; especially for the Precambrian);
3. Slumping; Restricted back-reef lagoon; Abundance of bioclasts;

Summary: identifying features of shallow-marine deposits
1. Extensive and diverse carbonates
2. Body and trace fossils (especially benthic and reefs)
3. Low-angle cross bedding, herringbone cross bedding, HCS
4. Extensive sand sheets (more laterally continuous)
5. Vertical sequences: alternating with deeper marine, fine-grained deposits (e.g., pelagic silt and mud)