GEOL 102 Historical Geology
Spring Semester 2014
Coastal & Marine Environments: Transgressions & Regressions
Coastlines are important: give us the interface between marine and terrestrial conditions.
VERY ephemeral: can move inland or seaward very quickly.
Different types of environments where land meets sea.
Barrier Island-Lagoon Complexes
- Make up much more of the shoreline than deltas.
- Composed of several different facies:
- Salt Marshes: paludal setting, where terrestrial and marine water mix
- Tidal Flats: often muddy sand or mud, alternately exposed to air and water,
lots of plant debris, mudcracks, etc.
- Lagoons: sheltered by barrier islands, low energy, often muddy (laminated if
low oxygen levels; otherwise homogenous), some channel deposits running through it.
- Barrier Islands: composed of well-sorted sandstones, possibly with paludal
deposits on the lagoonal side.
- If sea level is constant and no tectonic changes, will tend to prograde
(although not as fast as deltas).
Sabkha - Arid coastlines
- Similar to barrier island-lagoon complexes, but may contain carbonate muds,
smaller salt marshes, evaporites common above tidal flats. Often (but not
exclusively) found with reef-lagoon complexes. Tidal flats may contain stromatolites.
- As barrier island complexes, but instead of barrier island have some sort of
carbonate reef. Form in warmer environments. Lagoon and sometimes shore dominated
by carbonate muds.
- Regardless of organism that makes reef, structure tends to be the same:
- Talus: broken reef rock on seaward side
- Front reef: durable structures
- Reef flat: grows to almost low sea level (within a meter or so)
- Back reef: often with more ornate structures
- Patch reefs: grow in lagoon
Coastlines are ephemeral: they move towards the ocean basin or towards the continent due
to a number of factors:
- Tectonic uplift or down-dropping of the region
- Isostatic uplift (i.e., rebound) of the region
- Eustatic (worldwide) changes in sealevel (due to glaciers or mid-ocean
All these produce one of two net effects:
- Transgression: the landward migration of the shoreline (i.e., flooding of
- Regression: the oceanward migration of the shoreline (i.e., draining of
- During a transgression the sequence of rocks will show an onlap sequence (the
facies will become deeper-water environments as you move up through the sediments).
- During a regression the sequence of rocks will show an offlap sequence (the
facies will become shallower environments as you move up through the sediments).
Transgression-regression events allow geologists to see contemporary marine and
terrestrial rocks due to intertonguing of facies.
Most of the planet's surface is underwater. In fact, the present has one of the lowest
sea levels of Earth History (lowest of all seems to be recent interglacials). In other w
ords, we are in an oceanic lowstand and the continents are mostly emergent.
During other time periods, seawater covers most of the shallow parts of the continents:
an oceanic highstand when the continents are submerged. Seas that cover
continental rock are called epicontinental or epeiric seas.
Deep ocean basins are almost never preserved in the rock record (and when they are they
are pinched out and metamorphosed during continental collisions). However, some
relatively deep continental seas are preserved, and VAST amounts of sediment from epeiric
marine rocks are known.
On the continent, or along continental shelves:
Siliciclastic Shelf Environments:
- Can vary greatly: claystones, siltstones, sandstones.
- Animal burrows VERY common, as are animal body fossils.
Carbonate Banks & Platforms:
- Today relatively rare, but once covered VAST areas.
- Generally only form in warmer waters (think "Bahamas" or "Caribbean")
- As carbonate sediment accumulates, it builds upwards to stand higher than surrounding
- Often surrounded by barrier or fringing reefs.
- Shallow water typically has ooids (abiotic) and
- Produce limestones of various subtypes.
Continental Slope Environments:
- Dominated by mudstones.
- Charactistic pulses of Turbidity Currents (submarine avalanches), which produce
graded beds that coarsen upwards (coarser greywacke sandstones with scour
marks/sole marks on bottom, fineing upwards to laminated or bioturbated muds).
- If these are preserved as rock, become turbidites. Often found with many
turbidites stacked on top of each other.
- Some terrigenous sediments along edges, and deep-sea clay (fine grain fraction from
continents that make it out to deep sea) at centers of basin.
- Also organic oozes (carbonate or siliceous). Calcareous oozes only down to
~4000 m, below which calcite dissolves due to particular pressure/temperature conditions.
- Where glaciers have spread out, often find dropstones and other glacially transported
Pelagic sedimentary environments known mostly through deep-sea drilling, and are only rarely part
of terrestrial rock record.
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Last modified: 27 February 2014