World's highest tides: Hypertidal coastal systems in North America, South America and Europe

Sedimentary Geology 284–285 (2013) 1–25

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World's highest tides: Hypertidal coastal systems in North America, South America and Europe Allen W. Archer ⁎ Department of Geology, Kansas State University, Manhattan, KS 66506, USA

a r t i c l e

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Article history: Received 4 May 2012 Received in revised form 17 October 2012 Accepted 7 December 2012 Available online 21 December 2012 Editor: G.J. Weltje Keywords: Macrotidal Hypertidal Lamina Rhythmites Neap–spring cycles Annual cycles

a b s t r a c t Hypertidal systems can be defined as areas where spring tides have ranges greater than 6 m. These very high tidal ranges results in unique patterns of sedimentation within hypertidal estuaries. Such systems are not common but they do occur on a number of continents. This report will discuss six areas that have the highest tides in the world. North America hypertidal systems occur within Cook Inlet in Alaska, USA, Leaf Basin in Ungava Bay, Quebec Province, Canada, and the Bay of Fundy, Nova Scotia and New Brunswick, Canada. In South America, the Straits of Magellan and associated Atlantic coastal settings exhibit hypertidal conditions. European hypertidal systems include Bristol Channel and Severn estuary in southwest England and the Gulf of St. Malo in Normandy, France. These six areas have the highest tides in the world and spring tidal ranges that regularly exceed 10 m. All the six areas can be divided into intertidal sedimentological zones. Zone 1 is the outermost zone and contains longitudinal bars. Zone 2 exhibits laterally extensive sand flats. Zone 3 includes the innermost extent of tides and estuarine point bars. Annual and neap–spring cycles have been documented in Zone 3 and are probably the most indicative features of hypertidal systems. The North American systems occur in high-latitude cold climates where winter ice can have a minor or major impact on the development of sedimentary facies. Conversely, the European and Patagonia systems have climates minimal ice formation. © 2012 Elsevier B.V. All rights reserved.

1. Introduction

1.2. Frequency distribution of global tidal ranges

1.1. Study areas

Based upon maximum spring-tidal range, the subdivision of tidal systems by Davies (1964) has proved to be popular and has become widely utilized. His system defined microtidal as having a tidal range of 0 to 2 m, mesotidal as having a tidal range of 2 to 4 m, and macrotidal as having a tidal ranges greater than 4 m. No upper limit for macrotidal was originally defined. The frequency distribution of global tidal heights indicates that most coastal areas are dominated by micro- and mesotidal ranges (Fig. 2). Although macro- and hypertidal settings are rare by comparison, the dynamics of sediment erosion, transport, and deposition cause these high tidal-range areas to have extreme, short-term variability. As applied herein, hypertidal settings are defined as having tidal ranges that exceed 6 m. This places an upper limit of 6 m for the macrotidal category originally defined by Davies (1964). There are very high energies and dynamic intertidal sedimentation in hypertidal systems. In order to make simple comparisons between these very high tidal-range areas, the hypertidal range can be subdivided into 2-m intervals (Fig. 2): hypertidal-A (6 to 8 m), hypertidal-B (8 to 10 m), hypertidal-C (10 to 12 m), hypertidal-D (12 to 14 m), and finally hypertidal-E (14 to 16 m). Hypertidal systems have enormous tidal ranges coupled with a tremendous potential energy. A complete understanding of the

This report will compare tidal dynamics within the intertidal zone of six areas that have the highest recognized tidal ranges on Earth (Fig. 1). Other areas, of course, also have very high tidal ranges. For example, the summary of Archer and Hubbard (2003) discusses additional areas, particularly in Asia, with hypertidal ranges. Since that earlier review, all the areas discussed in this report have been visited and at least preliminary observations have been made. North America sites include: (1) Turnagain Arm within Cook Inlet in south-central Alaska, USA, (2) Leaf Lake in Ungava Bay, northern Quebec Province of Canada and (3) Salmon River estuary in the Bay of Fundy, Nova Scotia Province of Canada. In South America, hypertidal settings include: (4) several estuaries in the Patagonia region (southeastern Atlantic coast of Argentina). Areas in Europe include: (5) Bristol Bay and the Severn River estuary, southwestern UK and (6) and MontSaint-Michel Bay in Normandy, France.

⁎ Tel.: +1 785 532 2244; fax: +1 785 532 5159. E-mail address: [email protected]. 0037-0738/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.sedgeo.2012.12.007

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Fig. 1. Location of areas with highest tides in the world (black text). Hypertidal settings in North America include, from west to east: Cook Inlet, Alaska, USA; Ungava Bay, northern Quebec, Canada and Bay of Fundy, Nova Scotia, Canada. Settings in South America include the hypertidal estuaries of Rio Santa Cruz, Rio Coyle and Rio Gallegos. Study sites in Europe include the Severn River estuary in southeastern UK and the Mont-Saint-Michel Bay in Normandy France. Note that except for Cook Inlet all settings are part of the Atlantic Ocean margins. Other prominent areas of hypertidal ranges (green text) have been discussed by Archer and Hubbard (2003).

Fig. 2. Frequency distribution (histogram) of global tidal ranges extracted from NOAA (1998) tidal tables. This navigational database includes maximum-spring-tidal ranges from 6896 primary and secondary sites around the globe. Color bars delineate the widely used microtidal (0 to 2 m), mesotidal (2 to 4 m) and macrotidal (4 m and above) ranges. The macrotidal division as currently in use is open-ended and incorporates a tremendous range of tidal heights and potential tidal energies. For purposes of this comparative study, an upper limit for macrotidal is set at 6 m and all higher tides are defined as hypertidal. A tidal range of 6 to 8 will be termed Hypertidal A or abbreviated to hyper-A. Tidal ranges from 8 to 10 m will be referred to as Hypertidal B, and so on. Similar color bars will be used within subsequent tidal range diagrams in order to indicate the various hypertidal ranges. Projected tidal-power curves based upon changes in tidal heights taken to the 3rd, 4th, and 5th powers. These curves indicate that tremendous potential for tidal-power generation within hypertidal systems.

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dynamics of these extreme systems is important for at least two reasons. First, they serve as modern analogs for ancient systems that may have been formed within extremely dynamic tidal systems. Secondly, from a more pragmatic standpoint, tidal power is a relatively untapped nonpolluting and renewable source of energy. Currently online operational tidal-power stations have been constructed almost entirely within hypertidal coastal settings. It is almost certain that future developments will also be concentrated in these energetic hypertidal coastal settings. 1.3. Short-term tidal periods When plotted for the same time interval, the tidal curves for the 6 sites exhibit a number of significant variations (Fig. 3). The interval from July 1 to August 3, 2005 was chosen because it included a close alignment of full moon and perigee (closest approach of moon to the earth) occurred during July 21 and 22. This astronomical alignment results in perigean-spring tides, also termed proxigean-spring tides (Woods, 1986). The tidal system within Turnagain Arm is strongly influenced by Pacific Ocean tides and thus has a mixed, predominantly semidiurnal system (Defant, 1961). Pacific tides tend to exhibit a prominent diurnal inequality — a significantly higher, high tide is followed by a significantly lower high tide. In Turnagain Arm, Alaska, the lower high tide in the pair can be as much as 1 m lower than the higher-high tide (Fig. 3a). The diurnal inequality (DI) is even more pronounced in the lowest tides. The DI in this case can approach 3 m. There is also a significant difference between apogean and perigean tidal-height inequalities (API). For Turnagain Arm this can be as much as 2 m. Starting with the Leaf Lake in Ungava Bay, the other study areas are influenced by Atlantic Ocean-type tides. Briefly, this means a much reduced DI. In the Leaf Lake estuary of Ungava Bay, the DI and API is 90 cm (Fig. 3b). Bay of Fundy tides are unusual in that the neap tides are not that much lower than the spring tides (Fig. 3c). Thus, the API is minimal when compared to the other system. For tidal power consideration, this system has the most consistent tidal ranges and could produce the most consistent power output. The Patagonian sites share similar tidal curves when compared to the North American areas. The Patagonian sites have an average API of 2.6 m. This is the highest API for all areas. Visual inspection of the tidal curve (Fig. 3d) illustrates the strong response of the tidal system during proxigean alignment. In Europe, tidal curves for the Severn estuary (Fig. 3e) and Mont-Saint-Michel Bay (Fig. 3f) are similar and are typical North Atlantic Ocean tidal curves. Mont-Saint-Michel Bay, however, is much more affected by proxigean effects than is the Severn estuary. One of the effects is that the duration of subaerial exposure in the high intertidal, during neap-tide periods, is much more pronounced at the French site. 1.4. Effects of inland tidal amplification Areas that have hypertidal ranges have a complex combination of unique coastal, bay, and estuarine configurations. When there is a significant bay between the hypertidal system and the open ocean, rotating or residual currents can occur. Rotational currents cannot be maintained as the estuary narrows in an inland direction. This narrowing allows the development of a progressive wave. Such systems have been studied extensively in the Bay of Fundy. The progressive wave can result in significant inland amplification of the astronomical tides. In most cases, the open-coastal areas tend to have smaller tidal ranges. For each study area, the location of each tidal station is mapped and used to portray a simple diagram of the trend of the inland tidal amplification. This amplification diagram is simplistic and has little utility for oceanographic studies.

Fig. 3. Comparison of tidal heights for the six study areas during the interval of July 1 to August 3, 2005. Symbols includes tropical (declinational) tidal periods based upon maximum northerly lunar declination (No), equatorial passage (Eq) and maximum southerly lunar declination (So). Lunar phases are shown by dark circle (new moon), white circles (full moon) and the quarter phases. Apogee (Ap) and perigee (Pe) are also shown. This interval of time had an unusually close temporal alignment of the full moon and perigee. The height difference between apogean spring tides and perigean spring tides is the apogean–perigean inequality (API). API is referred to as the semimonthly or fortnightly period by some workers (Woods, 1986). The difference in the lower high tide and higher high tide is the diurnal inequality (DI). (a) Tidal curve for Sunrise (Turnagain Arm, Alaska) showing a DI of 1.1 m and an API is 2.2 m. (b) Tidal curve for Leaf Lake showing a DI of 0.9 m and API of 0.9 m. (c) Tidal curve for Bay of Fundy showing a DI of 0.4 and an API of 0.9 m. (d) Tidal curve for Rio Gallegos showing a DI of 0.4 m and a API for 2.6 m. (e) Tidal curve for Avonmouth (Severn River, UK) exhibiting a DI of 0.4 m and an API of 2.6 m. (f) Tidal curve for Granville (France) exhibiting a DI of 0.6 m and an API of 1.8 m.

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Such diagrams, however, can be extremely useful as models to aid in the understanding of ancient examples of hypertidal systems. 1.5. Satellite imagery and sedimentary facies In 2007, the United States Geological Survey (USGS) in conjunction with NASA began to provide all available Landsat imagery at no cost. Compared to more recent earth-imaging satellites the resolution of Landsat images is quite coarse (60- to 30-m pixel sizes). However, the available time-series of images allows comparison of site changes over, in some cases, a 40-year period of time. For this analysis, all available Landsat images were previewed. The best images taken at low tides were selected for the following section. Conversely Google Earth, in order to reduce what could be termed coastline shift, overwhelmingly uses highest-tide images. On the following LandSat images, the boundaries of the intertidal sedimentological zones were determined in the field and subsequently geolocated onto the satellite images. The boundaries of interest are the main areas of intertidal sedimentation. Thus, Zone 1 is defined by the presence of shore-parallel and linear bars. Dune-scale features are common. Mudflats and marshes that fringe the linear bars are also included in Zone 1. Zone 2 is characterized by extensive, ripple-covered sand flats and includes lateral mudflats and marsh. Marsh sediments include annual cycles in Zone 2. The innermost zone is Zone 1. It includes estuarine point bars, a variety of neap– spring tidal rhythmites, and extends upstream to the inland tidal limit. 2. Cook Inlet — Turnagain Arm, Alaska South-central Alaska was greatly affected by continental- and alpine-scale glaciation during the Pleistocene Epoch. Turnagain Arm, located at 61° north latitude, was originally an ice-eroded fjord carved by a large alpine glacier. This area is tectonically active and within a back-arc extensional zone related to subduction of the Pacific Plate under the North American Plate. In 1964, a quake in this area registered a Richter magnitude of 8.4 and caused about $500 million in property damage

(Eckel, 1970). The seismicity of the area has affected the long-term deposition and tidal-channel stability within Turnagain Arm (Brown et al., 1977; Bartsch-Winkler, 1988). Within the eastern end of Turnagain Arm, the 1964 earthquake created regional subsidence of 2 m or more.

2.1. Tidal stations and inland amplification Because of the importance of maritime navigation, there are a number of tidal stations in this region (Fig. 4a). Cook Inlet is a prominent embayment along the southern-central coast of the US state of Alaska. The inlet connects to the Gulf of Alaska in the northern Pacific Ocean. This is an area of coniferous forest coupled with a stormy and humid temperate to subarctic climate. Rainfall increases dramatically from west to east — eastern parts of Turnagain Arm have a temperate rainforest. The dense understory and forest reduces the amount of sediments brought into Turnagain Arm. There is extensive formation of winter ice, particularly in the more inland parts of the inlet by streams and rivers. This is an area of generally low population density and anthropogenic alterations are minimal except in the areas surrounding the city of Anchorage. At Anchorage, the system bifurcates into the northern Knik Arm and the southern Turnagain Arm. Tidal bores are well developed within Turnagain Arm. Aspects of sedimentation have been discussed in detail by Sharma (1969) and Bartsch-Winkler (1988). Tidal-range data compiled from NOAA indicates a systematic increase in tidal range along a southwest–northeastern transect for stations in the Cook Inlet area (Fig. 4b). At the southern end of Cook Inlet, the spring-tidal ranges are slightly higher than 4 m and thus are low macrotidal (Fig. 4b). Approximately half-way up Cook Inlet, stations such as Nikiski (2047) have ranges that are hypertidal-A. The furthest inland station is at Sunrise (2053) and this area has a spring-tidal range slightly higher than 10 m. This places the tidal range at Sunrise in the hypertidal-C range. Inland of Sunrise, the land-surface elevation is higher than the low-tide levels. In these areas, measurements were made of the depth flooding during high tide.

Fig. 4. Tidal stations and tidal amplification within the Cook Inlet area of Alaska. (a) Location of NOAA stations used for maximum spring-tidal-range data. Red dashed line is an arbitrary zero line that serves to delineate a seaward boundary for the Cook Inlet embayment. Distances to more inland tidal stations are measured perpendicularly from this zero line. (b) West to east amplification of tidal ranges within Cook Inlet and Turnagain Arm. Low macrotidal ranges occur along the outer coast and the seaward areas of Cook Inlet. Tidal ranges increase to 9 m at Anchorage and exceed 10 m at Sunrise. East of the abandoned town site of Sunrise, the effective tidal flux decreases since the land surface rises and as such is located above the level of low tides. Within the upstream limit of the estuarine zones, the depth of tidal flooding is reduced to zero. This is because only the highest tide results in inundation of the tidal flats. This results in zones of effectively meso- and microtidal conditions, in terms of actual depth of tidal flooding, within the inner portions of the depositional system. Tidal bore heights (yellow line) are estimated based upon field observations. Adapted from Archer and Hubbard (2003).

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2.2. Intertidal sedimentological zonation From the Pacific Ocean mouth of Cook Inlet, tidal influences extend inland for 500 km (Fig. 4b). Much of the outer coastline is dominated by erosional sea cliffs. Near Anchorage, the sediment influx from several rivers has resulted in extensive mudflats that fill much of eastern Cook Inlet and Knik Arm (Fig. 5). Because of the normal

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salinities coupled with relatively low rates of deposition, these mudflats tend to be highly bioturbated and can commonly exhibit a greenish coloration related to biofilm growth. Within Turnagain Arm itself, the western margin of Zone 1 is defined as the farthest western extent of linear, shore-parallel bars. These bars are only exposed during the lowest-low tides. Owing to short-term exposure, dangerous currents and extremely cold water, no attempts were made to visit these bars. Use of helicopters by USGS staff, however, allowed extensive mapping of surficial bedforms (Bartsch-Winkler et al., 1975, 1985). 3. Ungava Bay — Leaf Lake, Quebec, Canada Ungava Bay is located in northern part of the province of Quebec in Canada. It lies along the southern part of Hudson Strait, which leads westward to Hudson Bay. This entire area is mostly tundra and is only sparsely populated. Leaf Lake lies at 59° north latitude and the area was extensively scoured by continental-scale glaciation. Because of the proximity to the Greenland ice sheet, Ungava Bay is much colder than the other North America sites and is well north of the treeline. This area has a multifaceted and very complex tidal system. There has been a long-standing debate as to whether the Bay of Fundy actually has higher tides than Ungava Bay. The official view of the Canadian Hydrographic Service is that the two areas are for all practical purposes tied in terms of having the highest tides in the world (Canadian Hydrographic Service, 2004). Regarding the potential for tidal energy, Godin (1973) delineated several locations for combined hydroelectric and tidal-power generation. Ungava Bay is also located on the Proterozoic-age Canadian Shield. This area has extensive banded-iron formations (BIFs). Its remote location, however, hinders mining or tidal-power installations. 3.1. Tidal stations and inland amplification Because of the remote nature of Ungava Bay, there are few tidal stations (Fig. 6a). All the tidal stations along the shoreline of Ungava Bay itself are hypertidal. A well-developed amplification is developed within the Ungava Bay system when distances and ranges are compared based on a zero line that connects the outer most stations (151 and 167). There is significant amplification within Ungava Bay, from north to south, and additional amplification occurs within individual estuaries. Stations on the more open Hudson Straits, such as Sorry Harbor on Resolution Island (123), have upper macrotidal ranges of only 5.36 m. The highest tidal ranges are within the estuaries at the southwestern limit of the Bay (Fig. 6b). The highest range occurs at Leaf Lake (161) where there is a spring tidal range of 12.19 m (hypertidal-D). A range of 11.09 m occurs at the mouth of the Koksoak River (163). This would fall into the hypertidal-C category. At Leaf Bay (159), the range is 10.97 m and at Hopes Advance Bay (157), the range is 10.49 m. Thus, both these sites have hypertidal-C ranges. Conversely, at the northern mouth of Ungava Bay, the tidal range is greatly reduced. At the eastern margin of Ungava Bay, the tides at Button Islands (167) are macrotidal (4.69 m). At the western margin, the spring-tidal range is 3.2 m. 3.2. Intertidal sedimentological zonation

Fig. 5. Landsat 7 image of the entire Turnagain Arm acquired on June 2, 2001. Cook Inlet splits into two arms at the city of Anchorage, Alaska. The northern split is Knik Arm and the southern split is Turnagain Arm. The highest reported tidal range occurred at the abandoned town site of Sunrise within Turnagain Arm. Zonal boundaries are delineated by dashed red lines. Zone 1 includes, shore-parallel linear bars. Zone 2 includes laterally extensive sand flats and Zone 3 is characterized by estuarine point bars. Image has been highly processed to accentuate areas of tidal deposition. Light tidal flats are tan to light brown in color. Non turbid water, including lakes, ponds and marine bays are very dark brown in color. (USGS image code: LE70680172001153AGS00).

Usable satellite images of Ungava Bay are limited because of storms, snow, ice coverage and low-sun angles at 59° N latitude. Much of Ungava Bay and most of Leaf Lake are frozen over during the winter (Fig. 7a). The entire area is undergoing relatively rapid post-glacial emergence (Gray et al., 1980; Lauriol and Gray, 1987) and this reduces the potential for shoreline development. The coastal areas of Ungava Bay are mostly tundra (Fig. 7b). The area is only sparsely occupied by Inuit (Eskimos) and a number of permanent villages have been

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Fig. 6. Tidal stations located within Ungava Bay area and the development of regional tidal amplification. (a) NOAA stations utilized for tidal-range data and (b) tidal amplification for Ungava Bay and surrounding areas of Arctic Canada. In Ungava Bay in northern Quebec Province in Canada, hypertidal systems are particularly well developed in the Leaf Lake area. Turnagain Arm has a higher latitude than Ungava Bay but has a much more temperate climate. Inland distance (North to South) from the red-dashed zero line together with maximum spring-tidal- range data were used to plot tidal amplification within Ungava Bay of northern Quebec, Canada.

established at the coast and along major rivers. The primary area of study was located near the village of Tasiujaq (Fig. 7c), which had a population of about 250 in 2004. This village is located at the mouth of the Bérand River estuary. This river empties into Leaf Lake, which in turn connects eastward to Leaf Bay. Leaf Bay forms the most extensive embayment along the southern coast of Ungava Bay. Tidal flats are common within embayments along the Ungava Bay shoreline. Discontinuous bodies of permafrost, which reach thicknesses of 5 m, occur within the silty sediments in the upper part of the intertidal zone (Sequin and Champagne, 1979; Michel et al., 1992). In general, salt marshes are a minor component of the North American Arctic. This limited distribution relates to a very short growing season, a general lack of fine-grained sediment, and generally low tidal amplitudes (Bliss and Matveyeva, 1992). The area was deglaciated only about 5000 years ago and there are few, low-lying or wave-cut platforms on the coast. The Leaf Lake area is an exception and many of the embayments that comprise the lake have noteworthy areas of salt marsh (see Fig. 7c). 4. Bay of Fundy — Salmon River estuary For over the past four decades, this area has been the subject of a number of detailed research projects (Klein, 1970; Dalrymple et al., 1978; Amos and Long, 1980; Dalrymple, 1984). At 45° north, the Bay of Fundy has a much less extreme climate when compared to Cook Inlet or Ungava Bay. Much of the available land surrounding the Bay is used for agricultural purposes and a strong anthropogenic overprint exists particularly within the more inland areas. Many areas were diked, as far back as the late 1600s, to prevent estuarine flooding of agricultural land.

4.2. Intertidal sedimentological zones The primary depositional zones are to the east and inland of Burntcoat Head. West of Burntcoat Head, the sedimentary dynamics are largely erosional and the coastline is commonly lined with bedrock cliffs. An extensive and en-echelon series of bars characterize Zone 1 and these extend inland to the mouth of the Schubenacadie River (Dalrymple et al., 2012). Because of the very high tidal range, the bar tops are well exposed during lowest-low tide and shore-attached bars can be readily accessible (Fig. 9). Flood-tide rise is much quicker than ebb-tide fall. Thus, excursions onto these bars are dangerous and the bars must be vacated well before the ebb-to-flood turnaround. The guidebook by Dalrymple and Zaitlin (1989) contains much useful information regarding timing of site visits. Zone 2 is characterized by extensive sand flats (Fig. 9). These flats are covered with a tremendous variety of ebb-directed, ripple-scale bedforms. Within channels simple two-dimensional dune-scale features can sporadically occur. Despite the widespread occurrence of these surficial bedforms, trenching indicates that these features are not necessarily preserved (Dalrymple et al., 2012). In its place the internal features of the sand flats consist of pervasive fine-scale laminae related to ripple erosion and subsequent deposition within upper-flow-regime conditions (Dalrymple et al., 2012). As the width of estuary decreases the sand flats are gradually replace by meandering of the fluvial channels and the formation of estuarine point bars of Zone 3. Tidal bores generate high turbidities in Zone 3 and rhythmites are commonly exposed in cutbanks. The eastern boundary of Zone 3 is defined as the tidal limit. The geographic position of the tidal limit, of course, varies related to changes in tidal heights and also varies with seasonal changes in amount of fluvial discharge. Thus, the inland boundary of Zone 3 is not static.

4.1. Tidal stations and inland amplification 5. Patagonian amplified tidal systems There are a number of tidal stations that surround the Bay of Fundy and the open-coastal area (Fig. 8a). Within the inner arms of the bay tidal flow is essentially bimodal with a SW to NE direction (Fig. 8b). The highest historical tides were reported at Burntcoat Head. Unfortunately, this historical tidal range includes the coincidence of a hurricane-related storm surge referred to as the Saxby Gale. This extreme tidal datum also includes estimated heights of storm waves that overtopped dykes in the area.

Argentine systems discussed herein are on the South Atlantic coast and are within the region termed Patagonia in the Argentine Province of Santa Cruz (Fig. 10a). The systems include, from north-to-south, the Rio Santa Cruz, Rio Coyle and Rio Gallegos estuaries. These sites are developed within a semi-arid region (Soriano et al., 1983). All these estuaries open directly to the South Atlantic Ocean. Unlike many of the other study areas, these systems do not have any type of intermediate bays or

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winds generated to the west in the Andes Mountains. Because the winds blow predominantly offshore, the potential marine-wave effects are reduced. The very wide continental shelves in the southwestern Atlantic Ocean are the source of the primary amplification. In the study area, even the open coastal areas tend to have hypertidal ranges. No other areas in South America have such high tidal ranges over such a wide region. The only other area along the east coast of South America with significant regional tides is the Amazon River mouth area (Archer, 2005). In that area the regional widening of the shelf in the Amazon area relates to the wide shelf created by tremendous output of sediments from the Amazon River (Rio Amazonas). By comparison, much of the west coast of South America lies along a convergent plate margin where the plates of the Pacific Ocean plate are being subducted under the South American continent. The plate geometry results in a very narrow continental shelf and this greatly reduces the potential for significant tidal amplification (Klein and Ryer, 1978). 5.1. Tidal stations and inland amplification Several Pacific-coast embayments have macrotidal ranges but no extensive hypertidal systems have been documented. For an analysis of tidal amplification in the Patagonian estuaries, a zero line can be defined that extends from Santa Cruz, Argentina (5259) to the north and then southward to Bahia Thetis, Argentina (5275). This includes the Straits of Magellan as well as other rivers and coastal stations in Argentina and Chile (Fig. 10b). The three estuaries within southern Patagonia share many similarities. The Rio Santa Cruz and Rio Coyle systems have been characterized as rias (flooded river valleys) by Piccolo and Perillo (1999). Conversely, because of faulting the Rio Gallegos estuary can be classified as a structural estuary. Less obvious structural controls, such as basement faults, may affect the fill geometries of all three estuaries. Patagonia underwent major climatic changes during the Cenozoic Era. The continuing uplift of the Andes Mountains, along the west coast of South America, has greatly reduced the amounts of precipitation that occur to the east. In the Patagonian study area, Rio Coyle is now dry for a significant portion of the year. If this continues, the Rio Coyle estuary could be losing its estuarine character. Patagonia is also notorious for the straight-line winds that descend from the Andes. These winds can reach 100- to 200-km/h and have significant effects on the sedimentary dynamics within the hypertidal depositional systems. 5.2. Rio Santa Cruz estuary

Fig. 7. Satellite image of the Ungava Bay–Leaf Lake study area. (a) Landsat 7 image taken on March 30, 2002 showing Arctic icepack totally covering Ungava Bay. Dark areas in the icepack are leads (fractures) that are forming ice breakup. Leaf Lake (dotted line) is totally iced over. Along the coast only the lower reaches of Koksoak River and Leaf Bay are unfrozen related to the zone of highest tides (USGS image code: LE70150192002089EDC00). (b) Landsat 7 image taken July 17, 2001 showing a complete lack of snow and ice. Red dashed line indicates the upper Leaf Lake study area. (c) Location of the village of Tasiujaq on Leaf Lake. (USGS image code: LE70150192001198EDC00).

inlets that might separate them from the open Atlantic Ocean. Thus, there is a much greater potential for marine-wave effects in the mouth areas. Patagonia, however, is renowned for its straight-line

Although the modern Patagonian estuaries are relatively small, they exhibit the highest tidal ranges in the Southern Hemisphere. In the compilation of Archer and Hubbard (2003), these estuaries and the eastern parts of the Straits of Magellan were listed as the 5th-highest tides in the world. All the Patagonian estuaries in this compilation have a restricted mouth and the estuary of the Rio Santa Cruz is no exception (Fig. 11). This constriction reduces the velocity of the incoming tide and greatly reduces the potential for tidal bores. Approximately 50 km from the coast, the estuarine system bifurcates into the Rio Santa Cruz and the smaller, northern tributary is termed the Rio Chico (Fig. 11). The inland limit of Zone 1 is about 6 km ocean-ward of this confluence. Extensive sand flats are characteristic of Zone 2 and the Santa Cruz retains a major fluvial channel along the east side of the estuary. Conversely, the sand flats of the Rio Chico nearly fill this smaller estuary. Estuarine point bars are quite small and cutbanks exhibit abundant, normally graded sand–mud laminae and thin beds. Some of the earliest and most extensive English-language observations were published by Darwin (1906) based upon information collected during the voyage of the Beagle. Darwin's observations are briefly summarized below. He noted that the river maintained nearly a constant width of

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Fig. 8. Tidal stations and tidal amplification within the Bay of Fundy, Canada. (a) Regional NOAA stations in the Bay of Fundy area of Nova Scotia and New Brunswick provinces of Canada. Because of the importance of navigation, particularly for marine fishing, there are many tidal stations in this area. (b) West to east tidal amplification based upon spring-tidal ranges and station distance from the red-dashed zero line. Areas at the bay mouth have spring-tidal ranges that are less than 4 m (mesotidal). Blue areas are snow cover and light-green areas are tree-filled valleys below the snow line. Extensive half-way up the bay, tidal ranges fall into the hypertidal-A range. Burntcoat Head has the highest reported tidal ranges and falls into the hypertidal-D category. Zonal classifications as defined by Zaitlin (1987) are utilized in this analysis. The Bay of Fundy in eastern Canada is world famous for high tides and the tidal range is commonly considered to be the highest in the world. Ungava Bay tides, however, have a similar magnitude.

approximately 300 m and with a channel about 5 m deep. It had rapid currents, ranging from 2 to 3 m/s. The river valley ranged from 8 to 16 km in width and was bounded by step-formed terraces rising to a height of 150 m. Another early account of the estuary was presented by Fitz Roy (1837). He described the estuary mouth as confined and there was a 5-hour flood tide followed by a 7-hour ebb tide. This type of tidal-duration asymmetry is characteristic of a wide spectrum of tidal settings. Fitz Roy described a tidal range that would be hypertidal-D. The mouth of the river was described as having a number of banks, which were composed of gravel and mud. At the mouth, the tidal currents were estimated to range from 1 to 2.5 m/s. In the mid-channel, the currents were estimated qualitatively to be as great as 3 to 4 m/s. Upstream, where the river was about 600 m wide, the tidal range was only 1.2 m. At this point, the flow was 3 m/s during the ebb and was 1 to 2 m/s downstream during the flood tides. Thus, at this point there were no reversing currents. The deepest channel was described as being about 5.5 m deep. More recently, the geomorphology of this estuary has been described by Piccolo and Perillo (1999). An ebb-tidal delta is formed at the 2-km-wide mouth. More inland at Santa Cruz City, the estuary expands to be 6 km in width (Fig. 11). The ebb dominance of the system, at its mouth, is reinforced by the prevailing very strong westerly winds and wind-generated waves. The coasts, particularly south of the estuary, are highly embayed and commonly have extensive sea cliffs. Intertidal flats within the widest part of the estuary are 2.8 km in width and extend for 12 km along the estuary. The tidal limit is approximately 70 km upstream from the coast for the Rio Santa Cruz and 47 km for the smaller Rio Chico tributary.

prior to European immigration and establishment of extensive sheep ranching. Periodic ash falls from Andean volcanoes results in soils contaminated by glassy ash shard. This material is extremely abrasive to grazer's teeth. These factors combined with a decreased global market for wool have forced abandonment of many ranches throughout the Patagonian region. 5.4. Rio Gallegos estuary The Rio Gallegos is the 2nd largest river, when compared to Rio Santa Cruz, in the Patagonian area. The tidal limit on Rio Gallegos estuary is approximately 50 km inland and is approximately 30 km inland for the much smaller Rio Chico (Fig. 13). This Rio Chico is not the same Rio Chico of the Rio Santa Cruz system. At the mouth, the system is only 3.3 km wide, and the oil-loading facility at Punta Loyola is located on the southern bank. Open coastal areas outside of the estuary have prominent, high-angle shingle beaches. 6. Bristol Bay–River Severn estuary In southwestern England, the system defined by the Bristol Channel to River Severn estuary has very high inland tidal ranges. This area has a humid, maritime climate with relatively warm winters. Thus, there is little to no winter-ice cryoturbation. The degree of anthropogenic influences is very high and the system has been extensively diked along all of the inner reaches. A number of changes within the channels were related to improvements to navigation. Nearly all modern river navigation is within canals that mostly parallel the Severn.

5.3. Rio Coyle estuary 6.1. Tidal stations and inland amplification The Rio Coyle estuary (Fig. 12) is by far the least accessible of the Patagonian systems. General aspects of this system have been previously described (Perillo, 1995; Piccolo and Perillo, 1999). A small village was formerly situated near the estuary mouth but it is now abandoned. The Rio Coyle is also known as Rio Coig and the system does not have any major tributaries. Throughout the Patagonian region, gradual desertification has occurred (Oliva and Borrelli, 1994; Oliva et al., 1994). There is almost a complete lack of baseline studies

There are numerous high-tide areas in Europe. Factors contributing to the high tidal ranges include the wide continental shelf that surrounds the UK and the relatively shallow English Channel. The Bristol Bay–River Severn estuary is commonly described as having the highest tides in Europe (Fig. 14a). Conversely, the highest tides in continental Europe occur along the English Channel in the Normandy region of France at Mont-Saint-Michel Bay.

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6.2. Intertidal sedimentological zonation The Bristol Bay–Severn River estuary has definitely had the most extensive study of all the sites discussed in this summary. Its historic navigation use and modification extends back over several centuries. For this area the primary research interests were primarily focused on the inland rhythmites (Zone 3). Descriptions of the more seaward zones were compiled from previously published materials. Similar to the Bay of Fundy discussed above, this area has undergone pervasive anthropogenic modifications (Fig. 15). The highest tides occur at Avonmouth (mouth of the Avon River). As compared to the overall size of the Severn estuary Avonmouth is a very small component and is located seaward of limits defined herein as Zone 1. The outermost boundary of Zone 1 is estimated to be near the confluence of River Wye (Fig. 15). This lower limit of Zone 1 is categorized by the first occurrence of mid-channel, non-attached bars. Zone 2 extends from this point upstream to Tites Point. Rhythmites in Zone 2 can exhibit annual cyclicity (Allen, 1990, 2004). Upstream of this point, Zone 3 is defined by a strongly meandering stretch of the river. This includes several double-type meanders. These features occur in several other systems and appear to be characteristic of bimodal flow around Zone 3 estuarine point. 7. Gulf of Saint Malo–Mont-Saint-Michel Bay

Fig. 9. Landsat 7 image taken on May 19, 2003 of the Cobequid Bay and Salmon River estuary in the Bay of Fundy, Canada. Highest tidal ranges have been recorded at Burntcoat Head. Great Village is near the site of an accessible linear bar in Zone 1. Powerline is an inner Zone 3 bar with sandy rhythmites, and Hill-Crowe is an inner Zone 3 bar containing silty rhythmites. Image was taken during a period of exceptionally low tides. These site terms were defined by Dalrymple et al. (1991a,b). (USGS image code: LE70080282003139C00).

Tidal bores within the Severn have been reported to reach heights of 2.5 m. Bores move at about 16 km/h. Bores form only during the highest spring tides and not all spring tides are high enough to produce bores. At Gloucester, the inland progressive of the bores, which formerly reached nearly to Worcester, has been eliminated by construction of small channel-crossing weirs (dams). Locally it is debated that extensive surfing and kayaking of the bore reduces the actual bore height. In the western parts of this area, high tidal ranges occur even on the open coast where spring hypertidal ranges exceed 8 m (Fig. 14b). There is progressive inland amplification with maximal ranges of 13.26 m at Avonmouth (997). Further upstream, tidal range undergoes a dramatic decrease and a range of 8.44 m occurs near Wellhouse Rock (1001) within the Severn River. Tides continue within the Severn for more than 50 km inland (Rowbotham, 1983). Tidal effects, at least in the past, extended to Worcester, where a 30 cm rise in the Severn River level was related to the downstream tides impeding the free outflow of river water.

The second European site is located in the coastal areas of Normandy, France. For convenience, this site is referred to as the Bay of Mont-Saint-Michel after the famous medieval village and abbey located in this area. The abbey and village were, before the addition of a causeway, regularly surrounded by water during high tides. As compared to the restricted, funnel-like shapes of most of the other sites, the Bay of Mont-Saint-Michel has a very broad and flaring geometry. Such geometries are commonly described as funnel shaped. Two small rivers enter in the upper reaches of the estuary. These rivers are bringing in essentially no terrigenous sediments. All the silt- and sand-sized sediment within the depositional system has been derived from finely comminuted shelly materials (Larsonneur, 1989). Shelly beach-like banks and cheniers occur along the southern coast of the bay and wave activity, over time, gradually reduces the shells to silt-sized sediments. Thus, the origin of the calcareous sediment is biogenic. This type of sediment provides a useful counterpoint to the other sites that all have terrigenous hypertidal systems. This difference illustrates that there is little difference between the bedforms and other sedimentary features produced within a calcareous depositional systems. 7.1. Tidal stations and inland amplification Very high tidal ranges occur throughout the English Channel that separates the southern coast of England from the northern coast of France. Hypertidal ranges occur within the Gulf of St. Malo (Fig. 16a) and this includes the cities of St. Malo (725) and Granville (729). To the east, extensive tidal flats surround the famous abbey of Mont-Saint-Michel (3). Within the Bay of Mont-Saint-Michel the sedimentary facies have been extensively studied (Larsonneur, 1989; Tessier et al., 1989; Tessier, 2012). The finer-grained sediments and sedimentary structures share many similarities with equivalent facies of the Bay of Fundy. Siltation has been very rapid and this has reduced the tidal ranges and rates of sedimentation within the system. The area remains a valuable modern analog for tidal rhythmite production (see Tessier, 1993). In the western part of the English Channel, a progressive tidal wave is formed that moves to the east. Throughout this entire area, coastal tidal ranges are macrotidal. Owing to the Coriolis deflection, which is towards the right in the northern hemisphere, higher tidal ranges tend to occur along the coast of France as compared to similar longitudinal positions on the English coast. This can be seen in a comparison of the higher ranges at the Morlaix River (705) and Paimpol

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Fig. 10. Tidal stations located within southeastern South America. This includes tidal stations on the South Atlantic coast, stations within the Straits of Magellan and stations along the Chilean coast. (a) NOAA stations that were utilized for tidal-range data. (b) Tidal amplification for areas including and surrounded the Straits of Magellan. Inland distances, from east to west, were measured from the red-dashed zero line. This data together with maximum spring-tidal- range data were used to plot tidal amplification within these areas of southern South America.

(717) in France as compared to the lower ranges at Fowey (963) and Salcombe (955) in the UK (Fig. 16a). The effects of tidal amplification are best expressed by the ranges along the French coast. Conversely,

Fig. 11. Landsat image of the Rio Santa Cruz area taken on September 5, 2005. Note restricted opening to the Atlantic Ocean. This reduces the speed of flood tides and precludes development of tidal bores within the estuaries. Zone 1 delineates area of shore-parallel linear bars. Zone 2 includes laterally extensive sand flats and shallow channels. Zone 3 includes estuarine point bars that contain tidal rhythmites. Upstream boundary of Zone 3 is the inland tidal limit. (USGS image code: LE72280952000248AGS00).

tidal ranges along the coast of England show a general decrease from west to east (Fig. 16b). Although it is not a simple funnel-shaped system, there are funnel-shaped components within the Gulf of St. Malo. The world's

Fig. 12. Landsat 7 image of Rio Coyle estuary taken on April 2 of 2001. System appears to be prograding as the estuary is filling. Ebb delta is beginning to form in the mouth area. Absence of freshwater flux may allow an influx of marine-derived sediments. Sand flats are particularly extensive in the upstream part of Zone 2. Zone 3 includes large, fluvio–estuarine point bars. (USGS image code: LE72280962001122AGS00).

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longest running tidal-power generating station is on the Rance estuary in St. Malo. This power station was built from 1961 to 1967 (Frau, 1993) and has been operational since that time. Within this complex system, the tidal ranges are highest at Granville (729) and Cancale (727), which have hypertidal-C ranges. Tides within this area are reported to reach up to hypertidal-E (Tessier, 1993). At St. Malo, spring tidal ranges have been reported to be hypertidal-D (Macmillan, 1966, p. 177). Tidal ranges decrease dramatically to the east and the tidal flux at Mont-Saint-Michel (3) is only mesotidal. At Avranche (5) and Pontabault (4), tidal flux falls into the microtidal range (2 m and less) and the inland tidal limit is within a few km to the east. 7.2. Intertidal sedimentological zonation Anthropogenic changes have greatly affected this region (Fig. 17), particularly around Mont-Saint-Michel. Large areas of salt marsh have been reclaimed by the construction of dykes. The relict salt marshes are used extensively for the grazing of sheep. Aquiculture of oysters and mussels is carried on throughout the intertidal zone of the Gulf of St. Malo. Zonal boundaries are not as clearly delineated as most of the other hypertidal systems. The occurrence of Zone 1 is somewhat patchy and is very gradational to Zone 2. The lateral extensive sandflats, that characterize Zone 2, are well developed throughout the estuary. The estuarine pointbars, which delineate Zone 3, are best developed after the inner estuary bifurcates near Arranche. The point bars are much better developed and laterally more extensive in the southern channel (Selune River). 8. Sedimentological zonation

Fig. 13. Landsat 7 image of Rio Gallegos estuary taken on April 2 of 2001. The city of Rio Gallegos is situated at the bifurcation of the Rio Chico (southern arm) and Rio Gallegos (northern arm). The term Rio Chico is widely applied smaller tributaries of larger rivers. Thus, the Rio Chico of the Rio Gallegos is a completely different river than the Rio Chico of the Rio Santa Cruz. Here the lower Rio Chico is almost totally blocked by extensive Zone 2 sand flats. Freshwater throughput in the Rio Chico is minimal. (USGS image code: LE7228092001122AGS00).

Sedimentary features within the three zones share a number of similarities as well as differences. For example, within the North American sites the effects of stranded winter ice can produce a strong deformation overprint. Conversely, within the South American and European sites, the effects of winter ice are minimal to nonexistent. 8.1. Zone 1 The highest tidal ranges tend to occur near the seaward boundary of Zone 1. In Turnagain Arm the highest ranges occur near Bird Point.

Fig. 14. Tidal stations located within Bristol Channel area and tidal amplification in this area of the UK. (a) NOAA stations utilized for tidal-range data. (b) Tidal amplification for Bristol Bay and the Severn River. Inland distance, from west to east, from the red-dashed zero line together with maximum spring-tidal- range data were used to plot tidal amplification. Open coastal tides are in the macrotidal to hypertidal-A ranges. At Avonmouth (997), extreme amplification can occur and this results in hypertidal-D ranges.

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they are not as commonly subaerially exposed because of the lower tidal range. Commonly the linear bars are not attached to the shoreline and form within the deeper, generally mid-channel regions. Linear bars tend to form more-or-less immediately inland of the location with the highest tidal range. Tops of these bars may only subaerially exposed during the lowest tides. Bar tops can be covered by dune-scale features that can be either flood or ebb oriented (Fig. 18a, b). Geomorphic controls can greatly influence the location and development of Zone 1 bars. In the Rio Gallegos estuary (Patagonia), the largest bars are concentrated along the northern, faulted margin of the estuary. Bank-attached bars are smaller along the non-faulted southern estuarine margin (Fig. 18c). Within the Salmon River estuary (Bay of Fundy, Canada), the linear bars are very well developed and can be readily observed in satellite images (Fig. 9). The bars form complexes and km-scale drainage channels are frequently oblique to the linear-bar orientations. These shallow channels delineate the individual components of the bars. The bar complexes reach lengths of 8 km and widths of 3 to 4 km. The sedimentology of these features has been described in detail (Zaitlin, 1987; Dalrymple et al., 1991a,b, 2012). The outer parts of the Salmon River estuary contain an abundance of sand and gravel. Much of this coarse-grained sediment was originally derived from erosion of sea cliffs along the more seaward parts of the Bay of Fundy. Dune-scale bedforms are commonly covered with ripples (Fig. 18d). Because of the high-energy depositional system, biogenic sedimentary structures (trace fossils and ichnofossils) are not commonly preserved. Shallow burrows do occur within the ripple troughs, which can remain damp for several-hour duration of subaerial exposure (Fig. 18e). In the Leaf Lake area (Ungava Bay, Canada), the abundance of ice-rafted boulders (Dionne, 1994, 2002) impedes the development of well-defined bar- and dune-scale sedimentary structures. In sand-rich areas, or boulder-free zones, dune-scale, ebb- or flood-directed bedforms are common (Fig. 18f). More commonly, Zone 1 is covered with ice-rafted boulders and these serve to greatly impede bedform development (Fig. 19a). In Leaf Lake, scours and erosion generates furrows around the boulders are common and gravel trails are indicative of the high velocities developed during flood tides (Fig. 19b). Ice rafting of large boulders also occurs, at a lessor scale, in Turnagain Arm (Fig. 19c) and in the Bay of Fundy (Fig. 19d).

Fig. 15. Satellite image taken on October 5, 1999 of the Severn estuary in southwestern UK. Inland parts of Bristol Bay are located near the bottom of the image. Highest tides have been recorded at Avonmouth. Zone 1 includes elongate, en-echelon bars. Zone 2 consists of extensive sandflats and shallow-water drainage channels. Estuarine point bars characterize Zone 3. Inland tidal limit demarcates the most-inland part of Zone 3. (UGSG image code: LE72030241999278AGS00).

A similar relationship is associated with Burntcoat Head in the Bay of Fundy area. Thus, there is a tendency for the depositional areas to form inland of the actual areas of the highest tidal ranges. 8.1.1. Longitudinal bars Because of the gradual inland narrowing within hypertidal systems rotational and amphidromies cannot be form. Rotary tides, however, can occur in systems where there is a wider and more marine outer bay, such as Cook Inlet (Alaska), Bay of Fundy (Canada), Bristol Bay (UK), and the Bay of St. Malo (France). Within the inland parts of the estuaries, which include respectively, Turnagain Arm, Salmon River estuary, River Severn estuary, and Bay of Mont-Saint-Michel, the tidal flow will exhibit current reversals. In such cases, the flood-directed tidal flow will have an angle of approximately 180° to ebb-oriented tidal flux. Thus, deposition in Zone 1 tends to create a series of longitudinal bars that are typically more-or-less parallel to the long axis of the estuary. Although similar bars occur within systems with lower tidal ranges,

8.1.2. Fringing mudflats Within most of the systems described herein, mudflats can occur along the shorelines of Zone 1 if there is sufficient low-lying topography. Unless the linear bars lie close to the estuarine shore, they can be very difficult and dangerous to reach on foot. Fringing mudflats in this zone tend to be very highly bioturbated and, as a result, very thixotropic (Fig. 20a). These soupy mudflats can make it difficult to reach the bars from the shoreline. The intense bioturbation results in the destruction of many of the internal sedimentary structures. As compared to the inner zones the mudflats in Zone 1 are much more stable in terms channel positions and lower rates of sedimentation and erosion. Thus, the Zone 1 flats are commonly covered by green-colored biofilms (Fig. 20b). These organic films serve to reduce the effects of erosion on the mud flats. Because Zone 1 is within the wider, more open part of the estuarine embayment, wave-generated ripples are common where sands have been erosively concentrated on the flats (Fig. 20c). The surfaces of the wave-rippled sands common are covered by a variety of marine algae and the shells of burrowing bivalves. 8.1.3. Marsh Fringing and erosional sea cliffs are common along much of the sedimentary system. Only the most inland parts of the systems tend to be actively depositional. This is particularly true at Turnagain Arm and the Bay of Fundy. In these systems, there is only a limited amount of low-lying and low-slope area that is available for

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Fig. 16. Tidal stations located within the western part of the English Channel. This includes the Gulf of St. Malo and Mont-Saint-Michel Bay in France together with coastal areas along the southern coast of the UK. Data from these stations was used to depict regional tidal amplification. (a) NOAA stations utilized for maximum spring-tidal-range data. Highest tides occur at Granville (729). (b) Tidal amplification from east to west within the English Channel. Inland distance (west to east) from the red-dashed zero line together with maximum spring-tidal-range data were used to plot the include increase in tidal amplification.

salt-marsh development. Within small embayments along the estuarine margin, mud-rich sediment accumulates and allows the growth of small patches of salt marsh. Marsh development has also been greatly reduced by anthropogenic activity. The original salt- to brackish-water marsh areas in the Salmon River estuary (Bay of Fundy), Severn River estuary, and Mont-Saint-Michel Bay are now behind dikes that prevent brackish-water tidal flooding. These areas are now extensively utilized for livestock grazing. In other areas, the marshes are still essentially pristine. This is true in both the Leaf lake estuary and the Patagonian estuaries. In the Santa Cruz estuary of Patagonia, the fringing marsh is dominated by halophytes such as Salicornia (Fig. 21a). Within the uppermost part of these salt marshes, there are a number of aligned, elongate, non-connected pools surrounded by vegetation (Fig. 21b). In the Rio Gallegos estuary, the pools are aligned parallel to the strong inline winds that blow from west to east in this area. Thus, the prominent orientation is apparently related to wind erosion (Perillo et al., 1996). Either the wind is actively eroding the pool margins or wind-generated waves are eroding the edges of the pools. Salt marshes are rare in Arctic Canada but they do occur in the hypertidal areas of Leaf Lake. These marshes are covered with a distinctive, low-lying grass type of vegetation (Fig. 21c). Aligned pools (Fig. 21d) are similar to the wind-eroded pools in the Rio Santa Cruz estuary.

Fig. 17. Satellite image taken on April 16, 2003 of the study area in the Gulf of St. Malo and Bay of Mont Saint Michel, France, and surrounding areas. Highest tides recorded in this area occur at Granville. Near St. Malo, La Rance tidal power station has been operational since 1966. The tidal barrage and reservoir are immediately south of St. Malo. This area has an 8-m tidal range. Zone 1 and Zone 2 boundaries are somewhat arbitrary. Distinct linear bars are not evident, however tidal channel development is better defined in Zone 1. Zone 2 sand flats are very extensive and continue for a great distance to the western part of the bay. Two small tidal rivers constitute Zone 3. (USGS image code: LE720202003106EDC00).

8.1.4. Tidal bores Because of the greater width of the hypertidal systems in Zone 1, bores generally do not form. Commonly bores form more inland and also tend to form where there is a significant geomorphic constriction. In Turnagain Arm, the bores, observable on the northern side, form up estuary from near Bird Point (Fig. 5). Conditions within Zone 1 do not favor bore development. Conversely, the widespread depth reduction across the Zone 2 sand flats appears to favor bore developing. Accordingly, more information regarding bores is presented in the Zone 2 and Zone 3 discussions. 8.2. Zone 2 8.2.1. Sand flats Extensive sand flats characterize Zone 2. In Mont-Saint-Michel Bay, the flooding of these flats is traditionally described as faster

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Fig. 18. (a) View to south of upper portion of linear bar (lower Zone 1) exposed in Turnagain Arm. These bars are emergent only during the lowest-low tides. View is from north shore of Turnagain Arm looking to the SSW. Dune-scale bedforms on the bar top are ebb oriented (to the left) and have a horizontal spacing of approximately 1.5 m. Turnagain Arm is approximately 7 km wide at this point (2001/05/25). (b) Linear bars, covered with dune scale features, near the seaward limit of Zone 1 in the Severn estuary. View to south from north bank of estuary. Bars are separated by a broad tidal channel. Dunes are oriented in the ebb direction (to the right). Dark material in the foreground is a bank-fringing, highly bioturbated mudflat (2009/09/03). (c) View to the north in the Rio Gallegos estuary showing (from foreground to background) gravel beach (shingle), fringing mudflats and linear bar. Bar top exhibits a number of small, oblique drainage channels. Positions of the drainage channels are controlled by low-amplitude dunes, which have a wavelength of approximately 6 m. The northern faulted margin of the estuary can be seen on the far side of the estuary (2005/03/25). (d) View to the south from Zone 1 in the Salmon River estuary in the Bay of Fundy, Canada. Bar top exhibits ebb-directed dune-scale bedforms. The ebb-oriented upper surface of the dunes is commonly only superficial. Most of the underlying portions can exhibit flood orientations (Dalrymple et al., 2012). Bedforms are covered by multidirectional drainage-related ripples. Machete is 50 cm tall (1989/08/14). (e) Ladder-back ripple marks formed on top of dune-scale bedforms in the Salmon River estuary, Bay of Fundy, Canada. Snail-type shallow burrows are concentrated in still damp, ripple troughs. Both dunes and ripples, in this view, are ebb oriented. The ripples smaller were formed during late-stage emergent runoff. Because of the topography of the dunes, ripple-mark orientations exhibit a wide spectrum. Ripple tops have been flattened by wind-generated waves during the final phases of late-stage emergence (1989/06/11). (f) Relatively boulder-free area in Leaf Lake (Ungava Bay, Canada) estuary displaying dune-scale bedforms. Ripple fans, which formed during late-stage emergence, exhibit a variety of flow directions. Scale is 50 cm long (2005/07/22).

than a horse can gallop (B. Tessier, pers. comm., 1989). That would entail velocities of approximately 20 km/h or more. All areas of hypertidal systems are potentially dangerous to visit. Rapid rates of tidal flooding occur across the low-relief, Zone 2 sand flats. This can make these areas particularly hazardous. The laterally extensive flats are composed of silt and sand. The Patagonian estuaries are generally smaller in scale than the North

American and European systems. Nonetheless, there is a considerable areal extent of sand flats in the Patagonia estuaries (Fig. 22a). Sand flats within the Bay of Fundy study area have a similar extensive distribution. Within any of the localities, the effects of wind reworking are evident on the sand flats (Fig. 22b). In such cases, the current-generated ripples can be significantly modified by eolian reworking.

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Fig. 19. (a) Series of dune-covered bars within the Leaf Lake estuary. Concentration of pack-ice rafted boulders apparently has greatly reduced the potential for ripple development. View to the northwest (2005/08/22). (b) Gravel-filled furrows, in the Leaf Lake estuary, created during flood-directed flow. Ripples are ebb-directed. Scale directly above largest boulder is 40 cm long and boulder diameter is approximately 60 cm. Heavy minerals are concentrated in furrows and ripple faces (2005/07/22). (c) Fringing Zone 1 mudflats in Turnagain Arm, Alaska, containing ice-rafted boulders. Large boulder on right is approximately 1 m in diameter. Surrounding muds are very thixotropic because of extensive bioturbation. This area exhibits low rates of sediment accumulation. (d) Ripple-covered dunes in Zone 1 of the Cobequid Bay, Nova Scotia, Canada. View is to the south. Ice-rafted boulders, up to 1 m in diameter, occur primarily in the dune troughs. Erosional pools in lower right are rimmed with mud drapes that will most likely be eroded during next flood tide (2012/06/15).

In Turnagain Arm, the transition from the linear bars to the laterally extensive sand flats (Zone 2) occurs near a bedrock ridge named Bird Point (see Fig. 5). Here a bedrock ridge extends well into the estuary and the width of Turnagain is greatly reduced. Most tidal bores initiate near and to the east of Bird Point. Within this part of Turnagain Arm, the estuary width decreases from 4.7 to 3.2 km at Bird Point. The main tidal channel (2–3 km in width) splits into two channels, each with dramatically smaller widths (>0.5 km) than the main channel of Zone 1. Examination of historic aerial photos and maps by Bartsch-Winkler (1986, 1988) indicated that there has been significant variability within the number, width, sinuosity and position of channels within Zone 2. Channel shifting that occurred after the 1964 earthquake is inferred to have been caused by the extensive subsidence along the north shore of the arm (Bartsch-Winkler, 1986, 1988). Subsidence of 2 to 3 m also forced the relocation of Girdwood and abandonment of the village of Portage and a rise in sedimentation rates (Ovenshine et al., 1976). Tidal bores tend to first appear on the sand flats because of the general, nearly channel-wide shoaling (Fig. 22c). The tidal channels are much shallower than the channels that separate the bars in Zone 1. Thus, a hydraulic jump occurs at the Zone 1–Zone 2 transition. This relatively abrupt change in bottom topography serves to initiate bore development. Bores are also common in Zone 2 of the Severn estuary and attain their maximum height in channels that commonly form the landward edge of the sand flats (Fig. 22d). Within Turnagain Arm, the sand flats cover nearly 90% of the Zone 2 during low tide and are completely flooded during the subsequent high tide (Fig. 5). Turnagain Arm tidal bores are impressive because of the lateral widths of the sand flats. Bores commonly exceed 2-m in height during spring tides (Fig. 22e). Even during neap tides, the bores are commonly at least 50 cm in height (Fig. 22f). The consistency of bore formation within nearly every flood tide sets Turnagain Arm apart from the other systems.

The flats in Leaf Lake estuary are also unique because of the large amounts of boulder-rich sediments. This material is rafted into the depositional system by winter-season pack ice (Fig. 7a). Because of winter and spring ice transport, the flats are covered by a great variety of relatively evenly spaced boulders (Fig. 23a). The boulders are lifted up as the ice freezes at the bottom and melts (sublimates) at the top. This process can continue until the boulders actually sit on top of an ice floe. Large boulders can be ice transported for considerable distances until the ice melts. Because of the very high tidal range in Leaf Lake, gigantic boulders are lifted by winter ice and then deposited on the flats during spring melt (Fig. 23b). In addition to granitic compositions many of the ice-rafted boulders are composed of Proterozoic-age, banded-iron formations. Iron formations have a specific gravity about twice that of granite. The weight and size of such boulders are phenomenal. 8.2.2. Ripple marks A tremendous variety of ripple marks occur in the laterally extensive sand flats of Zone 2. Although sand flats are extensively covered with ripple marks, these features in some settings may not have a very high preservation potential. Trenching into the Bay of Fundy sand flats indicates that ripple-scale features are rarely preserved. Instead, upper-flow regime, planar bedding is common (Dalrymple et al., 1991a,b). Most of the ripples are formed by ebb-directed flow and are produced during late-stage emergent runoff. One of the most indicative features of reversing flow is the common occurrence of double-crested ripples (Fig. 23c). The secondary crest is usually initiated near the top of the primary crest. Ripples with rounded and reworked crests are also very common (Fig. 23d). Ladder-back ripples are particularly common on the sand flats and indicate that the flow direction during late-stage emergence was at approximately 90° to the dominant flow (Fig. 23e). Bioturbation is generally lacking within ladder-back ripples. Ladder-back ripples

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Fringing mudflats also occurs laterally to the sand flats in the Severn estuary. In some areas, outcrops of Paleozoic-age bedrock occur within the intertidal mudflats (Fig. 24b). Bedrock pebbles scattered throughout the modern estuarine sediments are good indicators of the antecedent, fluvial-incised valley. 8.2.4. Fringing marsh In the areas with a high degree of anthropogenic modification such as the Bay of Fundy, Severn River and Mont-Saint-Michel Bay, much of the salt marsh has been eliminated and converted to agricultural uses. The most common type of modification is the construction of dykes that are utilized to prevent brackish-water flooding. Over time, the original salt marsh will become a freshwater environment. Despite this anthropogenic overprint patches of marsh sediments still occur. In actively accreting, upper-intertidal marsh sediments mm-thick laminations are common. In the Severn estuary, these laminae consist of alternating layers of sands separated by mud drapes. A series of sand–mud laminae are separated by a thicker (1 to 2 cm) mud-rich zone. Many of the sand-rich laminae exhibit an erosional lower boundary (Fig. 25a). This type of rhythmite has been interpreted as yearly and related to seasonal, temperature-related changes in water density and viscosity (Allen, 1990, 2004). In the Selune estuary of Mont-Saint-Michel Bay, marsh deposits exhibit a very fine scale of lamination (Fig. 25b). Because there are more laminations than would occur within a single neap–spring cycle, such cyclicity has been interpreted as annual (Tessier, 1998). Within cutbanks in the fringing mudflats in the Bay of Fundy, there is a similar cyclicity that has also been interpreted as a yearly period (Fig. 25c). Thin laminated zones, which contain more sand, serve to separate thicker, bioturbated zones. In the Rio Santa Cruz estuary in Patagonia there are similar cycles seen in marsh sedimentation. In this case, well-defined sand–mud laminations are separated by more massive, sand-rich zones (Fig. 25d).

Fig. 20. (a) Highly thixotrophic mud containing abundant burrows of Arenicola sp. The burrowing activity tends to be extensive and few physical sedimentary structures are evident. (b) Fringing mudflat undergoing erosion in the outer part of Zone 1. The light green color indicates development of a sediment-binding biofilms. From Turnagain Arm, Alaska. Scale is 30 cm (2005/07/18). (c) Wave-generated ripples from the shoreline of Zone 1 in Turnagain Arm. Several types of algae are stranded and the ripple troughs contain concentrations of macerated plant materials (2005/07/15).

can be further eroded during the subsequent flood tide so the only a small pit remains where the troughs of the small-scale ripples previously existed (Fig. 23f). 8.2.3. Fringing mudflats A relatively narrow band of mudflats can form along the coastal margins of the sand flats. These mudflats occur within the upper-intertidal zone. In Turnagain Arm, regional subsidence of 2 to 3 m occurred during the large earthquake of 1964. The subsidence lowered a number of supratidal features down into the intertidal. Extensive groves of dead spruce trees, now within the intertidal mudflats, have recorded the abrupt environmental change associated with tectonism. These trees are still standing after nearly 5 decades (Fig. 24a). The cold climate and density of old-growth spruce wood serve to reduce the rates of wood decay in the subarctic setting.

8.2.5. Tidal bores Significant narrowing of the estuarine channels commonly results in a dramatic transition from Zone 2 to Zone 3 (Fig. 26a). This narrowing can result in an increase in bore heights (Fig. 26b). Bores are common only within estuarine hypertidal systems and are essentially similar to shock or explosive waves. Bores tend to form immediately following the lowest, ebbing tide. The incoming flood tide forms a wall of water the moves upstream at speeds ranging from 10 to 20 km/h. Local, day-to-day factors such as wind speed, wind direction, and amount of fluvial discharge can greatly affect bore heights as well as bore-arrival times. Turnagain Arm (Alaska) has exceptionally consistent bores. The bores can attain nearly 3-m heights during spring tides (Fig. 22e) and approximately 50 cm during neap tides (Fig. 22f). Tidal bores are also common throughout the Bay of Fundy area and reach heights of 2 to 3 m. In the Severn estuary, bores are only developed during the highest spring tides. Neap tides do not generate bores. Construction of low dams (weirs) across the Severn has eliminated bores from the upper parts of Zone 3. Similar to Turnagain Arm, it is possible to follow a single Severn bore and view it a number of times as it progresses upstream. The Severn bores reputedly attain heights of nearly 2 m. Within the innermost parts of the Bay of Mont-Saint-Michel bores that approach 1 m in height can occur during spring tides. 8.3. Zone 3 8.3.1. Estuarine point bars The transition from Zone 2 to Zone 3 is gradual, but is characterized by a change from laterally extensive tidal flats to laterally restricted estuaries. There is also a narrowing of the estuary and widening of bordering marsh formed in the uppermost intertidal. Erosion of estuarine point bars exhibits a high degree of laterally

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Fig. 21. (a) Lower part of eroded fringing salt marsh in the Rio Santa Cruz estuary. Halophytes are exhibiting fall colors. High-water gravel beach (gb) in left-side background. Mudflats (mf) will be submerged during high tide. Above this is zone of semiarid-shrub vegetation. Cliffs in background, covered by semi-arid shrubs, contain Miocene sedimentary rocks (2010/03/ 14). (b) Fringing salt marsh undergoing erosion in the lower estuary (Zone 1) of the Rio Santa Cruz estuary in Patagonia. Marsh plants, mostly halophytes of the genus Salicornia, are exhibiting fall coloration. Mud flats (mf) are in upper left and a gravel beach-berm (gb) is on the left. Irregular, unconnected ponds formed by wind erosion (described by Perillo et al., 1996. The zone of shrubby vegetation above the beach is the typical semi-arid flora for this area. Background cliffs are composed of Miocene sedimentary rocks (2010/03/14). (c) View to the north of the lower salt marsh covered with Puccinellia phryganodes in the Leaf Lake area of Ungava Bay. Large boulders were ice-rafted and then dropped onto the marsh surface. Linear trails in the vegetation are caribou (reindeer) trackways formed when herds of 100,000 or more migrated through this area to more northerly summer range. Because of a lack of suitable low-lying topography and because of low tidal ranges, salt marshes are rare in most other areas of the Canadian Arctic (2005/07/14). (d) Irregular water-filled pools in the uppermost salt marsh of the Leaf Lake area. These elongate features are similar to wind-eroded features formed in Patagonian (2010/03/14).

continuity in the laminated sediments (Fig. 27a). Similar cutbanks with laterally continuous thin beds also occur in the rivers that empty into Mont-Saint-Michel Bay (Fig. 27b). Because of a lack of clay within Turnagain Arm, tidal bedding (such as lenticular, wavy, and flaser bedding) is practically absent (Bartsch-Winkler and Ovenshine, 1975; Bartsch-Winkler and Schmoll, 1984). The surface of the estuarine point bars can exhibit ebb-directed ripples and other bedforms (Bartsch-Winkler and Ovenshine, 1975, 1988; Bartsch-Winkler et al., 1975); however, these are commonly reworked by the flood tides. Thus, the primary physical fabric that is preserved within the sediment on point bars is dominated by planar beds (Fig. 27c). Following the 1964 earthquake, tidal silts were rapidly deposited in area of quake-related subsidence (Ovenshine et al., 1976; Ovenshine and Bartsch-Winkler, 1978; Bartsch-Winkler and Garrow, 1982; Atwater et al., 2001). Channel shifting within this zone of Turnagain Arm has been related to earthquake-related aggradation (Bartsch-Winkler, 1988). In the Rio Gallegos estuary in Patagonia, cutbanks within the estuarine point bars commonly exhibit fine-scale lamination (Fig. 27d). This lamination is similar to thick laminae and thin beds that can be observed in the Severn estuary. Each lamina and thin bed is normally graded. Extensive bioturbation is almost always absent within this facies. A number of different types of surficial trails and trackways can be found, including those produced by insects, fish, birds, and mammals (Archer, 2004). These surficial markings are very similar to those described from the Bay of Fundy and Bay of Mont Saint Michel in France (Tessier et al., 1995) and other modern and ancient

hypertidal settings (Archer, 2004; Archer and Greb, 2012). Surficial marks can be made by fish at high tide when the bar tops are flooded. Conversely, during low tide subaerial exposure, insects and birds will land and create trackways. A variety of ripples occurs on the tops and margins of the estuarine point bars. Double mud-draped ripples were observed in the Rio Gallegos estuary (Fig. 28a). Double crested ripples from Turnagain Arm are also common. In areas near the small, glacial-melt-water rivers, gravel-sized sediments can be concentrated in the ripple troughs (Fig. 28b). Within areas of rapid sedimentation in Turnagain Arm, mud-draped ripples can be preserved (Fig. 28c). Tiling experiments confirmed that each ripple and associated mud drape were deposited during a single, semidiurnal tidal event. Margins of the estuarine point bars likewise exhibit a variety of scale of drainage rills. These rills can be superimposed on any precursor sedimentary structures (Fig. 28d). Close to the inland tidal limit (innermost Zone 1), ebb- or flood-oriented, climbing-ripple lamination is common (Fig. 28e, f). In this feature, the ripples have rounded crests and can have a mud-rich drape formed over each set of ripples. Similar features have been described from paleotidal facies of Carboniferous Period (Kvale and Archer, 1991) and modern settings (Lanier and Tessier, 1998; Choi, 2011). In modern settings, these features can be observed to form on different sides of a single bar and can exhibit bimodal current orientations (Choi, 2011). Thus, this type of climbing-ripple lamination is probably not very reliable as a paleoflow indicator. Thick–thin pairs may indicate a diurnal inequality (Fig. 28e) and tidal periodicities can be formed within Zone 3 (Fig. 28f).

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Fig. 22. (a) View to northwest of laterally extensive sand flats in Zone 2 of the Rio Gallegos estuary in Patagonia. Scarp in background is fault scarp in Miocene age sedimentary rocks. Note gravel (shingle) beach (gb) on mid-left part of photograph. (b) Blowing sand on sand flats of the Salmon River estuary, Bay of Fundy, Canada. View is to the east and wind is blowing from the east. Because of high winds and low humidity, the ripped surface is being eroded by 15 km/h wind. Ripple troughs are collecting dry and wind-blown sand. The characteristic red-brown color of sediments is related to weathering of Triassic redbeds that fringe the Bay of Fundy. (c) Tidal bore, approximately 30 cm in height crossing the inner portion of Zone 2 in the Salmon River estuary. This is near the initiation area of most of the bores. Because of the width of estuary at this point, the bore is not well organized and consists of at least 3 distinct lobes. Upstream in Zone3, the separate lobes can combine to form a 1-m-tall or higher bore. Large sand flat on the left and view is to the south (2006/07/14). (d) Laterally extensive, mid-channel sand flat in Zone 2 of the Severn estuary. Standing waves in the channel are characteristic of fluvial flow during late-stage ebb. View from top of a cutbank that is eroding into the marsh. (e) A 2-m-high tidal bore within Turnagain Arm, Alaska. Note kayaker and surfer (ks) riding the bore. Bore in lower right is a breaking-wave type and bore in upper part of photograph is largely comprised of whelps. Standing waves (sw) in left-center are within a shallow channel and are being generated by ebb-directed fluvial flow (2007/05/18). (f) Neap-tidal bore crossing sand flats in Zone 2 of Turnagain Arm with 3 distinct lobes. Bore has formed 3 distinct lobes. The uppermost lobe is a non-breaking, whelp-covered bore. Lowest lobe is traveling down a shallow channel. Middle lobe is slower because it is crossing a ripple-covered sand flat. Erosion of low-water mud drapes, in the middle lobe, is creating very high turbidity along the margin of the bore lobe (2001/05/017).

8.3.2. Rhythmites Within Zone 3, both flood and ebb tides can deposit sediments on the point bars. The ebb-related sedimentation is also closely related to the fluvial throughput that occurs during low tides. In Zone 3, very narrow cross-sectional geometry of the estuary greatly reduces the potential for erosion by wind-generated waves. Much of Zone 3 in all the study areas can exhibit rhythmite deposition. Such conditions can also occur in protected embayments throughout Zones 2 and 3. The presence of a variety of rhythmites is probably among the best indicators of hypertidal conditions. For neap–spring rhythmites to form, very high rates of sedimentation are necessary. Some of the earliest descriptions of tidal rhythmites were from Mont-Saint-Michel Bay (Tessier et al., 1989; Tessier, 1993). These rhythmites included sub-daily to daily laminations along with a

systematic thickening and thinning of thicknesses of successive laminations (Archer, 1995). The systematic thickness variations in successive laminae were related to neap–spring tidal cycles (Fig. 29a). Such features were also described essentially simultaneous from the Salmon River estuary in the Bay of Fundy (Dalrymple and Makino, 1989) and from rocks of the Paleozoic and Proterozoic (Archer and Johnson, 1997). These rhythmites include planar-laminated forms (Fig. 29a). In addition, thinly bedded silty rhythmites with bed thicknesses of approximately 1 cm were reported. Cm-scale, bedded rhythmites in Mont-Saint-Michel Bay also contain impressions of hoof prints (bioturbation) made by grazing sheep on the halophyte-covered upper-intertidal flats (Fig. 29b). In the Patagonian estuaries, similar planar-laminated, normally graded rhythmites were observed in cutbanks that dissected estuarine point bars in Rio Gallegos estuary (Fig. 29c).

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Fig. 23. (a) Sand flat along margins of Leaf Lake estuary in Ungava Bay. Boulders have been ice rafted on the flat. Note that the boulders are sitting on the flats and are not even partially buried. In foreground is a pebbly, storm-water beach; black material forming a strandline is desiccated seaweed. Growing plants indicate initial marsh development (2005/07/14). (b) Large granitic boulder at Leaf Lake stranded on sand flat during spring melting of floating pack ice. Boulder is surrounding by smaller boulders, cobbles and gravel. Muddy sand is accumulating in the low areas surrounding. The unusual size of such ice-rafted boulders is probably related to the very high tidal range. The entirety of Leaf Lake is frozen over during the winter and early spring (Fig. 7a). (2005/07/12). (c) Flood-tide ripples with reworked, secondary ebb-oriented crests in Turnagain Arm. Extended part of scale is 30 cm. Such double-crested ripples are a good indicator of reversing flow within sediments (2005/07/15). (d) Flood-directed ripples in the sand flats of the Rio Gallegos estuary in Patagonia. Light-color sediment concentrated in troughs is volcanic ash. The origin of the ash is from large eruptions of Andean volcanoes. High-level winds can transport such pyroclastic sediments across the entire width of the continent of South America (2005/03/20). (e) Larger-scale ripples almost totally reworded by smaller, ladder-back type ripples. From Turnagain Arm. Drainage rills are also forming in the troughs of the larger ripples. Note total lack of surficial bioturbation or macrobiota (2006/08/05). (f) Ladder-back ripples that have undergone erosion and been converted to nearly flat-topped ripples. Small ovoids on the flat-top surface are the remains of the original ladder-back ripple troughs. The sequence of events was forming of large ripples, then development of ladder-backs during late-stage-emergent runoff, and finally erosion by the subsequent flood tide that planned off the ripple tops. From Turnagain Arm (2004/07/07).

Of all the study areas, the best developed and most consistently generated rhythmites are within the estuarine reaches of Turnagain Arm in Alaska. This probably relates to the fine-grained nature of the sediments that are dominated by quartz silt (rock flour) produced by the extensive glacial activity in the area. Typically, the rhythmites consist of mm-thickness, normally graded laminae (Fig. 29d). Neap–spring cycles can be commonly observed in cutbanks formed by the erosion of estuarine point bars. Other, more complex tidal cycles, such as apogean and perigean tidal periodicities, have been reported (Greb et al., 2011). Thin intervals of extensive soft-sediment deformation are also common (Fig. 29e). Some earlier interpretations, made before the rapid nature of the sedimentation was recognized, suggested that the soft-sediment deformation (SSD) was more-or-less directly tied to seismic events. Thin zones of SSD bounded by non-deformed laminae definitively indicate a non-seismic causation of this deformation (Greb

and Archer, 2007). Similar planar-laminated rhythmites occur in the sandy flats of Leaf Lake (Fig. 29f). Because of the general lack of fine-grained sediments in the Leaf lake area, the rhythmites were observed only to be forming within scoured depressions created by erosion surrounding large, ice-rafted boulders. 9. Discussion Given the diversity of the locations with hypertidal system, it should not be surprising that there are many dissimilarities as well as similarities. One prominent feature is that all the systems have formed within bedrock-confined paleovalleys. Because of their proximity to coastal area, these bedrock valleys were generally eroded during Pleistocene sea level lowstands. As such, these systems provide potentially useful modern analogs for the study and analysis of

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Fig. 24. (a) View from Zone 2 in Turnagain Arm inland to Zone 3. Modern salt marsh in lower left and blocks of root-stabilized marsh sediment has been lifted from marsh surface and carried down onto muddy flats. Dead Sitka spruce in foreground and spruce grove in background were killed when regional subsidence of 2 to 3 m, related to the 1964 Alaskan earthquake, allowed brackish-water intrusion into what had previously been a freshwater marsh (2003/08/24). (b) Erosional cutbank/slump in the Severn estuary Zone 2 to Zone 3 transition. Marsh sediments, held together by roots, are slumping into the channel. Apparent “bar” in left-middle part of photograph is outcrop of folded limestone bedrock (2009/08/27).

Fig. 25. (a) Annual cycles (white lines) in Zone 2 of the Severn River estuary, UK. Scrapped surface shows cm-scale annual banding. Scale bar in cm. Note the alternation of bundles of thin sand–mud laminae that are separated by cm-thick massive mud layers. There are numerous small-scale, erosional truncations (1989/06/07). (b) Finely laminated sediments exposed in a cutbank along the Selune River in the Mont-Saint-Michel area. Light-colored areas are sand ripples and thin beds. Overlying marsh has abundant halophytes. Thicker, indented areas on the cutbank face are composed primarily of more easily eroded sand. Sand-rich zones appear to delineate annual cycles (white lines). (c) Yearly cycles in marsh sediments in cutbank in Zone 2 of the Salmon River estuary. Massive zones are overlain by 2- to 3-cm of mm-scale lamination (indicated by white lines). Bioturbation include grass roots and vertically oriented bivalve burrows. (d) Annual cycles (white lines) in Zone 2 of the Rio Santa Cruz estuary in Patagonia (Argentina). Five distinct units (3 laminated and 2 massive) are evident in this naturally etched, cutbank exposure. Despite the high degree of rooting, the sediments have retained their original, horizontal depositional fabric (2005/ 03/24).

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Fig. 26. (a) Transitional area between Zone 2 and Zone 3 in Turnagain Arm, Alaska during late-stage ebb. View is to the east and taken from a large ripple-covered fluvio–estuarine point bar. Note washed-out ripples were formed by sediment reworking during ebbing tides. Glaciers on the mountains in the background provide much of the freshwater input for the estuarine system. Cut banks occur along both sides of the estuary. Owing to very low salinities in the head of the estuary, there is no brackish- or saline-water marsh. Brown-colored, low shrubs along the cutbanks have not yet leafed out (2001/05/19). (b) Extremely turbid bore crashing onto the cutbank in Zone 3 of the Salmon River estuary. The mid-channel bore exhibits whelps as well as a breaking-wave morphology. Highly turbid bore in foreground is eroded previously deposited high-water mud drape. Lower margin of bore is being refracted by the steep cutbank (2006/07/14).

incised-valley facies (IVFs). Consequently a potentially major sequence boundary formed at the base of the estuarine–fill sequence (Dalrymple et al., 1991a,b, 2012; Tessier, 2012). Ancient hypertidal systems have yet to be extensively recognized. This is, at least in part, due to a lack of comprehensive study and descriptions of modern analogs. Stratigraphic sections that contain abundant mm- to cm-scale rhythmites have traditionally and commonly been interpreted as lacustrine, distal turbidites, or a host of other

interpretations. Traditional interpretations commonly invoke lowenergy, waning-flow settings. Re-evaluation of such sections may reveal here-to-forth unreported tidal influences. This ongoing study of hypertidal systems was originally undertaken in order to find analogs for rhythmite-rich, Carboniferous-age paleotidal systems. These paleotidal deposits are common within the Eastern Interior Coal Basin (Illinois Basin) of the US (Kvale et al., 1989; Kvale and Archer, 1990, 1991; Archer, 2004; Archer and Greb, 2012).

Fig. 27. (a) Cutbank, approximately 2 m tall, in Zone 3 of the Severn estuary. Note lateral continuity of the thin beds. The cyclicity within these beds has been interpreted as annual (Allen, 1990, 2004). Patches of plants in foreground are root-bound masses that were erosionally undercut and subsequently redeposited at a lower positional within the tidal frame. This type of reworking of marsh sediments can cause problems in the application of C-14 dating (2009/08/28). (b) Active cut bank (Zone 3) in marsh sediments at Mont-Saint-Michel Bay, France. Lighter zones of rippled-sand beds can be traced in the cutbank for at least 100 m laterally. These zones delineate the annual cycles (2006/04/30). (c) Cut bank in small tidal creek in Zone 3 of Turnagain Arm, Alaska. Spruce grove was killed by brackish water intrusion that occurred after area of down dropped by 2 m during the 1964 earthquake. Spruce peat is undergoing erosion and slumping into the channel (2003/08/24). (d) Small cutbank showing dipping, cm-scale tidal rhythmites in Rio Chico estuary in the Rio Gallegos system. This area has slightly brackish water and the marsh plants include both halophytes and grasses. Plant roots help to hold the sediments in place until they become undercut.

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Fig. 28. (a) Complex ripples with two distinct mud drapes from sand flats in Zone 3 of the Rio Gallegos estuary in Patagonia (2001/05/23). (b) Double crested ripples formed in Turnagain Arm. Dark materials are granules of, soft metasedimentary rock being eroded from surrounding mountains (2005/07/10). (c) Mud-draped ripples (vertical cut) in Zone 2–Zone 3 transition area of Turnagain Arm. Each sand–mud couplet was deposited during a single, semidiurnal tidal event. Note variability in the thickness of the mud drapes (lighter coloration). Specific tides may have higher turbidity related to the occurrence of wind-generated waves. Scale in cm. (d) Small-scale linear ripples being erosionally cut by anastomosing erosional rills. From erosional upper surface of estuarine point bar, Turnagain Arm, Alaska. (e) Translating climbing-ripple lamination in Mont-Saint-Michel Bay. Ripples have well-rounded crests and are ebb directed. From the innermost part of Zone 1. Scale bar in cm. (f) Laterally, translating (“rhythmic”) climbing-ripple lamination from innermost Zone 1 of Turnagain Arm. Flow was from right to left and is ebb-dominant. Thick–thin pairing probably reflects the diurnal inequality within the tidal regime. Scale numbers in dm (1989/05/11).

Prior to the 1989 work of Bernadette Tessier in the Mont-Saint-Michel Bay and Robert Dalrymple in the Bay of Fundy, there were no even remotely similar modern analogs for the Carboniferous tidal rhythmites. Significant tidal amplification is commonly described as occurring within funnel-shaped systems. This is only partially true particularly if only the depositional areas, and not the erosion areas, are considered. Bay of Fundy–Salmon River estuary is unusually wide when compared to the other hypertidal systems. In fact, only the Bay of MontSaint-Michel has a funnel-shaped geometry, but its tidal amplification is significantly increased by the high-tidal ranges that are developed in the English Channel. The other hypertidal systems occur in very narrow paleovalleys and their geometry might be more strongly related to geomorphic rather than sedimentologic controls. The variability of features in the hypertidal systems can be simplified into three relatively distinct zones. Zone 1 includes linear or longitudinal bars that form the most seaward portion of the depositional system. This zone includes dune-scale depositional features. Zone 2, which is

inland of Zone 1, has extensive rippled sand flats. Internally, the ripples may not be preserved and instead a fabric dominated by upper-flow regime (UFR), primary-current lineation (PCL). Marshes formed laterally to the sand flats can preserve cm-scale cycles. These marsh cycles have been interpreted by a number of researchers as annual. The most-inland, component of deposition occurs in Zone 3, which is dominated by estuarine point bars. Ripples commonly cover these point bars but the most commonly preserved depositional fabric consists and mm-scale laminations and cm-scale thin beds. These features are normally graded and can be topped by well-developed mud drapes. Systematic thickening and thinning is common in series of stacked laminae and such cycles appear to be related to neap–spring tidal cycles. Sedimentation and erosion is very dynamic in all the zones and the linear bars, sand flats and estuarine point bars may form and subsequently be eroded during a single year. The six areas discussed have a wide range of differing climates. The South American and European hypertidal systems enjoy at least

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Fig. 29. (a) Neap–spring tidal rhythmites from Mont-Saint-Michel Bay. Photograph provided by Bernadette Tessier. Dark stripes related to development of biofilm during neap tides. Note that this area has a tidal system in which upper portions of the tidal flats are not submerged during neap tides. French 5-franc coin for scale. (b) Thick, silt-rich rhythmites within small tidal creek in Zone 3 of the Mont-Saint-Michel Bay (scale in cm). From a scraped and smoothed cutbank exposure. Disruption of the thin bed is related to soft-sediment deformation created by sheep-hoof bioturbation (1989/07/15). (c) Thin beds of fine-grained sand ranging up to 2 cm in thickness. Beds are draped by very thick mud drapes. Sand–mud couplets exhibit normal grading. From Zone 1 of Rio Chico in the Rio Gallegos estuary, Patagonia. (d) Cut bank exposure with rhythmites etched by flowing water during high tides. From Zone 3 of Turnagain Arm estuary, Alaska. Laminae thicknesses range from 1 to 10 mm and portions of 3 neap–spring cycles are delineated by lines marking neap periods. (e) Vertical surface (scrapped and smoothed) exhibiting a variety of Zone 3 rhythmites and associated depositional structures in Turnagain Arm, Alaska. Ra = reactivation, Cg = finely comminuted plant debris (“coffee grounds”), Lc = load casts. Scale in dm. (f) Sand–mud rhythmites from the inner sand flats of the Leaf Lake estuary. Rhythmites directly overlie a layer of coarse, gravel-rich sand. Thin layers of gravelly sand punctuate the rhythmites, these may be storm related.

somewhat moderate climates and the effects of winter ice are at worst minimal. The annual cycles reported in the Zone 2 marshes may be climate related (Allen, 1990, 2004). Conversely, the North American hypertidal systems are all affected by various degrees of ice formation. In the Bay of Fundy and Cook Inlet, floating ice blocks can be dropped onto intertidal flats during ebbing tides. This can result in significant cryoturbation on fringing mudflats and estuarine point bars. At the other extreme ice-related transport of gigantic boulders has a tremendous influence on the depositional systems in the Leaf Lake estuary. Despite these variations in climate, there are many similarities in the sedimentary tidal features among all 6

areas. The similarities indicate the predominance of hypertidal dynamics on erosion and deposition.

10. Summary and implications Because of their unusual features and general rarity, modern hypertidal depositional systems have not been widely discussed in the tidal literature. There is a unique suite of sedimentary structures and rhythmites that are formed within hypertidal systems. Additional study of such features can be used to interpret and re-interpret

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ancient depositional settings with potentially more appropriate modern analogs. The common occurrence of well-developed dune-, ripple- and laminae-scale features is a common feature of hypertidal systems. The general lack of extensive bioturbation is related to the rapid deposition and erosion as well as variations in salinity that can occur in hypertidal settings. The lack of bioturbation has, at least in some ancient counterparts, fostered a misinterpretation of hypertidal facies. Rhythmites are the most characteristic and unique feature of hypertidal systems. The large amounts of suspended sediment created by the very high tidal ranges and tidal bores allow rapid deposition during both flood and ebb tides. Mud deposited in the highest intertidal of Zone 3 has an extended period to compact and dewater before the next flood-tide event. Because of this and related factors, the potential preservation of neap–spring cycles in lamina is greatly enhanced. Lower rates of sedimentation occur in the Zone 2 sand flats. Rates of deposition are significantly lower than in Zone 3, but repetitive flooding of the marsh can result in annual cycles. Conversely, in Zone 1, the strong sedimentary dynamics only allow preservation of a few events over much shorter periods of time. A more complete understanding of these unusual, hypertidal systems could assist in the development of a tidal power. This is a significant and clean energy resource. Obviously, tidal-power generation can potentially play a significant role in our future power need. Hypertidal areas have the greatest energy potential. Acknowledgments This manuscript was greatly improved by a review by Kyungsik Choi and two anonymous reviewers. Initial study of European hypertidal settings was funded by National Science Foundation (NSF) grants EAR-9018079 and EAR-8903792. Aspects of this project were funded by NASA grant NCC5-234 entitled “Aspects and Controls of Tidal Sedimentation and the Extraction and Modeling of Tidal Parameters from Modern and Ancient Tidal Rhythmites.” This collaborative project was initiated by Bruce Bills of NASA/GSFC (Goddard Space Flight Center). Altimetry/bathymetry database from Smith and Sandwell (1997) was used to make tidal-amplification diagrams. Background information for the Severn estuary rhythmites was provided by discussions with J.R.L. Allen. Bernadette Tessier provided several excursions to the Bay of Mont-Saint-Michel. R.W. Dalrymple kindly provided information and localities for Bay of Fundy research. G.M.E. Perillo provided much background information and guidance for the study of the Patagonian estuaries. References Allen, J.R.L., 1990. Salt-marsh growth and stratification: a numerical model with special reference to the Severn estuary, southwest Britain. Marine Geology 95, 77–96. Allen, J.R.L., 2004. Annual textural banding in Holocene estuarine silts, Severn estuary levels (SW Britain): patterns, cause and implications. The Holocene 14, 536–552. Amos, C.L., Long, B.F.N., 1980. The sedimentary character of the Minas Basin, Bay of Fundy. In: McCann, S.B. (Ed.), The coastline of Canada: Geological Survey Canada Paper, 80–10, pp. 123–152. Archer, A.W., 1995. Modeling of cyclic tidal rhythmites based on a range of diurnal to semi-diurnal tidal-station data. Marine Geology 123, 1–10. Archer, A.W., 2004. Recurring assemblages of biogenic and physical sedimentary structures in modern and ancient extreme macrotidal estuaries. Journal Coastal Research 43, 4–22. Archer, A.W., 2005. Review of Amazonian depositional systems. Special Publication International Association of Sedimentologists 35, 17–39. Archer, A.W., Greb, S.F., 2012. Hypertidal facies from the Pennsylvanian period: eastern and western Interior Coal Basins, USA. In: Davis Jr., Dalrymple, R.W. (Eds.), Principles of Tidal Sedimentology. Springer, pp. 421–436. Archer, A.W., Hubbard, M.S., 2003. Highest tides of the world. In: Chan, M.A., Archer, A.W. (Eds.), Extreme Depositional Environments: Mega End Members in Geologic Time: Geological Society of America Special Paper, 370, pp. 151–173. Archer, A.W., Johnson, T.W., 1997. Modeling of cyclical rhythmites (Carboniferous of Indiana, Precambrian of Utah, USA) as a basis for reconstruction of intertidal positioning and paleotidal regimes. Sedimentology 44, 991–1010.

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