10.3: Clastic Marginal Marine Environments
Marginal marine environments form at the intersection of land and sea, where terrestrial and marine processes converge. In regions with a moderate supply of clastic sediment, these environments are shaped by the balance between waves, rivers, and tides which results in the development of beaches, deltas, and tidal flats, respectively. Relative sea level also plays a critical role in influencing these processes and the resulting depositional architecture. For instance, coastal areas dominated by fluvial systems are more likely to form deltas in regressive settings and estuaries in transgressive settings.
Beaches
A beach is a sandy, wave-dominated environment that is attached to the mainland. Waves are created when wind blows across the surface of the ocean; the resulting friction generates waves. Remember that waves cause circular motion in the water column and that motion translates down to a depth that is equal to one half of the wavelength. Fairweather wave base is the depth at which waves “feel” bottom under normal conditions; it marks the transition from marginal marine “beach” type deposits to the fully marine environments of the shelf.
Environments and Processes
Beaches are characterized by a distinct succession of environments, processes, and sedimentary structures that are summarized and illustrated below.
Backshore areas only submerged during storms or extremely high tides and are dominated by eolian processes as wind blows inland across the beach. Eolian dunes and vegetation are common and the resulting deposits are characterized by landward- and seaward dipping low angle laminae, cross-beds, well-sorted, wind-reworked sand, and possibly roots and minor bioturbation.
The foreshore lies within the intertidal zone, between high and low tide levels. Here, sedimentary processes are dominated by the action of swash – where breaking waves send shallow/high velocity pulses of water and sand up the beach profile. Some water and sediment might flow back downslope. This results in low-angle laminae and the formation of heavy mineral lags. Bioturbation is very minor as few organisms can live in this dynamic and high energy environment.
The shoreface is always submerged and is regularly reworked by wave activity. The shoreface can be divided into three distinct subzones based on sedimentary structures and biological activity:
- Upper shoreface : This zone is characterized by trough cross-bedding from landward-migrating dunes, low-angle laminae, and abundant erosion surfaces.
- Middle shoreface : Sedimentary structures in this zone include trough cross-bedding and reworked hummocky cross-stratification (HCS). Bioturbation becomes more abundant and diverse.
- Lower shoreface : This deeper zone is dominated by abundant HCS formed by oscillatory wave conditions during storms. Bioturbation becomes both intense and diverse - often to the point of destroying primary bedding structures. Fairweather wave base marks the boundary between the lower shoreface and deeper water deposits of the offshore transition (inner shelf).
Barrier Islands
Barrier islands are detached beaches separated from the mainland by low-lying brackish wetlands called lagoons . Barrier island deposits are largely similar to the beach deposits described above and are best diagnosed through their context and association with lagoons. Lagoonal deposits include muds deposited from suspension deposition, beach sands introduced via washover events, and tidal inlet channels. Lagoons are typically organic-rich and experience both oxygen and salinity stress.
Most barrier islands form via one of two models:
- Transgression by shoreface retreat: As sea level rises, storm washover processes move sand landward, resulting in the gradual migration of the barrier island toward the mainland. New sand is supplied by longshore drift of sediment supplied to the coast via rivers.
- In-place drowning: If sea level rises rapidly, waves may deposit sand in situ, leading to the development of a barrier island without significant lateral migration.
Deltas
Deltas are marginal marine systems formed where fluvial systems build into standing bodies of water; they have both subaerial and subaqueous deposits. Deltas grow through the progradation of lobes, with new lobes forming as rivers shift to steeper gradients during avulsions. This process distributes sediment evenly across the delta system. Although they can be shaped by waves and tides, rivers are the dominant force shaping deltas. The largest examples typically occur on passive margins or in slowly subsiding areas where sediment supply exceeds the rate of subsidence.
Environments and Processes
The delta plain is the subaerial portion of the delta, which can be divided into two zones. The upper delta plain lies above the limit of tidal influence and is shaped primarily by fluvial processes. It is composed of river and floodplain deposits and they commonly contain peat swamps (coal). The lower delta plain , between the low tide mark and the furthest extent of tidal influence, experiences a mix of fluvial, tidal, and marine processes. This area is a stressed environment due to salinity, oxygen, and water level fluctuations. It includes distributary channels and interdistributary bays and wetlands.
Distributary channels transport water and sediment to the sea, often featuring river-like deposits with tidal mud drapes. These channels may cut into underlying deposits. Interdistributary intervals are broad, shallow bays with a mix of fresh and marine waters. Sediments in these intervals include muddy, organic-rich deposits and sandy crevasse splays. Faunal diversity and the intensity of bioturbation is limited by environmental stresses.
The delta front is a shallow marine zone (typically < 10 m deep) where clastic sediments are deposited by both bedload and suspension processes. This area may experience additional modification by wave or tidal processes, resulting in conditions that range from fully marine to brackish. Mouth bars and subaqueous levees form where sediment-laden channelized flow slows down where it meets standing water.
The prodelta lies seaward of the delta front, where fine silts and clays settle. These fines are supplied by plumes of fresh, muddy water that are less dense than salt water and may extend considerable distances from where distributary channels meet the ocean. This zone is less influenced by tides and waves, creating stable, open marine conditions with abundant nutrients and organic matter. Bioturbation is common, although localized stresses from decaying organics can reduce oxygen levels.
Morphology and Geometry
In map view, deltas commonly display a triangular to lobe-shaped morphology and in cross-section view they exhibit a wedge-shaped profile that thins in both updip and downdip directions. Although dominated by fluvial processes, the shape of and processes within deltas can be modified by tides and waves. Overwhelmingly fluvial-dominated deltas, such as the Mississippi River Delta, form lobate shapes with sandy distributary fingers. These types of deltaic successions commonly show stacked shallowing upward successions that record the progradation, and eventual abandonment, of individual lobes. Wave-dominated deltas, like the Nile Delta, are shaped by wave action that results in a blunt overall shape with flanking beaches. Tide-dominated deltas, such as the Mahakam Delta, feature funnel-shaped, sand-filled distributary channels and prominent tidal indicators.
Deltaic deposits on progradational coastlines commonly exhibit coarsening-upward successions formed from shallowing and the advance of the delta into the standing body of water. Given the overlap with purely fluvial systems, deltaic interpretations are best made from multiple outcrops where one can more confidently identify sandy or silty fluvial deposits prograding into lacustrine or marine mudrocks or seismic lines where one can see seaward-dipping clinoforms (when viewed perpendicular to the coast) or mounded clinoforms (when viewed parallel to the coast).
Tidal Flats
Tides are periodic vertical changes in water level, caused by the combined effects of centrifugal and gravitational forces in the Earth-Moon and Earth-Sun systems. Most coastlines experience a tidal range (the difference in height between high and low tide) of just a few meters, but local conditions can cause it to be much higher in some locations. The world’s highest tides are present in the Bay of Fundy in Atlantic Canada where the average tidal range in some parts of the bay are ~15 m (50 ft).
Causes of the tides
As mentioned above, tides are caused by gravitational and centrifugal forces caused by the interaction of the Earth-Moon and Earth-Sun systems. Although the moon is much smaller than the Sun, it is much closer and exerts approximately twice the tidal influence of the Sun. For the sake of simplicity, we will first consider the interaction of the Earth and Moon in our initial discussion; the interaction of the Earth and Sun is much the same and is best considered in the discussion of neap and spring tides. A fuller discussion of the causes of the tides is available in Chapter 11.1 of Webb’s Introduction to Oceanography (also an OER resource).
The Moon’s gravity pulls water toward it which creates a tidal bulge that is greatest directly beneath it. A second, smaller tidal bulge is present on the opposite side of the Earth; it is caused by centrifugal force from the rotation of the Earth and Moon around their shared center of mass (the barycenter). The tidal bulges remain beneath and opposite the position of the Moon, but because the Earth spins on it's rotational axis and the moon rotates in the same direction, it takes 24.84 hours for a tidal bulge to return to the same spot on the surface of the Earth. The unevenness of the two high tides (and intervening low tides) is further enhanced by the inclination of the Moon’s orbital plane relative to the equator. Thus, many places experience two unequal high tides and two unequal low tides every 24.84 hours (about 50 minutes later each day).
Tidal range also changes because of the interaction of the lunar and solar tidal bulges. Spring tides are times with relatively high tidal ranges that occur when the Earth, Moon, and Sun are in alignment and the resulting tidal bulges experience constructive interference. Conversely, neap tides are periods of lower tidal range caused by the Moon being at 90° to the Earth-Sun line which results in destructive interference. Thus, there is a ~28 day cyclicity with spring tides occurring during new and full moons (about 14 days apart) and neap tides occur midway between them when the moon is at first quarter and last quarter, respectively.
Tidal Deposits
The influence of tides can result in regular changes in current velocity and direction which can, in turn, cause the formation of distinctive sedimentary structures that include:
- Tidal rhythmites : These deposits consist of sand-mud couplets, where mud is deposited during periods of low water movement (high and low tide) and sand is deposited when currents are stronger during the rising and falling tides. In some tidal systems, the ebb and flood currents are weaker, resulting in mud-sand couplets that are unequal in size.
- Heterolithic bedding : Tidal deposits often show more complexity than simple rhythmites. Alternations between sand and mud reflect fluctuations in flow strength due to changes in current velocity, water depth, neap-spring cyclity, and other factors.
- Mud drapes : In tidal environments, periods of slack water may allow the deposition of thin mud drapes over coarser sediments, even in areas with strong currents capable of forming dunes.
- Herringbone cross-beds : In situations where currents are consistently strong, sedimentary structures such as herringbone cross-beds may form. These features reflect changes in current direction, typical of tidal environments where the flow alternates between flood and ebb tides.
Tide-dominated coasts
In coastal areas dominated by tides we can consider three main zones:
- Supratidal areas are located above the high tide mark and are only inundated during exceptionally high tides or storms. These areas consist of muddy salt marshes that are colonized by salt-tolerant grasses, succulents, and herbs.
- The intertidal zone is effectively synonymous with the term “tidal flat” and is the low-gradient area that is variously exposed and submerged during a tidal cycle. Updip areas are typically muddy because they are only inundated around high tide when water velocity is relatively low. Tidal flats become progressively sandier downdip because these areas experience bedload transport during relatively high velocity flows during falling and rising tides. The intertidal zone is the area that is exposed and submerged with each tidal cycle. They often have a dendritic network of tidal channels that help drain water during low tide. These areas contain both sand and mud and heterolithic bedding is common; lenticular bedding dominates in updip areas and flaser bedding dominates in downdip areas.
- Subtidal areas are always submerged and are profoundly influence by currents from rising and falling tides and/or currents that parallel the shoreline. They are composed of sandy sediment organized into a variety of large dune and sandbar morphologies.
Tidally-influenced rivers
Rivers that drain into areas with high tides can be profoundly impacted by tidal cycles; in extreme cases the channel might be bankfull during high tide and flow inland and nearly empty and flow seaward during low tide. In certain regions, rising tides can create a tidal bore - wave that travels upstream, marking the point where flow reversal occurs. Tidal influence can extend a surprising distance upstream in larger systems and might be expressed through the influx of brackish waters or decreased flow velocity because of ponding during high tide. As mentioned above, tidally-influenced deltas often take on a funnel-like morphology with relatively straight, radiating distributary channels
Readings and Resources
- Boyd, R., Dalrymple, R., and Zaitlin, B., 1992, Classification of clastic coastal depositional environments. Sedimentary Geology. v. 80. p. 139-150.
- Dalrymple, R.W., 1992, Tidal Depositional Systems in Walker, R.G. and James, N.P. (eds.), Facies Models (2nd ed), Geological Association of Canada, St. John's, p. 195-218 a
- Kvale E.P., Sowder K.H., Hill B.T., 1998, Modern and ancient tides - Poster and explanatory notes, SEPM, Tulsa, OK, and Indiana Geological Survey, Bloomington, IN.
- Webb, P., 11.1 - Tidal forces in "Introduction to Oceanography". https://rwu.pressbooks.pub/webboceanography/chapter/11-1-tidal-forces/