جزر و مد-Tidalites
Sediments deposited by tidal processes. Until recently, “tidalites” referred to sediments deposited by tidal processes in both the intertidal zone (between normal low- and high-tide levels) and shallow, subtidal (permanently submerged), tide-dominated environments less than 200 m (660 ft) deep. Tidalites are now known also to occur within supratidal environments (above normal high tide and flooded only during storms or very high spring tides) and submarine canyons at depths much greater than 200 m. Common usage has drifted toward describing tidalites as ripple- and dune-scale features rather than more composite deposits such as large linear sand ridges of tidal origin present on continental shelves or point bars associated with migrating tidal channels. Both of these larger-scale features, however, would be composed of tidalites.
By identifying tidalites in either the modern or the ancient geological record, geologists are implying that they know that the sediments were deposited by tidal processes rather than by storms or waves. Tidalites are not always easy to identify with certainty, especially in the rock record. In order to do so, it is necessary to understand the basic tidal cycles that can influence sedimentation.
Tides are generated by the combined gravitational forces of the Moon and Sun on the Earth's oceans. Some sources are misleading in suggesting that the tidal forces from the Moon and Sun, in combination with centrifugal forces associated with the spin of the Earth, produce oceanic bulges on opposite sides of the Earth. While it is true that the combined gravitational forces of the Moon and, to a lesser extent, the Sun produce tides on the Earth, the Earth does not spin through two oceanic bulges that form on opposite sides of the Earth. This conceptual model has little bearing on real-world tides. Rather, water within each of the Earth's ocean basins is forced to rotate as discrete waves about a series of fixed (amphidromic) points (Fig. 1). For a fixed point along an ocean coastline, a tidal system is referred to as diurnal if it experiences the passing of the resultant tidal wave once every 24 h 50 min. The tidal system is semidiurnal if the resultant tidal wave passes the fixed point twice during the same time.
Fig. 1 North Sea amphidromic tidal system. Corange lines indicate equal tidal range. Cotidal lines show times of high water. Arrows show rotation directions of the tidal waves. (Modified from R. W. Dalrymple, Tidal Depositional Systems, in R. G. Walker and N. P. James, eds., Facies Models Response to Sea Level Changes, pp. 195–218, Geological Association of Canada, 1992)
In the open ocean, the motion of a tidal wave is largely expressed as a vertical movement of water masses. In shallow basins along the coast, water movements are more horizontal, with tides moving in and out of estuaries and embayments, resulting in a change in water level as the tidal wave passes. The daily or semidaily rise in tides is called the flood tide, and the fall is referred to as the ebb tide (Fig. 2a). Tidal currents are maximized between flood and ebb tides and minimized at highest flood or lowest ebb tides (Fig. 2b). The difference between the high tide and the low tide is called the tidal range.
Fig. 2 Flood–ebb cycle. Idealized (a) semidiurnal tidal cycle and (b) time–current velocity curve. (Modified from S. D. Nio and C. S. Yang, Recognition of tidally influenced facies and environments, Short Course Note Ser. 1, International Geoservices BV, Leiderdorp, Netherlands, 1989)
The intensity or height of the daily or twice-daily tides can vary in a number of ways. Cyclic semimonthly changes in daily tidal heights associated with neap–spring tidal cycles are the most pronounced of these. Spring tides occur twice a month when the tidal range is greatest, and neap tides occur twice a month when tidal range is least. Neap–spring cycles can be generated in two ways. The familiar neap–spring cycle is related to the phase changes of the Moon. Spring tides occur every 14.76 days when the Earth, Moon, and Sun are nearly aligned at new or full moon (Fig. 3a). Neap tides occur when the Sun and Moon are aligned at right angles from the Earth at first- and third-quarter phases of the Moon. The result is that spring tides are higher than neap tides (Fig. 3b). The time from new moon to new moon is called the synodic month, which has a modern period of 29.53 days. This type of neap–spring cycle is referred to as synodically driven, and it dominates the coastlines of western Europe and the eastern coastline of North America.
Fig. 3 Idealized models of origin of neap–spring tidal cycles: (a) Synodic month. (b) A segment of the 1991 predicted high tides from Kwajalein Atoll, Pacific. (c) Tropical month. (d) A segment of the 1994 predicted high tides from Barito River estuary, Borneo. (Modified from E. P. Kvale, K. H. Sowder, and B. T. Hill, Modern and ancient tides: Poster and explanatory notes, Society for Sedimentary Geology, Tulsa, OK, and Indiana Geological Survey, Bloomington, IN, 1998)
A second type of neap–spring cycle is less well known but no less common, and is related to the orbit of the Moon around the Earth. The Moon's orbital plane is inclined relative to the Earth's equatorial plane. The period of the variation in lunar declination relative to the Earth's Equator is called the tropical month, and is the time the Moon takes to complete one orbit, moving from its maximum northerly declination to its maximum southerly declination and return (Fig. 3c). In this type of neap–spring cycle, the tidal force depends on the position of the Moon relative to the Earth's Equator. The tide-raising force at a given location is greater when the Moon is at its maximum declination every 13.66 days. These periods correspond to the generation of spring tides (Fig. 3d). The neap tides occur when the Moon is over the Earth's Equator. The current length of the tropical month is 27.32 days, and neap–spring cycles in phase with the tropical month are referred to as tropically driven. These types of neap-spring cycles dominate coastlines in the Gulf of Mexico and many areas in the Pacific.
Besides generating neap–spring cycles in many parts of the world, the changing position of the Moon relative to the Earth's Equator through the tropical month causes the diurnal inequality of the tides in semidiurnal tidal systems. In tidal systems that experience two high tides and two low tides per day, the tropical monthly cycle results in the morning high tide being greater or lesser than the evening high tide. The diurnal inequality is reduced to zero when the Moon is over the Equator, resulting in the morning tide and the evening tide being of equal magnitude (Fig. 3b and d).
Other tidal cycles besides those mentioned above can influence sedimentation and have been documented in the geologic record. These include monthly, semiannual, and multiyear tidal cycles. See also: Earth rotation and orbital motion; Moon; Tide
Examples of tidalites
How the various tidal cycles manifest themselves in the geologic record and how geologists can identify their influence on sedimentation has been studied for nearly 75 years. To recognize tidalites in the geologic record, geologists look for evidence of one or more of the following:
1. Sediment deposited by reversing currents (that is, flood–ebb cycles).
2. A stacked sequence of sediments that show a recurring change from sediments transported (and deposited) by currents at maximum current velocity to sediments deposited from suspension at minimum current velocity (Fig. 2b).
3. Stacked packages of sediments in which each package shows evidence of subaerial exposure superimposed on sediments deposited in subaqueous settings (sediments transported and deposited during flood tides and exposed during low ebb tide).
4. A sequence of sediment packages in which the thickness or accretion of successive packages of sediments varies in a systematic way, suggesting diurnal, semidiurnal, and/or neap–spring tidal cycles.
An example of a small-scale tidalite can be found in the Mansfield Formation (Pennsylvanian Period) in Orange County, Indiana (Fig. 4). The sample shown is from a rock core taken through this interval. The lighter-colored layers are siltstone, and the thin dark layers are finer-grained claystone. The regular and repeating change in deposition from siltstone to claystone indicates systematic current velocity fluctuations related to the tidal cycle over a 12-h period (see item 2 above). The thick–thin pairing of the lighter bands of siltstone suggests the influence of the semi-diurnal inequality over a 24-h period (see item 4). In addition, the regular and systematic overall thickening and thinning of the siltstone layers, as shown in the bar chart next to the core in Fig. 4, suggests that neap–spring tidal cycles controlled the thicknesses of the silt layers. The higher spring tides resulted in thicker accumulations of silt than the lower neap tides. See also: Pennsylvanian; Sedimentary rocks
Fig. 4 The core shows small-scale tidalites from the Hindostan Whetstone beds, Mansfield Formation, Indiana. The chart shows thicknesses of layers as measured between dark clay-rich bands. The interval shows approximately one synodic month of deposition.
An example of a large-scale tidalite can be found in the Jurassic Sundance Formation of northern Wyoming (Fig. 5). This tidalite is the remnant of a migrating subtidal dune or sandwave. The preserved inclined beds of the avalanche face (I) indicate the migration direction of the dune from right to left (Fig. 5a). The evidence for tidal influence, however, lies within the inclined, less resistant, and more recessed lighter-colored bands (examples marked by arrows in Fig. 5a). In this interval (Fig. 5b), one sees evidence of (1) cessation of dune migration and a reversal of current direction from flood tide to ebb tide with small ripples migrating up the avalanche face (II); (2) a mud drape (III) resulting from fine-grained materials settling out of suspension when the current velocities reached zero as the tide reversed (Fig. 2); and (3) a reactivation of the migrating dune above the mud drape (IV) as current velocity increased during the next flood tide. The right-to-left migration of the dune was also controlled by the neap–spring cycle, with greater migration (interval between lighter-colored bands) occurring during spring tides and lesser migration during neap tides (Fig. 5a). In the example shown, the neap tide deposits are centered on line N. See also: Jurassic
Fig. 5 Photographs from Sundance Formation of northern Wyoming. (a) Example of large-scale tidalites. (b) Closeup of inclined light-colored band showing evidence of current reversals.
Deposits of tidalites are known from every geologic period from the modern back into the Precambrian and from depositional environments with water chemistries ranging from fresh to hypersaline. Studies of tidalites are important because geologists have used these features not only to interpret the original depositional settings of the deposits but also to calculate ancient Earth–Moon distances, interpret paleoclimates existent during deposition, and calculate sedimentation rates. See also: Depositional systems and environments; Geologic time scale; Marine sediments; Sedimentology
Erik P. Kvale
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E. P. Kvale