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.  Shown 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 (a.k.a. ooids, ooliths) 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 repeated rolling 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.

 


 

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