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

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Presentation on theme: "Clastic tidal sedimentology- with examples from Turnagain Arm (estuary), Alaska Stephen F. Greb Kentucky Geological Survey, University of Kentucky."— Presentation transcript:

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

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) When sun and moon align = additive lunar and solar forces = high (spring) tides Spring

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

9  Semidiurnal ( 2 tides each day)  Diurnal ( 1 tide each day)  Mixed ( 1 to 2 tides each day) 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

10 Tides 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 also varies Tidal Range

13 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 From Sand = white, Mud = black

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 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 Tidal structures

19 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 Dalyrmple et al (1992) 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 Definition

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

23 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) Models Estuaries

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

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

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

27 5 mi Gi An Ho 1 8* Bp Wi 5 mi Girdwood An Hope Bpt Wi Neap Salinity Spring Salinity Salinity measurements = Marine, = Brackish, 0 = Fresh * 12 Anchorage Bird Point Hope Girdwood Distance  In estuaries, salinity varies with neap- spring cycles and seasonal fluvial fluctuations

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

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

30 5 mi Gi An Ho 1 8* Bp Wi 5 mi Girdwood An Hope Bpt Wi Neap Salinity Spring Salinity Salinity measurements = Marine, = Brackish, 0 = Fresh * 12 Anchorage Bird Point Hope Girdwood Distance  Headward in estuaries, salinity decreases and bioturbation decreases

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

32 The headward end of estuaries is usually a stream or river. Turnagain Arm has three fluvial channels 20-mile Creek

33 Tidal structures and bidirectional bedforms give way headward to fluvial, down- dip-oriented structures 20-mile Creek

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

35 Glacier Creek 2003 channel The study flats are situated in a reentrant in the sedge marsh (light green) where Glacier Creek is deflected westward into the arm. Low tide rising 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/ Height (ft) 8/128/148/168/188/208/228/248/26 Spring Neap 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 1 tide D

40 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 1 day S D Tidal structures

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

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

43 8/2003 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 m Sedge marsh is ~2m above low tide level 7/2004

44 Completely buried tile Partially buried tile Placing washers and flags

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

46 Glacier Ck20 m 470 mm Trench Laminae thickness (mm) 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

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

48 Glacier Ck 20 m Soft-sediment deformation Alternating spring (S) bundles of thicker laminae couplets and thinner laminae couplets represent perigee (Sp) high-spring and apogee (Sa) low- spring tides

49 Tidal Range/ Height (ft) 8/128/148/168/188/208/228/248/26 Spring Neap 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 20 m Composite results of trenching showing changes in rhythmite signal laterally as flats thinned

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

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

53 Soft-sed def Decrease bundle thickness Decrease number of daily laminations Decrease neap cycle preservation Erosion of upper spring cycles Correlation of bundles and number of laminae per bundle Trench 2 Trench 3 Trench 4 Trench 5 50 cm Soft-sed def neap spring

54 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 Sedimentation

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

56 An essentially rotational current is produced in the reentrant, which may help to keep sediment in suspension and facilitate vertical accretion

57 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 Rhythmite preservation

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

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

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


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