Tidal Systems (SA)


Bay of Fundy

Figure 1: Difference in low and high tide at the Bay of Fundy (Photo Credit: Steve Brown).

    Tidal systems are energy-dynamic environments located on coasts/beaches. This system is dominated by a high (flood) and low (ebb) tide (Fig.1). A high tide occurs when the ocean swells in and the water level rises up to the top of the beach, or nearly to the mudflat. This is a supratidal zone/facies. Low tide takes place when the ocean water pulls out and the water level drops of the sandflat or the subtidal zone/facies. When the tide is transitioning from high to low or low to high, the ocean water is in the intertidal zone/facies. Tides are caused by the gravitational interaction between the Earth and Moon (Fig. 2). A bulge in seawater (high tide) occurs in the direction of the Moon. Therefore, low tide will occur once the Moon moves to the other side of the Earth. Since the Earth is rotating, high and low tide will (respectively) occur twice a day (Col, 1996). Shorelines that experience two equal high and low tides daily are referred to as having a semi-diurnal tide. Shorelines that experience two 

Spring vs. Neap Tides

Figure 2: Difference in spring tide (a) and neap tide (b) (Col,1996).

length high and low tides are referred to as having a mixed tide. The duration and amplitude of tides are affected by the position of the Sun and the Moon, the tides in the deep ocean, and the depth of the water in the ocean, or bathymetry. In order to record these differences in tides, humans use tide gauges at fixed stations. This records amplitude and duration of the tides. 
    Spring tides are stronger high tides that occur when the Sun, the Moon, and the the Earth are in a line (fig. 2). It is stronger because both the Sun and the Moons gravitational pull effects the ocean. This occurs during the full and new moon. On the contrary, ebb tides are weaker low tides, that occurs when the Sun and the Moon are perpendicular to one another (with respect to the Earth) (Col, 1996). The resulting gravitational pull of the Sun and the Moon contradict one another to produce a weaker tide. 

Differences in Tide Amplitude

Figure 3: Differences in tide amplitude (height) is represented by color. The white lines are cotidal differing my one hour. Blue is the least extreme tidal amplitude 
and red is the most extreme tidal amplitude (Accad and Pekeris,1978)

Geomorphology & Modern Analog 

    Tidal systems have a very interesting geomorphology. One of the key factors that

Modern Tidal System

Figure 4: Diagram of modern tidal system with a barrier island
illustrating tidal flats, creeks, and sandbars. Courtesy of Brian Currie. 

effects tidal geomorphology is the amplitude of the tidal range. For example, if a beach (e.g., modern analog of the coast of Western Australia) has only ~0-10 cm tidal range, then the beach itself will not be influenced much by the tides. On the other hand, a beach with a large tidal range (e.g., modern analog of the Bay of Fundy), will be largely influenced by the tides which will be shown by the geomorphology of the coast. The Bay of Fundy has an incredible tidal amplitude, which can be seen in this video link: Bay of Fundy Tidal Range. Large scale geomorphological features that will be influenced by tides are tidal creeks, flats, and sandbars (Fig. 4). 
    On a smaller scale, certain tidal systems sedimentary structures and fossils dominate the geomorphology in specific tidal facies. For example, the subtidal facies will mainly have cross bedding, ripples, and burrows. The intertidal facies will have a combination of ripples (Fig. 4), lateral accretion bedding, flaser bedding, wavy bedding, fossils (e.g., bivalves) and burrows. The supratidal facies will have ripples, dunes, (Fig. 3) lenticular bedding, rooted muds, and burrows. These features can be seen in modern tidal systems across the world. Below are some modern examples for these sedimentary structures. 

Wave Ripples

Figure 4: Wave ripples on an intertidal sand flat. Courtesy of Brian Currie. 

Ripples and Dunes

Figure 4: 2-Dimensional dunes with super-posed ripples, Dee Estuary, England. Courtesy of Brian Currie. 

Tidal Creeks

Figure 5: Mud flat with tidal creeks. Courtesy of Brian Currie. 

Depositional Processes and Depositional Facies 

The main depositional process in tides is the change in energy from high to low tide. Sedimentary structures produced from energy fluctuations include interbedded coarse and fine laminae (tidal bundles), mud draped ripples, and mud on foresets.

There are three main facies present within a tidal system that develop from energy fluctuations:
  1. Subtidal: This is the low (ebb) tide level, therefore this facies has the 
    highest energy level due to all the wave energy in the shallow water. This results in the determination of sand in this facies with sedimentary structures such as ripples. Normally, this facies occurs nearshore. In the rock record, subtidal facies can be composed of shales, lime muds and sands, lime sand shoals, stromatolites, and reefs (Walker, 1984).
  2. Intratidal: This facies occurs with the change in tides, resulting in a mixture of sand and mud. In this facies, there will be a variety of sedimentary structures such as mud-draped ripples, mud on foresets, etc., that will show change in energy. This facies becomes increasingly mud-rich as you move landward. Mud drapes indicate an increase in mud due to rising sea level  (e.g., energy change from high to low). In the rock record, intertidal facies has mixture of sand- and mud-stone to the low-high energy fluctuations (Walker, 1984).
  3. SupratidalThis facies developed only during storms or the highest tides. This is the lowest energy facies due to the deepest water, resulting in the main lithology being mud. Normally, this facies occurs in the marsh which means there will be lots of roots and burrows. This facies has a variety of subenvironments including algal marshes and arid sabkha because of well-drained and elevated nature (Walker, 1984). 

Tidal Facies

Figure 6: Tidal facies in a detailed diagram with sedimentary structures and a stratigraphic column. 

Subtidal, Intertidal, and Supratidal in Stratigraphic Column

Figure 7: This stratigraphic column illustrates what each facies (subtidal, intertidal, and supratidal) look like in relation to a surf zone and larger beach environment. (Walker, 1984).

Controls on Depositional System Evolution 

    There are many controls on a tidal system. First of all, climate will have a major factor in the tidal systems. As seen in the figure 8, if there climate is extremely arid (like a desert), then there will not be any rivers/creeks flowing into the ocean . Sabkhas will form in this climate (salt flat) and other salt minerals like anhydrite, gypsum, and halite will precipitate. If there is a moderately-humid climate, then there will be marshes, ponds, and creeks from the excess rainwater in the environment. This will effect the geomorphology on the system as well. There will not be many salt minerals precipitating, resulting in less extreme environment regarding salinity. This means that more animals will be able to inhabit this environment causing more burrows and other various trace fossils. 

Geomorphological Controls

Figure 8: Differences in geomorphological controls (Walker, 1984).

Facies Models 

    The main facies produced by a tidal system are the subtidal, intertidal, and supratidal. Below are two figures that show the surface geology of these facies and the facies within stratigraphic column. The subtidal zone will be mainly composed of sand with fossils present and ripples. The intertidal zone will be composed of sand and mud and will have mud cracks, burrows, and ripples, The supratidal zone will have mainly mud with mud cracks and burrows and potentially some roots (Fig. 10).

Tidal Facies

Figure 9: Diagram of subtidal, intertidal, and supratidal facies. 

Tidal Stratigraphic Column

Figure 10: Stratigraphic column produced by the different facies. 


Accad, Y. and Pekeris, C.L.,1978, Solution of the Tidal Equations for the M2 and S2Tides in the World Oceans from a Knowledge of the Tidal Potential Alone, Philosophical Transactions of the Royal Society of London, Series A, Mathematical and Physical Sciences v. 290, p. 235–266, doi:10.1098/rsta.1978.0083.

Col Jeananda, 1996, Enchanged Learning, http://www.enchantedlearning.com.

Currie, B., In-class lecture. 

Brown, S., https://www.flickr.com/photos/sjb4photos/.

Walker, Roger G., 1984, Facies Models: Geoscience Canada, v. 2, 317 p.