LACUSTRINE SEDIMENTS
Lacustrine refers to sediments deposited in lake environments. The
type of sediment deposited in lakes varies widely, depending on the size of the
lake, the climate, and the nature of the surrounding countryside.
Many beaches along Lake Michigan and Lake Superior in
the Great Lakes region of North America have nice, very quartz-rich
sands. Below are two examples. The first photo shown below is from
a beach along the southern shores of Lake Superior, the largest lake on Earth
in terms of areal extent. The grains shown here are all quartz, but the
entire sample also has a very small lithic sand component (<1%). The
grains are rounded to subrounded in shape. This is, in part, the result
of abrasion by shallow-water wave action. This quartz sand is almost
entirely recycled from Mesoproterozoic and Cambrian quartzose sandstones that
outcrop in the area (e.g., Jacobsville Sandstone & Munising Formation).
So, the grain shapes are principally inherited from ancient sedimentary rocks.
The sands in the Jacobsville & Munising Formations were principally abraded
by wave action in ancient shallow ocean environments.
Modern quartzose lacustrine beach sand, just below lake level at Miner's Beach, southern
shores of Lake Superior, Upper Peninsula of Michigan, USA.
Microphotograph by Sara Beth Kopczynski.
This second sample is from a beach along the
northeastern shores of Lake Michigan. The sand here consists principally
of subrounded to rounded quartz grains. The beach also has significant
pebble populations eroded from igneous, metamorphic, and sedimentary
rock. These pebbles of widely-varying lithologies were initially
deposited in this area by Pleistocene glaciers. Some of the more famous
pebble lithologies in this part of the world are Devonian fossiliferous
limestones, especially rounded clasts of colonial corals (e.g., Hexagonaria Petoskey Stones).
Bells Bay beach, northeastern entrance to Fishermans Island State
Park, northwestern Charlevoix County, northwestern Lower Peninsula of Michigan,
USA.
Modern quartzose lacustrine beach sand, swash-zone at Bells Bay beach, west of Charlevoix,
northwestern Lower Peninsula of Michigan, USA. Microphotograph by Sara
Beth Kopczynski.
Some of the oddest lacustrine sediments on Earth are
nonmarine oolites at Great Salt Lake in Utah, USA. Oolites are moderately
common in marine carbonate platform environments (now & in the geologic
past), but nonmarine oolites are not common. Great Salt Lake oolites (aka
ooids) are typically whitish to off-white to grayish colored, very well-rounded
grains composed of finely concentrically-layered aragonite (CaCO3 -
calcium carbonate).
In both marine & nonmarine settings,
oolites form by grains repeatedly rolling around on very shallow-water
seafloors or lakebeds by wave action. Calcium carbonate gets
inorganically precipitated onto the surfaces of oolites as they get rolled
around. Some researchers have suggested that the presence of bacterial
films on oolite surfaces play a critical role in CaCO3
precipitation. (When in doubt, blame bacteria!)
Modern lacustrine aragonitic oolitic sand from a beach on Antelope Island, Great Salt Lake,
northern Utah, USA. Collected & donated by Lin Jih-Pai
("Alex" Lin). Microphotograph by Sara Beth Kopczynski.
Modern lacustrine calcareous oolitic sand - broken oolites showing internal structure
consisting of concentrically layered aragonite (CaCO3 - calcium
carbonate).
Modern lacustrine calcareous oolitic sand on the southern shores of Great Salt Lake at Saltair,
Utah.