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Clastic tidal sedimentology- with examples from Turnagain Arm (estuary), Alaska Stephen F. Greb Kentucky Geological Survey, University of Kentucky Slide 2 Outline Tides introduction Estuaries introduction Turnagain Arm, Alaska Glacier Creek study area Tidal rhythmites Scales of rhythmicity Controls on rhythmite preservation Slide 3 Deltas How many of you have seen this diagram before? http://www.pnas.org/content/104/43/16804/F1.large.jpg Slide 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 Slide 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 Slide 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) Slide 7 Tides Major tidal forces (constituents) When sun and moon align = additive lunar and solar forces = high (spring) tides Spring Slide 8 Tides Major tidal forces (constituents) When moon is at right angles to sun = opposing tidal forces = low (neap) tides Neap Slide 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 isnt covered by water uniformly. Continents in the way, different bathymetries, slope, etc. so tides act differently in different parts of the world Slide 10 Tides http://en.wikipedia.org/wiki/File:Diurnal_tide_types_map.jpg Distribution of tidal types varies Slide 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. Slide 12 Tidal range also varies Tidal Range http://en.wikipedia.org/wiki/File:M2_tidal_constituent.jpg Slide 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? Slide 14 Tidal structures A continuum of different types of ripple bedding occur in tidal facies Flaser Wavy Lenticular From http://www.kgs.ku.edu/Current/2008/Enos/gifs/fig10.gif Sand = white, Mud = black Slide 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 troughsbe careful interpreting herringbone!!! Slide 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 Slide 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 Slide 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 cant be transported and silt- sized particles fall out of suspension leading to mud drapes on foresets Fine- grained drape Tidal structures Slide 19 http://www.seddepseq.co.uk/DEPOSITIONAL_ENV/Tidal/Picture2.png Bundled foresets: Tides dont only increase and decrease and reverse daily, they also change during a neap-spring cycle Slide 20 Tidal structures http://www.seddepseq.co.uk/DEPOSITIONAL_ENV/Tidal/Picture2.png Bundled foresets: Thicker foresets (d) form during higher energy (and velocity or duration) spring tides and thinner foresets form during neap tides Slide 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 Slide 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) Slide 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 Slide 24 Turnagain Arm branch of Cook Inlet Cook Inlet You are here Alaska Anchorage Cook Inlet 2019 2041 2047 2053 Seward 050100 km From Archer and Hubbard (2004) Bathymetry and elevation maps from http://ibis.grdl.noaa.gov/cgi-bin/bathy/bathD.pl 02550 km Anchorage Knick Arm Turnagain Arm 6th largest tidal range in the world Slide 25 Tides are amplified partly because they are driven into funnel-shaped estuaries Cook Inlet You are here Alaska Anchorage Cook Inlet 2019 2041 2047 2053 Seward 050100 km From Archer and Hubbard (2004) Bathymetry and elevation maps from http://ibis.grdl.noaa.gov/cgi-bin/bathy/bathD.pl 02550 km Anchorage Knick Arm Turnagain Arm Slide 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 2019 2041 2047 2053 Seward 050100 km 02550 km Anchorage Knick Arm Turnagain Arm Bathymetry and elevation maps from http://ibis.grdl.noaa.gov/cgi-bin/bathy/bathD.pl Hypertidal Slide 27 5 mi Gi An Ho 1 8* 10 9 7 6 5 5 0 0 0 Bp Wi 5 mi Girdwood An Hope 1 10 11 6.5 8 7 6 5 4 0 1 1 0 0 0 Bpt Wi Neap Salinity Spring Salinity Salinity measurements 12 - 7 = Marine, 6 - 1 = Brackish, 0 = Fresh 0 3 6 9 * 12 Anchorage Bird Point Hope Girdwood Distance In estuaries, salinity varies with neap- spring cycles and seasonal fluvial fluctuations Slide 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 Slide 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 Slide 30 5 mi Gi An Ho 1 8* 10 9 7 6 5 5 0 0 0 Bp Wi 5 mi Girdwood An Hope 1 10 11 6.5 8 7 6 5 4 0 1 1 0 0 0 Bpt Wi Neap Salinity Spring Salinity Salinity measurements 12 - 7 = Marine, 6 - 1 = Brackish, 0 = Fresh 0 3 6 9 * 12 Anchorage Bird Point Hope Girdwood Distance Headward in estuaries, salinity decreases and bioturbation decreases Slide 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 1.2-1.8 m high bore 1 Study area 0 2550 km Knick Arm Turnagain Arm Slide 32 The headward end of estuaries is usually a stream or river. Turnagain Arm has three fluvial channels 20-mile Creek Slide 33 Tidal structures and bidirectional bedforms give way headward to fluvial, down- dip-oriented structures 20-mile Creek Slide 34 Heres 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 Slide 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 Slide 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 Slide 37 Tidal Range/ 2003 0 5 10 15 20 25 30 35 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. Slide 38 Here are trenches of the flats, which contain stacked parallel-laminations, and soft-sediment deformation Slide 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 Slide 40 A single days tide resulted in a thick-thin couplet consisting of two sand laminae draped by mud drapes = Half days dominant tide followed by a subordinate tide 1 day S D Tidal structures Slide 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 Slide 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 Slide 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 2004 20 m Sedge marsh is ~2m above low tide level 7/2004 Slide 44 Completely buried tile Partially buried tile Placing washers and flags Slide 45 Natural weathering shows two distinct thickness trends in preserved tidal bundles Bundles with thin couplets Bundles with thick couplets Slide 46 Glacier Ck20 m 470 mm Trench 2 0 2 4 6 8 10 12 14 1713192531374349556167 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 Slide 47 Soft-sediment deformation is likely due to dewatering of the flats Slide 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 Slide 49 Tidal Range/ 2003 0 5 10 15 20 25 30 35 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 dont cover the flats Slide 50 20 m Composite results of trenching showing changes in rhythmite signal laterally as flats thinned Slide 51 20 m So not everywhere preserves complete tidal signal. Usually, you see only part of the signal preserved. Slide 52 20 m How much is preserved is dependent on the space available for sedimentation = Accommodation space Slide 53 4 8 2 7 3 10 11 4 8 4 14 4 10 14 3 8 2 Soft-sed def 8 5 3 10 14 3 8 7 6 2 1+ 7 6-7 1-2 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 40 30 20 10 Soft-sed def neap spring Slide 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 Slide 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 Slide 56 1 2 3 4 5 An essentially rotational current is produced in the reentrant, which may help to keep sediment in suspension and facilitate vertical accretion Slide 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 Slide 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) Slide 59 Modern tidal flatHazel Patch Ss., KY In modern and in rock Slide 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. 179-190. Klein, G.D., 1970, Depositional and dispersal dynamics of intertidal sandbars: Journal of Sedimentary Petrology, v. 40, p. 1095-1127. 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. 191-246.