Deltas and Estuaries - Introduction
When rivers reach standing water such as a lake or the ocean, the flow speed slows down dramatically. And when flows slow down, sediment is deposited. Almost all of the sediment transported in a river is thus deposited close to the river mouth, with the exception of grains that are fine enough to remain in suspension. Lacustrine and marine processes can rework the deposited sediment to distribute it along shorelines.
Deltas form at the mouths of rivers that transport enough sediment to build outward. (Building outward is a key component of the definition of a delta. In contrast, estuaries are present where the ocean or lake waters flood up into the river valley.) Deltas require substantial accumulation of sediment, in contrast to estuaries which do not build outward. Sedimentary facies are similar to other depositional environments, but the association of subenvironments are recognizable as deltas. Some of the sub environments include: river facies with all the associated sub environments; shore line deposits including beaches, marshes/swamps, etc.; submarine shelf and slope facies, including storm deposits and turbidites; etc.
I will draw cross section and map views of a delta showing the delta plane, delta slopes, and prodelta. Rivers flow through delta planes and slow when reaching water, producing a mouth bar. Grain size decreases with distance away from the river mouth.
Because deltas are sites of sediment building outward from the coast, they are progradational; the landward depositional environments move seaward over more marine/lacustrine deposits. Thus, delta sequences in the rock record start with deep water, marine (or lacustrine), fine grained sediments and grade upward into shallower water, possible more freshwater, coarser grained sediments. This is one of the distinguishing aspects of deltas that let you define them in the sedimentary record. These changes in grain size and environment typically occur over 1’s to 100’s of meters in the rock record and include many beds.
Sediment Transport Type
All deltas (by definition) have their sediment transported to the delta by rivers. Thus, fluvial deposits are always associated with them. In addition, depending on marine (or lacustrine) conditions, waves and tides can redistribute the fluvial sediment changing the morphology and facies of deltas. There are three main end member categories of deltas when characterized by processes: 1) River dominated; 2) Wave influenced; and 3) Tide influenced. We will come back to these delta types after discussing the marine processes.
Most of Earth is covered with oceans, there is abundant life in the oceans, most sediments eventually get transported into the oceans, and shallow marine deposits are the most abundant in the in sedimentary record due to their large volume and the low erosion rates in shallow marine environments. You need tectonics to uplift them above sea level to get significant erosion. This happens commonly, so that we can also see them exposed.
Several processes are unique to shallow marine deposition (and some large lakes): Waves, storms, and tides
Waves have oscillating current directions every few seconds. The flow in both directions is equal in deep water, but not necessarily near shore. Draw a picture of wave water motion. (Water at the top of the wave moves in the direction the waves move.)
Wave Ripples - Wave ripples are different from current ripples because that they experience transport in both directions over the time scale of seconds. Draw a picture with the laminar boundary layer, etc. Wave ripples can be recognized in rocks by their symmetric shape (if flow in each direction is the same speed) and most importantly, the presence of cross-laminations dipping in two directions. This is the truly distinctive feature and can be present even if the ripples are not very symmetric. At low flow, the boundary layer doesn’t have enough speed or momentum to remove the crest of the ripple and the grains that are moved are deposited right on the upper part of the lee slope. Thus, crests are sharp. At higher flow, the crests erode and deposition occurs farther down the lee slope. Thus, high flow ripples have rounded crests.
In shallow water, currents from the waves can be strong enough to flatten out the ripples, but they are not consistent enough in one direction to form dunes; the flow switches directions too frequently for dunes to build up. Thus, where flow speeds are too fast to form ripples, the sedimentary surface tends to be planar or broadly scalloped as the waves are focused into certain areas. This produces a flat lamination (not upper planar lamination) where waves are in very shallow water relative to their height, e.g. from the breaker zone towards the shore.
KEY POINT FOR WAVES: Bi-directional flow every few seconds
Storms produce both large waves and strong, irregular currents. Combined waves and currents produce unique deposits that can be used to recognize ancient storms in a marine sequence. Storms generally start far from shore and get stronger at the shoreline through time. Then they either die out or move on. Thus, near shorelines, storms tend to start out with low energy flows, increase in flow speed until they are erosional (if strong enough) and then decrease back to lower energy flows. For example, sharp crested wave ripples might mark the start of a storm, then transition into round crested wave ripples, followed by cross stratification due to large waves and strong currents, followed by erosion. As the storm wanes, the coarsest sediment would be deposited, then finer sand deposited by waves and currents, and then wave ripples, e.g. the reverse of the sedimentary structures seen as the storm approaches. There is usually little sediment being deposited at the beginning of a storm because not much sediment has been eroded yet and flow speeds are increasing through time. Thus, there is usually no record of the first half of a storm sequence in the rock record. It is only the second half that gets preserved. Storm deposits almost always consist of an erosion surface with sediment of decreasing grain sizes upward - like turbidites, but with different sedimentary structures.
HCS - The cross stratification that is deposited as a combination of strong currents and large waves is unique to storms (and is found only in medium to fine sands). It is called hummocky cross stratification (HCS) and swaley cross stratification. When currents are washing eroded sand into an area with strong oscillatory flow from waves, rounded mounds or hummocks of sand develop on the sea floor separated by lows (swales). These mounds are a few to 10 cm high and 10’s of cm across. See Figures 14.3 and 4 in Nichols. Variations in current strength cause erosion locally, and the locations of the hummocks and swales change through time. This produces erosional surfaces which truncate the older laminae (note that Fig 14.2 has the wrong laminae truncated). HCS is characterized by low angle laminae truncated by low angle surfaces. There are abundant concave and convex up laminae and many fewer flat laminae.
Storm Sequence - A sample stratigraphic column consists of: Mud, scoured surface, sole marks, (gravel at base), normally graded, HCS, flat laminae or wave rippled top, return to suspension settling. Contrast this to a turbidite - I will ask you to do this!
KEY POINT FOR STORMS: Multi-directional flows over seconds, low to high to low energy in deep water
Two key characteristics that are unique to tides: 1) flow changes direction 1 or 2 times per day; and 2) The speed of flow is cyclical with flow going onshore, stopping at hight tide, then flowing offshore, and stopping at low tide. There is lots of variability in tides depending on geography. Flow speeds vary, producing different sedimentary structures. In the Bay of Fundy, which has the highest tides recorded in the world (up to 16 m - a 5 story building), the water moves up to 15 km/hr (417 cm/sec) which is fast enough to transport boulders. At the low end, tidal currents are essentially non-existent. Also, there are times of slack tides when the water is essentially still or wave-dominated. Thus, the range of sedimentary structures is wide, including dunes (often called tidal bars when very large) and ripples. The main characteristic to look for, though, is variations in flow speed and DIRECTION.
Tidal sedimentary structures - Due to changing flow directions, two sediment transport directions are common, one for onshore flow and one for offshore flow. Often the onshore and offshore flows are not in the same location, but they shift around. This gives rise to current ripples showing transport in two directions and dune migration in two directions producing herringbone cross stratification. See figures 11.6 and 11.7 in Nichols. If the dunes are small and sedimentation rates are very high, you can get herringbone cross stratification in one tidal cycle in a modern environment. It is usually not preserved in the geological record because it is eroded prior to lithification. It is almost always the longer term changes in current locations that gives rise to preserved herringbone cross stratification. Dunes migrate in one direction for a while, and then currents patterns change and they migrate in the other direction. Herringbone cross stratification is almost always due to tidal processes, although it is not all that common in the sedimentary record. Commonly, one tidal current is much stronger than the others or the flow locations aren’t systematically shifting, so tabular cross stratification is more common. It is not unique to tidal environments, however.
Reactivation Surfaces - Reactivation surfaces form when flow in one direction is stronger than the other, but the other flow is strong enough to modify the bedform shape. See figures 11.6 and 11.9 in Nichols. Reactivation surfaces are erosion surfaces within the sets of cross stratification. They look like irregular surfaces that are similarly oriented to the foresets, but usually do not dip quite as steeply. Also, the foresets above and below the reactivation surface commonly have a slightly different orientation. Reactivation surfaces indicate varying flow directions, which is very common in tidal environments.
Mud Drapes - Flow speeds are also cyclical. During slack tides (low or especially high), fine grained sediment can fall out of suspension draping tidal bedforms with mud. Because mud is cohesive, it does not necessarily erode during the next tidal flow, particularly in the separation zone where flow is slow, e.g. at the bases of ripples and dunes. Thus, sand foresets coated with mud are very common in tidal environments as well. See figures 11.6 and 11.8 in Nichols.
KEY POINT FOR TIDAL PROCESSES: Bi-directional flow with varying speeds over hours