Sed Strat Week 10

estuaries

Estuary influences

Fluvial, wave, and tidal influence

Estuary

Brackish, semi-enclosed body of water with connection to the open sea where fresh water is mixed with saline water. Most are transgressive (drowned river mouth) and temporary, as when water recedes again the area becomes a delta. Margins are often sand/mud flats

Link between estuaries and incised valleys

Regression (low BL) exposes river valleys, allowing for gradients to steepen and rivers to break their channels, carving out valley walls to make them wider. Ocean water then fills this space when BL rises, creating the estuary

Base level

lowest elevation to which a river can erode. Lowers with regression and raises with transgression

Lagoon

Has no strong river-valley association, filled entirely with marine water and sediment. Not the same as an estuary

Coastal classification chart

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Estuary energy

Fluvial energy decreases moving closer to the coast due to proximity to the ocean, decrease in gradient, and widening of river channel

Marine energy decreases moving up-river due to increased fluvial input, steepening of gradient, and narrowing of river channel

Outer estuary

High energy zone dominated by marine processes

Central estuary

relatively low-energy central zone, where marine and river currents are approximately equal in strength. Calm, tends to be finest grained

Inner estuary

High energy, inland zone dominated by fluvial processes

Wave dominated estuary

One maxima occurs at the estuary mouth due to wave energy, and another at the head, due to river current. A central energy minimum occurs between, becoming less pronounced as the estuary fills

Marine, horizontal sand bodies occur at the mouth, which are cut by tidal inlets flowing into the central estuary (calm, muddy area). Limit of the shoreface is the marine limit of the estuary

Bayhead delta

Sand and gravel are deposited at the head (top) of the estuary; typical deltaic form. Often river-dominated

Tide dominated estuary

Mostly macrotidal; sediment is finest close to shore. Flood tide inundates coastal mud flats. Sand bodies occur in elongate, submarine channels at the mouth rather than long, horizontal bars. Bayhead delta is not present at estuary head

May be marked by mud drapes, herringbone cross-strat (rare), and reactivation surfaces. Areas of strongest current are marked by upper plane bed flow

Heterolithic (IHS)

Consist of more than one grain size, like sand and mud (tidal estuary channel sands and PB deposits)

Sequence stratigraphy

At the low stand (regressive), there is an erosional unconformity, in which the water was at its lowest and after this point began to rise (fluvial deposits, sand)

At the high stand (transgressive), there is a maximum flooding surface, in which the water was at its deepest and after this point began to lower (muddy deposits, shale, silt)

Submarine canyon

Steep-walled canyon cutting across the continental shelf that funnels coarse sediment into deeper water (turbidity currents). Often formed by river incision during LSTs

often several km deep and 10s of km wide

Submarine fan

Sediment that submarine canyons transport into deeper water extends out onto the continental rise, forming underwater fans that extend in a similar manner to an alluvial fan

Submarine fan stacking pattern

Suprafan lobes prograde similarly to a delta. An upward coarsening sequence is present, but lobe switching is difficult to define (unlike a delta)

upward fining packages occur in fan channels due to lateral accretion

Passive margin types

Normal clastic, carbonate (steep), delta w/ tectonic features (growth faults), salt tectonics (diapirs)

Breakup unconformity

major erosional surface separating older faulted rift sediments (syn-rift) from younger, undisturbed post-rift sediments. Marks transition from rifting to spreading

Salt tectonics

Salt tends to flow upwards due to density imbalance; it is lighter than surrounding material. As a result, it pierces upper layers of sediment and may become exposed as a diapir at the surface, with strata subsiding between diapirs and creating mini basins

Bouma sequence

Upward fining sequence describing the flow of a turbidite; distal sediments are finer while proximal sediments are coarser. Often not fully preserved

Flow stripping

The top of the turbidity current is separated from the heavier underlying sediment as it runs over an underwater feeder channel, which captures that coarser sediment. The finer overlying sediment runs into the outer fan

4 flow types (and their mechanisms)

debris flows (internal strength of matrix), liquefied flows (upward escape of fluid), grain flows (grain-to-grain impacts), turbidity currents (turbulence of flow)

Pelagic (biogenic)

Biogenic sediment not derived from weathering

Hemi-pelgaic (non-biogenic)

Fine, clay sized sediment from continental weathering that can stay in suspension for long periods of time

Neap vs spring tide

Neap is tidal minimum; moon is directly above/below the earth and the sun is on the left

Spring is tidal maximum; moon is to the left or right of the earth and sun is on opposing side