GEOL 102 Historical Geology
Spring Semester 2017
Fluvial & Deltaic Environments; Walther's Law
Fluvial - Rivers
In the above, each particular subenvironment produces rocks of a particular facies
(particular combination of sedimentological, structural, and biological features).
- Make up a very large fraction of terrestrial sedimentary rocks
- Rivers are largely erosive, but some relatively continuous (pointbar) and
periodic (flood) deposition
- Braided streams - horizontal beds of conglomerates and sands with unidirectional
crossbeds, almost no mud.
- Meandering streams
- Stream channels migrate downstream over time
- Produce very distinctive sediments:
- Channel conglomerates
- Cross-bedded channel sandstones, well-sorted
- Pointbar sands, somewhat finer than channel sands
- Levee sands, ripple marks and finer grains
- Backswamp, non-laminated mudstones
- This package of sediments produces a sequence that fines upwards.
- Also associated with meandering streams
- Additionally, meandering stream sequences surrounded by flood-plain deposits of
paleosols interrupted by layers of flood muds. The whole package of stream deposits,
paleosols, and ox-bow lakes form fluviolacustrine deposits.
Walther's Law: As depositional environments migrate laterally, the sediments of
one environment come to lie on top of sediments of the adjacent environment.
Deltaic - deltas
- Form where rivers empty into a spot where the flow is zero:
- Typically a sea
(or lake or other still water)
- BUT can also be a river emptying into a desert, like Botswana's
Okavango Delta (and ancient deltas, as in the Karoo Supergroup of southern Africa or the
Nemegt Formation of Mongolia)
- Suddenly stream flow drops down to zero: material falls out of suspension.
- Deltas are modified in shape by waves, tides, and stream forces, but produce same
general facies of different sedimentary environments from inland towards the water body (discussion below assumes delta into water: substitute "desert deposits" and "desert fossils" as appropriate for Okavango model!):
- Topsets (delta plain): sand and silt, nearly horizontal, cut through
with stream channel deposits and with paludal facies, will contain terrestrial fossils
- Foresets (delta front): grade into silt and clay, slope seaward, will
contain marine fossils, but also some material washed in from terrestrial environment
- Bottomsets (prodelta): clay, horizontal beds, marine fossils dominate
- Deltas build seaward, or prograde. Thus, bottomsets are deposited at a
particular spot, then covered by foresets as the delta front moves past, then by topsets
as that spot has become filled with sediment to the surface. So, by Walther's Law,
deltaic sediments come in stacks that coarsen upwards (opposite of fluvial).
- Because active lobes of deltas change position from time to time, can accumulate many
episodes of deltaic sedimentation on any spot.
To Lecture Notes.
Last modified: 18 January 2017