32) Chemical Sedimentary Rocks

Essentially, "Chemical Sedimentary Rocks" are products of the precipitation of dissolved minerals in water.  The chemical constituents that serve as sources are the products of Chemical Weathering (the discussion begins at about 4:38 in the 11/02/19-posted video).  A second post from 11/03/19 provides a little more information (please take notes).  

The primary modes of Chemical Weathering are Oxidation, Dissolution, and Hydrolysis (see this post).

The most common Chemical Sedimentary Rocks (in no particular order): 

Limestones, Evaporites, Cryptocrystalline Quartz (aka Flint and Chert, along with Chalcedony, Carnelian, Jasper, ... have I forgotten anyone?).  

Limestones

To keep it simple, most marine limestones largely form due to the biochemical actions of algae removing Calcium Carbonate from warm, shallow seawater for their internal structures and the additional removal of Calcium Carbonate by other organisms for their external structures.  Upon the deaths of these organisms, their respective carbonate residuum becomes part of the shallow seafloor, often becoming the matrix enclosing the bioclastic remains of macroscopic organisms.

The most favorable settings for marine Limestones are usually relatively free of suspended "clastic muds" (or minute clastic quartz and clay particles), away from the influences of river deltas that drain large terrestrial exposures of silicates.  [Think of the modern Caribbean Sea, Bahamas, the various archipelagoes of the South Pacific.]   (See more in the video on Carbonate Platforms.)

There are a few freshwater limestones found in the Western United States (or in some cases, quartz sandstones with calcite cement).  Both the Green River Formation (UT, CO, WY) and the Verde Formation (central AZ) include some "freshwater" carbonate facies.

Evaporites

The next major class of Chemical Sedimentary Rocks is Evaporites, which include Halite, Sylvite, Gypsum, and Anhydrite.  The Evaporites are discussed beginning at 1:03 and ending about 4:03.  The best sites for deposition are Enclosed Basins, in areas of High Evaporation, and Warm Temperatures.  Within these Enclosed Basins, there is Inflow of water during storms, but no Outflow.  The Warm Temperatures and High Evaporation rates usually occur between 30 degrees Latitude North and South.

From the CliffsNotes:

"Evaporites are rocks that are composed of minerals that precipitated from evaporating seawater or saline lakes.  Common evaporites are halite (rock salt), gypsum, borates, potassium salts, and magnesium salts."

From this source is a partial history of the accumulation of several vertical kilometers of salt in the growing Mesozoic Gulf of Mexico:

"The Gulf of Mexico has always lain in the hot, arid subtropics.  As a small ocean basin peripheral to the Atlantic, the Gulf has experienced extended geological intervals when connection to the world ocean was restricted.  The first, and most dramatic result of aridity and restriction was deposition of widespread salt, the Louann, over much of the basin floor.  This salt layer formed the foundation onto which subsequent sediment would be deposited.  Salt deposition continued for several millions of years, creating a unit as much as several kilometers thick.  ..."

Siliceous 

The Silicate-based Chemical Sedimentary rocks are discussed between 4:03 and 5:10 in the video, primarily related to the "Where and How" the silicates are deposited.  From "The Quartz Page - Flint and Chert":

"In sedimentary rocks, the silica in flint is usually of biogenic origin: countless skeletons of tiny marine organisms, like radiolaria, foraminiferes, or diatoms that have sedimented on the ocean floor and are slowly buried under more chalk, silica and organic ooze. The silica skeletons consist of opaline silica (amorphous silica with some water) and are much easier dissolved in alkaline solutions than quartz." (Emphasis added.)

[The mechanisms of "How" silica replaces invertebrate fossils, fossil wood, and entire limestone units is getting pretty deep into Geochemistry and beyond the current scope of this blog.  If I find a source, I will add it.]

The video's author includes interactions of sedimentary and igneous rocks; and the actions of associated hydrothermal fluids and the deposition of secondary minerals.  (I will leave you with the narration beginning at about 8:10 in the video, otherwise, I might write five more paragraphs.)

This aspect (or branch) of Geology is one of the reasons that we are usually required to take multiple Chemistry courses to get our Bachelor's Degrees.  (In my case, I had to suffer through three of them as an undergrad.  Perhaps it was my "math troubles" that made Chemistry (and Physics) tough for me.)  

Anyway, once we have gained a "working knowledge" of "the Chemistry of Minerals", it helps us understand the "Hows and Whys" of "post-magmatic" mineral associations.

[Photos may be added.]

Additional Sources:

Cliff Notes: Chemical Sedimentary Rocks
GeoExpro Article Gulf of Mexico
The Quartz Page Flint and Chert Page