Geol 370: Sedimentology and Stratigraphy Topic 13b: Beach and Barrier Systems

1. Beach & Barrier Systems
Photo by W. W. Little

2. Simplified Models
Beaches are typically separated into shore-parallel environments based
on water depth and relationship wave processes.

3. Reineck & Singh, 1980
Common Shoreline Facies
Prograding shorelines are
typified by a coarsening-
upward succession of
deeper water to shallower
water to above water
facies all dominated by
sand. Facies are
distinguished primarily
by grain size and
sedimentary structures.

5. Photo by W. W. Little
Laminated Mud
Laminated mud forms offshore below storm wave-base just beyond
the beach system and in back-barrier lagoons through suspension-
settling of clay and silt.

6. Hummocky Cross-bedded Sand
Hummocky cross-bedded sand is produced mostly during storms by a
combination of current and oscillatory flow and is found primarily on the
lower shoreface and in the transition zone. Photo by W. W. Little
Photo by W. W. Little

7. Photo by W. W. Little
Trough Cross-bedded Sand
Small-scale trough cross-stratified sand is common in the surf
zone of the upper shoreface.

8. Photo by W. W. Little

9. Horizontally-stratified Sand
Deposition in the swash zone produces relatively continuous laminae
of sand that dip slightly basinward. Small cavities formed from
trapped air and water escape structures are common.

10. Photo by W. W. Little

11. Photo by W. W. Little

12. Shell Beds
Mollusc shells commonly accumulate during storm events, as the
shells are piled upon one another and finer sediment is winnowed
away.

13. Photo by W. W. Little
Edgewise Conglomerate
Edgewise conglomerate is an intraclastic conglomerate produced
during storm events as partially lithified sediment is “ripped up”
and redeposited.

14. Photo by W. W. Little
Thrombolites
Thrombolites are biological structures similar to stromatolites but
without internal structure.

15. Photo by W. W. Little

16. Photo by W. W. Little

17. Ripple Bedding
Ripple bedding commonly forms as part of the backwash; however,
because of the higher-energy swash part of the couplet, they have a
low preservation potential. Photo by W. W. Little

18. A: End of swash B: Beginning of backwash
C: End of backwash D: Rhomboidal ripples Photos by W. W. Little

19. Rhomboidal Ripples

20. Current Ripples
Photo by W. W. Little

21. Photo by W. W. Little
Coal
Coal forms through the accumulation of peat in back beach and back
barrier regions.

22. Coastal Depositional Systems
A variety of depositional systems are found along a shoreline. Their
distribution, geometry, and extent are determined by proximity to a
sediment source, sediment abundance, energy conditions, and
fluctuations in base-level.
This lecture
focuses on the
beach and barrier
island systems.

23. Progradational Beach Associations
Beaches are typically separated into shore-parallel environments based
on water depth and relationship to wave processes.

24. Beaches are typically separated into shore-parallel environments based
on water depth and relationship wave processes.
Retrogradational Beach Associations

25. Idealized Vertical Profiles
Vertical profiles for shoreline successions vary depending, in part, on
whether they are produced under transgressive or regressive conditions.
Progradational successions are more common, but transgressive
successions tend to be more complex.
BeachBeach Barrier IslandBarrier Island Tidal inletTidal inlet
Progradational ProfileProgradational Profile Retrogradational ProfilesRetrogradational Profiles

26. Barrier Islands & Lagoons
Barrier islands form mostly during transgressive conditions and are,
in essence, beaches that are separated from the mainland by a lagoon.
Modern barriers are commonly associated with estuaries.

30. Tidal Inlets
Tidal inlets are breaches in a barrier island that allow sea water to
move into and out of a lagoon. As the inlet migrates in the direction
of long-shore transport, a typical vertical profile would consist of
fining-upward channel deposits overlain by spit sediments.

31. Washover Fans
Storm surges will often erode sand from the basinward side of a bar
and redeposit it on the lagoonal side, producing a washover fan.

34. Tidal Deltas
Barrier islands often become breached by large storms. Tidal currents
then flow through the opening producing small “deltas.” Ebb deltas
are much more common than flow deltas because of the shallower
basin in the ebb side of the bar.

37. Back-barrier (bay mouth) Tidal Flat
Bay mouths behind barrier bars can become filled with fine-grained
sediment deposited by estuarine fluvial systems, producing tidal
flats.

39. Photo by W. W. Little
Large-scale Architecture
(progradational beach)
Progradational beach successions tend to be cyclical, in which each
cycle, as well as the entire succession coarsen upward. Each small-
scale cycle is referred to as a parasequence. Boundaries between
parasequences are flooding surfaces and represent relatively abrupt
rises in base-level.

40. Photo by W. W. Little

41. Photo by W. W. Little
Flooding surface/parasequence boundary

42. Photo by W. W. Little
Coarsening-upwardsuccession
Flooding surfaces

43. Photo by W. W. Little
Offshore Deposits
Above the flooding surface are deeper-water shelf mudstones and
poorly-developed limestones, both representing deposition below
storm wave base.

44. Photo by W. W. Little

45. Photo by W. W. Little
Transition Zone
Transition zone deposits are composed of interbedded offshore
mudstone and hummocky cross-bedded sand deposited during
storms. This zone represents sedimentation below normal and above
storm wave-base. Relative abundance and thickness of hummocky
beds increases upward.

46. Photo by W. W. Little

47. Photo by W. W. Little

48. Photo by W. W. Little
Lower Shoreface
Shoreface deposits consist primarily of stacked hummocky cross-
stratified sand beds deposited between normal wave base and the surf
zone.

49. Photo by W. W. Little

50. Photo by W. W. Little

51. Photo by W. W. Little
Upper Shoreface
Trough and low-angle trough cross-bedded sand transported by surf
comprises the upper shoreface.

52. Photo by W. W. Little
Foreshore
Swash deposition in the foreshore zone produces horizontally-laminated
sand.

53. Photo by W. W. Little

54. Photo by W. W. Little

55. Photo by W. W. Little
Backshore Deposits
Backshore deposits can take a variety of forms, including eolian dunes,
lagoonal mud, tidal sand or mud, and peat swamps.

56. Photo by W. W. Little

57. Photo by W. W. Little

58. Photo by W. W. Little