The document summarizes clastic marine shelf systems. Clastic shelves are typically pericontinental or epicontinental settings. Sediment transport on shelves is complex, influenced by waves, tides, currents, and density contrasts. Deposits become finer-grained away from the shoreline due to decreasing energy. Storm beds are interspersed with quiet water deposits and indicate transitions from shoreface to offshore facies.

1. Clastic Marine Shelf Systems
(continental shelf/shallow
marine/offshore)

2. Shelf Settings
Clastic shelves in the stratigraphic record mostly represent two settings,
pericontinental (continental passive margin) and epicontinental
(continental interior).

3. Shelf Processes
The interaction between waves (fairweather and storm), tides, and
contrasts in water density creates a complex set of processes
operating to transport sediment on the sea floor.

4. Geostrophic Currents
Geostrophic currents are set
up by pressure gradients
that cause flow along an
isobar that curves away
from the shoreline in
response to the Coriollis
Effect. In deeper water,
they often move as a
nepheloid flow of
suspended sediment
derived through
hypopycnal flow,
hyperpycnal flow, and
storm surges.

5. Photo by W. W. Little
Storm-dominated Shelves
The interaction between waves (fairweather and storm), tides, and
contrasts in water density creates a complex set of processes
operating to transport sediment on the sea floor.

6. Photo by W. W. Little

7. Photo by W. W. Little

8. Wave/storm-dominated shelves ideally exhibit a transition from sands
in the lower shoreface, to alternating sands and muds below
fairweather wave base, to muddy facies below storm wave base.
Shoreline to Offshore Model

9. Wave-base
Wave base is the depth to which waves make contact with the sea floor.
• Fair-weather wave base varies from approximately 5 to 15 m
depth.
•Storm wave base ranges from around 15 to 30 m depth.

10. Clastic marine shelves are flat and slope gently basinward, producing a graded profile
in which deposits become finer-grained and less susceptible to wave activity away from
the shoreline, reflecting an overall decrease in energy. The general pattern can be
complicated by tide and submarine currents. Often, depositional relics remain from
earlier base-level conditions.
Simplified Facies Model

11. Shanmugam 2008
Shanmugam 2000
More Complex Facies Model

12. Shore to Shelf Transition
Shelf sediment transport can be accomplished through quiet-water settling, storm
waves, storm swells, tides, submarine currents, and bioturbation. The relative
importance of these processes varies with distance from the shoreline, water
depth, basin geometry, and sediment supply (type and abundance).
Heckel 1977

13. Photo by W. W. Little
Laminated mud forms by suspension settling and is preserved in
abundance only below storm wave-base.
Laminated Mud

14. Photo by W. W. Little

15. Bioturbated Mud and Sand
Bioturbation is common in marine shelf deposits and can be
expressed in a variety of forms that are indicative of water depth.

16. Hummocky Beds
Thin storm beds are scattered throughout the proximal portion of the marine
shelf, becoming finer-grained basinward. Storm beds typically have erosional
bases. Graded bedding, hummocky cross-bedding, gravel intraclasts, and shell
concenrations are common structures.

17. Hummocky Cross-bedded Sand
Hummocky cross-bedded sand is produced during major storms and
typically forms thin beds scattered through a predominantly muddy
succession.

18. Photo by W. W. Little

19. Photo by W. W. Little

20. Photo by W. W. Little

21. Shell Beds
Shell beds accumulate during large
storms. They typical have a sharp,
erosional base and grade upward
into overlying sand.

22. Photo by W. W. Little
Megaripples
Cross-bedding in storm beds is related to the migration of megaripples.

23. Large scale cross-bedding
Large scale cross-bedding
diagram

24. Photo by W. W. Little
Limestone
Thin limestone beds are often scattered through the succession. These
represent condensed intervals (unconformities) formed during sea-level
highstands.

25. Photo by W. W. Little

26. Large-scale Architecture
Offshore marine deposits are often found in cyclical coarsening-upward
successions with shoreline deposits. The marine deposits typically
consist of monotonous laminated and bioturbated shales with
increasingly common interbedded storm beds in the upward transition to
shorface sediments.

27. Photo by W. W. Little
Offshore to Shoreface Succession

28. Photo by W. W. Little

29. Photo by W. W. Little
Offshore Zone

30. Photo by W. W. Little

31. Photo by W. W. Little
Offshore to Transition Zone

32. Photo by W. W. Little
Transition Zone to Shoreface

33. Photo by W. W. Little

34. EaES 455-6 34

35. EaES 455-6 35

36. Storm-dominated Shelf Profiles
A typical succession consists of interbedded quiet water and storm deposits. Quiet
water sediments are consist mostly of bioturbated and laminated mud that can be
glauconitic. Storm “beds” are typically erosional at the base and fine upward from
intraclastic gravel, through trough cross-bedded sand to hummocky cross-bedded sand.

37. Shanmugam 2000

38. Tide-dominated Shelves
On shallow shelves in areas with high tidal ranges, subaqueous tidal currents can
transport sand to create a variety of bedforms, depending upon tidal strength
(current velocity) and sediment supply. These range from mud-draped dunes to
sand waves to longitudinal ribbons to erosional surfaces with increasing current
velocity. Tidal bedforms can be tens of meters high and several kilometers wide
where sand is abundant. Storm beds can be interbedded with tidal deposits, and
bioturbation can be well-developed.

39. Tidal Dunes
Tidal dunes can be complex, showing evidence of bidirectional flow
internally, while maintaining their asymmetrical morphology
demonstrating prevailing sediment transport direction. Reactivation
surfaces are common.

40. Tide-dominated Shelf Profiles