1. Clastic tidal sedimentology- with examples from Turnagain Arm (estuary), Alaska
Stephen F. Greb
Kentucky Geological Survey, University of Kentucky

2. Outline Tides introduction Estuaries introduction
Turnagain Arm, Alaska
Glacier Creek study area
Tidal rhythmites
Scales of rhythmicity
Controls on rhythmite preservation

3. Deltas How many of you have seen this diagram before?

4. Deltas
Deltas are large sedimentary deposits formed when sediment from rivers dumps into a lake or sea
Deltas have different shapes depending on the relative strength of river vs. tide. Vs. wave energy

5. Deltas vs. estuaries River Deltas in previous diagram Estuaries
River-dom
Wave-dom
Tide-dom
Estuaries
(Wave-dom)
(Tide-dom)
Strand plains/beaches
Tidal flats
Wave
Tide
Dalyrmple et al (1992)

6. Tides Major tidal forces (constituents)
Principal lunar (1 high, 1 low) P= 12.4 hours
Principal solar (1 high, 1 low) P= 12 hours
Lunar tide creates a semidiurnal force with approximately two high tides each day (P=24.8 hours)

7. Tides Major tidal forces (constituents)
Spring
Spring
When sun and moon align = additive lunar and solar forces = high (spring) tides

8. Tides Major tidal forces (constituents)
Neap
Neap
When moon is at right angles to sun = opposing tidal forces = low (neap) tides

9. Tides Major tidal forces (constituents)
Principal lunar (1 high, 1 low) P= 12.4 hours
Principal solar (1 high, 1 low) P= 12 hours
But the world isn’t covered by water uniformly. Continents in the way, different bathymetries, slope, etc. so tides act differently in different parts of the world
Semidiurnal ( 2 tides each day)
Diurnal ( 1 tide each day)
Mixed ( 1 to 2 tides each day)

10. Tides Tides Distribution of tidal types varies
Distribution of tidal types varies

11. Tidal Range
Tidal constituent variations cause water to pile up in some areas more than others, so tides have different ranges:
Microtidal (0-2 m)
Mesotidal (2-4 m)
Macrotidal (4 m +) or (4-6 m)
Hypertidal (6 m+)
Tidal range is dependent on a variety of factors including the tilt of the earth, bathymetry, phase and amplitude of tides, shape of the shelf and coastline, rate of shallowing landward, etc.

12. Tidal Range Tidal range also varies

13. Tidal structures What are some typical tidal structures?
Daily to twice daily changes in direction and amplitude of water acting on the sediment bed creates a variety of sedimentary structures
What are some typical tidal structures?

14. Tidal structures
A continuum of different types of ripple bedding occur in tidal facies
Flaser
Wavy
Lenticular
Sand = white, Mud = black
From

15. Tidal structures
Herringbone cross stratification (xbeds or ripples) forms by current reversal*
But you need equal flow in both directions, and the space to stack one crossbed on another
Many so-called herringbone xbeds are really obliquely-aligned xbed troughs…be careful interpreting herringbone!!!

16. Tidal structures
Herringbone cross stratification forms by current reversal*
Much more common are unequal tides, where dominant tide moves large dunes to form crossbeds in one direction and then subordinate tide moves smaller ripples on crossbeds topsets or on crossbed foresets in the other direction

17. Tidal structures
Reactivation surfaces form by erosion of the bedform during flow reversal
Reactivation generally forms a slightly concavo-convex erosion surface, which cuts across multiple foresets

18. Tidal structures
Mud drapes on foresets form as fines settle out during slack water between higher flow
Also common, during reversing flow or waning flow as currents change can simply result in low velocities or stagnation in which sand can’t be transported and silt-sized particles fall out of suspension leading to mud drapes on foresets
Fine-grained drape

19. Tidal structures Bundled foresets:
Tides don’t only increase and decrease and reverse daily, they also change during a neap-spring cycle

20. Tidal structures Bundled foresets:
Thicker foresets (d) form during higher energy (and velocity or duration) spring tides and thinner foresets form during neap tides

21. Estuaries
Definition
Estuaries are partly enclosed bodies of water, which are open to the sea at one end, and to rivers or streams on the other
Hence, they are influenced by shallow marine to fluvial processes
Dalyrmple et al (1992)

22. Estuaries
Definition
In some estuaries, headward narrowing causes tides to funnel landward, which tends to increase the amplitude of the tidal range (tides get higher)
From Dalyrmple and Choi (2007)

23. Estuaries
Models
The relative interaction of waves, tides, and rivers forms successions of sedimentary facies, which vary with the energy input into the system
From Allen (1991)

24. Turnagain Arm branch of Cook Inlet
Alaska
You are here
Anchorage
2053
2047
Cook Inlet
2041
Seward
2019 50
100 km
Knick Arm
From Archer and Hubbard (2004)
Turnagain Arm branch of Cook Inlet
Anchorage
6th largest tidal range in the world
Turnagain Arm
Bathymetry and elevation maps from 25
50 km

25. Cook Inlet
Alaska
You are here
Anchorage
2053
2047
Cook Inlet
2041
Seward
2019 50
100 km
Knick Arm
From Archer and Hubbard (2004)
Tides are amplified partly because they are driven into funnel-shaped estuaries
Anchorage
Turnagain Arm
Bathymetry and elevation maps from 25
50 km

26. Hypertidal Tidal range (m)
Alaska
Turnagain Arm
You are here
Anchorage
Hypertidal
Anchorage
2053
2047
Cook Inlet
2041
Tidal range (m)
Seward
2019 50
100 km
From Archer and Hubbard (2004)
Knick Arm
Headward tidal amplification in the Turnagain Arm branch of Cook Inlet exceeds 10 m (35 ft)
Anchorage
Turnagain Arm
Bathymetry and elevation maps from 25
50 km

27. Salinity measurementsAn
Salinity measurements 10 11 11 3 6 9 * 12
Anchorage
Bird Point
Hope
Girdwood 8
6.5
Bpt
Girdwood 7 6 5 4
Hope 1 1
5 mi 1 Wi
Neap Salinity
Distance
= Marine, = Brackish, 0 = Fresh An
In estuaries, salinity varies with neap-spring cycles and seasonal fluvial fluctuations 8* 10 7 9 Bp Gi 6 5 5 Ho
5 mi 1 Wi
Spring Salinity

28. IntertidalFlats at low tide
5 mi A 1
Chugach St. Pk. Headquarters
Hope
Girdwood
Bird Pt.
Bioturbation-Arenicolites
Outer estuaries where salinity is higher tend to have bioturbated tidal flats
IntertidalFlats at low tide

29. Bioturbation often destroys original sedimentary structures
5 mi A 1
Chugach St. Pk. Headquarters
Hope
Girdwood
Bird Pt.
Bioturbation-Arenicolites
Bioturbation often destroys original sedimentary structures
Flats at low tide

30. Salinity measurementsAn
Salinity measurements 10 11 11 3 6 9 * 12
Anchorage
Bird Point
Hope
Girdwood 8
6.5
Bpt
Girdwood 7 6 5 4
Hope 1 1
5 mi 1 Wi
Neap Salinity
Distance
= Marine, = Brackish, 0 = Fresh An
Headward in estuaries, salinity decreases and bioturbation decreases 8* 10 7 9 Bp Gi 6 5 5 Ho
5 mi 1 Wi
Spring Salinity

31. An
Headward funneling and shallowing increases the relative tidal range resulting in a breaking wave = tidal bore
Study area
Bird Point
Indian
Bore viewing
Turnigan Arm
Girdwood
Hope
20-mile 25
50 km
Knick Arm
Turnagain Arm
5 mi
Portage 1
Placer
Anchorage
m high bore

32. 5 mi An 1 8* 10 9 7 6 5 Wi
Spring Salinity
Hope
Bird Pt.
Girdwood
20-Mile Creek
20-mile Creek
The headward end of estuaries is usually a stream or river. Turnagain Arm has three fluvial channels

33. 5 mi An 1 8* 10 9 7 6 5 Wi
Spring Salinity
Hope
Bird Pt.
Girdwood
20-Mile Creek
20-mile Creek
Tidal structures and bidirectional bedforms give way headward to fluvial, down-dip-oriented structures

34. Here’s another type of bedding
Here’s another type of bedding. The study flat is located where Glacier Creek empties into Turnagain Arm (near Girdwood).
Girdwood
3 mi
5 km
Anchorage
Study area
Turnagain Arm
5 mi 1

35. The study flats are situated in a reentrant in the sedge marsh (light green) where Glacier Creek is deflected westward into the arm.
2003 channel
Glacier Creek
rising tide
Low tide

36. The current peat marsh (which is the high high tide mark (fresh water) is 2.74 m above a gravel bed the flats are accreting on

37. Tidal Range/ 2003 Spring Neap35
Spring 30
Neap 25 20
Height (ft) 15 10 5
8/12
8/14
8/16
8/18
8/20
8/22
8/24
8/26
The tidal flats in the reentrant are influenced by twice daily (semidiurnal) tides.

38. Here are trenches of the flats, which contain stacked parallel-laminations, and soft-sediment deformation

39. A flag and washer was placed on one day, and then we came back to dig it up after one half day; another at one day; another after a week D
1 tide

40. Tidal structures
A single day’s tide resulted in a thick-thin couplet consisting of two sand laminae draped by mud drapes =
Half day’s dominant tide followed by a subordinate tide S D
1 day

41. Tidal structures
These are stacked in groups or bundles of rhythms representing longer time periods
1 day
2 week
1 month S N D

42. Tidal structures
Rather than bundled foresets in laterally migrating crossbeds, these are vertically-accreted laminae
1 day
2 week
1 month S N D

43. 8/2003
7/2004
In 2004, we monitored sedimentation on the flats for 10 days and trenched flat in several locations to examine controls on rhythmite deposition and preservation
Glacier Ck
2004
20 m
Sedge marsh is ~2m above low tide level

44. Placing washers and flags
Completely buried tile
Placing washers and flags
Partially buried tile

45. Bundles with thin couplets
Natural weathering shows two distinct thickness trends in preserved tidal bundles
Bundles with thin couplets
Bundles with thick couplets

46. Flags with washers showed twice daily sedimentation
470 mm
Trench 2 14
Soft-sediment deformation 12 10
Laminae thickness (mm) 8 6 4 2 1 7 13 19 25 31 37 43 49 55 61 67
Soft-sediment deformation
Flags with washers showed twice daily sedimentation
Trenches showed cyclic bundles of 14 to 10 laminae (5 to 7 laminae couplets)
Also, zones of soft-sediment deformation
Glacier Ck
20 m

47. Soft-sediment deformation is likely due to dewatering of the flats

48. Laminae thickness (mm)
Trench 3 2 4 6 8 10 12 1 7 13 19 25 31 37 43 49 55
320mm thick
Soft-sediment deformation n Sa Sp
Alternating spring (S) bundles of thicker laminae couplets and thinner laminae couplets represent perigee (Sp) high-spring and apogee (Sa) low-spring tides
Glacier Ck
Soft-sediment deformation
20 m

49. Tidal Range/ 2003 Spring Neap 35 30 25 20 Height (ft) 15 10 5
8/12
8/14
8/16
8/18
8/20
8/22
8/24
8/26
There is 2 m of relief between the marsh (spring high tide) and the alluvial gravel apron, which means that 3 to 4 days of neap tides don’t cover the flats

50. Composite results of trenching showing changes in rhythmite signal laterally as flats thinned

51. So not everywhere preserves complete tidal signal
So not everywhere preserves complete tidal signal. Usually, you see only part of the signal preserved.
20 m

52. How much is preserved is dependent on the space available for sedimentation=
Accommodation space
20 m

53. Correlation of bundles and number of laminae per bundle
50 cm
Trench 2 8 40 3 10
Trench 3
neap
spring 3 8 30 14 3 10
Trench 4 5 4 14 8 20 14 4 4 11 3
Soft-sed def 14
Trench 5 10 10 2 7 8 2 7
Soft-sed def 6 2 4
6-7 8
1-2 7 1+
Decrease bundle thickness
Decrease number of daily laminations
Decrease neap cycle preservation
Erosion of upper spring cycles

54. Sedimentation 1 2
Flood tides stay on the north side of Glacier Creek (mutually evasive) and are deflected into a small channel on the west side of the flats
Flood tide entering drainage creek

55. 1 4 2
Water levels rise in the creek and the southern gravel bar is inundated.
Flats become a bar between drainage creeks and main river channel 3

56. 1 4 2 5 3
An essentially rotational current is produced in the reentrant, which may help to keep sediment in suspension and facilitate vertical accretion

57. Rhythmite preservation
Rhythmites are preserved in upper flats in the fluvial-estuarine transition of Glacier Creek
As much as 12 mm/ day
As much as 160 mm/ month

58. (Greb and Archer, 1998)
Similar rhythmites are preserved in Carboniferous tidal flats and abandoned channel fills, and these also show strong accommodation space influences

59. In modern and in rock
Modern tidal flat
Hazel Patch Ss., KY

60. Some good textbooks for understanding tidal structures:
Reineck, H. E-, and Singh, I.B., 1973, Depositional Sedimentary Environments: Spring Verlag
Clifton, H.E., 1982, Estuarine deposits, in Scholle, P.A., and Spearing, D., eds., Sandstone Depositional Environments: American Association of Petroleum Geologists, p
Klein, G.D., 1970, Depositional and dispersal dynamics of intertidal sandbars: Journal of Sedimentary Petrology, v. 40, p
Weimer, R.J., Howard, J.D., and Lindsay, D.R., 1982, Tidal flats, in Scholle, P.A., and Spearing, D., eds., Sandstone Depositional Environments: American Association of Petroleum Geologists, p