A "New" Geologic Term?

Figure 1.

It appears that along with complex structural features in heavily-weathered metamorphics, this site, at Laurel Park at Lake Lanier in Hall County, Georgia, has a "Tortured Pegmatite" (right half of photo).  The host rock was probably an amphibolite or maybe a biotite schist (my choice is the former) that has undergone extensive Ductile Deformation in addition to the Chemical Weathering.  The Pegmatite itself seems to be primarily Quartz & K Feldspar, with minor Muscovite.

The park is located on a peninsula that extends roughly southeast into Lake Lanier and from the perspective of nearby Cleveland Highway (US Hwy 129/GA Hwy 11), the feature above is on the southwest side of the peninsula, near the distal end.

I was drawn to the park by way of a TV news report.  Due to a protracted drought (in 2007) and Corps of Engineers' mistake, water levels were about 12 feet below the Full Pool elevation of 1071.00 feet above MSL.

On the other side of the peninsula, close to the tip, were exposed a portion of the concrete grandstands of an old, submerged raceway, locally known as Looper Speedway.  As I am a fan of stock car racing, I went to get a few photos and wound up wandering the park shoreline for a couple of hours, looking at the geologic features. 

Also known as Gainesville Speedway, it was a 1/2 mile dirt raceway that hosted stock and modified stock car races from 1949 until it closed during the filling of the lake in 1956.  I think I heard Lee Petty's name mentioned as having raced there, as well as (probably) the Flock brothers (from Atlanta). 

Returning to the lakeside features, Figure 2 is a broader view of the Wavecut Bench shoreline of the west side of the peninsula, with other exposures of "Tortured Pegmatite" amidst the heavily "saprolized" country rock discussed above.  As with Figure 3, here the metamorphic saprolite is overlain by sands and rounded quartz pebbles, derived from the overlying eroded paleo river gravels (containing small amounts of heavy minerals and gold).

Figure 2.

Further northwestward along the west side of the peninsula were miniature Wave-Cut benches in the shoreline sands, temporarily recording the decline of lake water levels during the multi-year drought (2006 - 2009).

Figure 3.

Figure 4.

Some of these sands may have been introduced as "Beach Nourishment" for the nearby park.

Above the old concrete bleachers was this Nonconformity (Sedimentary over Metamorphic Saprolite). 

 Figure 5.

There are other interesting places along the shores of Lake Lanier that I have visited during Protracted and Winter Seasonal Droughts, including the west side of Van Pugh Park (where there are exposures of small-scale Ductile Deformation and a Tourmaline-K Feldspar Pegmatite), the subject of a future post.

Careers in the Geosciences

Careers in Geology

Earth Science Careers Not Just Rocks USGS

Should I major in Geology? and Other FAQ!

One Would Think ...

... That self-quarantine during the Coronavirus outbreak would be a good time to blog and a good time for others to read blogs, to fight the "Coronavirus cabin fever".  True, but from my end, I am still getting used to the things that I "can't currently do".

[I am thankful for our household and that of close family members - thus far - being bypassed by the Coronavirus.]

Among those things I can't or shouldn't do include visiting my adult kids and grandkids, going to my adult Sunday School, having Mexican food (in a restaurant) at least once a week, regularly visiting beer stores in search of new Georgia craft beer cans, visiting new local breweries, sharing a craft beer (or two) with a nearby friend, visiting used book stores, substitute teaching, ...  I am still "wrapping my head around all of this", that it may be going on for weeks.  I am worried about the survival of some restaurants and newly-opened breweries, as well.

Theoretically, I could still go out and do "rockhounding" and nature photography, as that usually doesn't include personal contacts with other people and I may do that in a few weeks.  For the short-term, I need to remain at home while my wife recuperates from total knee-replacement surgery last week.

Add to that is the normal Spring allergy season and above-average late-Winter rainfall and the fatigue and disillusion that those events bring.  Any prayers and kind thoughts are appreciated.  

Sedimentary environments

Sedimentary Structures and Environments of Deposition

05 - Sedimentary Rocks: Depositional Environments

Physical Geology, Sedimentary Depositional Environments

Changes Coming ...

After neglecting this blog for several years, as well as my other blog, the travel-related Itinerant Geologist, late last year I embarked upon a binge of postings, partially to force myself to keep busy.  

Due to some home- and family-related issues and responsibilities, those attempted daily posts were my attempt to stay in touch with Geology and related sciences, that have been part of my life for 50+ years.  I also have some personal narratives as writing projects about cross-roads events in my life.  And sporadic gardening, carpentry, and photography projects, when weather, time, circumstances, and inspirations allow. 

When I post videos - from other people - I intend to include some of my own comments and images, where pertinent.  But those additional comments (on both blogs) take time, energy, inspiration, and concentration to articulate, compose, and edit.  In other words, those "isolated" or "orphaned" videos are intended to have something else added.  As time permits, I hope to add comments and summaries, after-the-fact.

There are a few more of these orphaned videos "in the hopper" for the next few days, as my wife is undergoing knee-replacement surgery tomorrow and will be recuperating after that at home.  Thus she will need at-home assistance for a few days, at least.  Other than checking email, I may not have the time or the attention span to blog.

So please bear with me.  Additionally, I am considering merging the younger Itinerant Geologist blog posts into this one, as I am not sure of the efficacy of trying to maintain its separate-but-related goals as a geo-travel blog (as present circumstances don't allow much travel).  There is intended to be more text and notes presented along with the travel videos (as suggested above).

37) Depositional Environments

36) Secondary Sedimentary Structures

35) Primary Sedimentary Structures Pt. 2

34) Primary Sedimentary Structures Pt. 1

33) Clastic Sedimentary Rocks - Part 1

A few points-to-remember:

Clastics are derived from the Mechanical Weathering (aka Physical Weathering) of pre-existing rocks.  (Chemical Weathering plays a subsidiary role in breaking down the rocks, too.  It will be partially addressed below.)

A shown, Physical Weathering is akin to smashing a rock with a sledgehammer.  The mineral makeup of the fragments (clasts) is not changed during the breakage, however, the increase in surface area (and creation of micro-fractures within the clasts) provides additional sites for subsequent Chemical Weathering in humid climates and in semi-arid/arid climates following their rare precipitation events.  

[In arid and semi-arid climates, the paucity of water restricts Chemical Weathering and soil formation.  In those settings, Physical Weathering dominates the natural "destruction" of rocks.] 

Modes of Physical Weathering include:

• Ice Wedging (water held within fractures and micro-fractures expands during freezing).
• Thermal Expansion and Contraction - Diurnal and Seasonal.  [There is (or has been) some disagreement over the relative importance of Diurnal Expansion and Contraction.]
• Exfoliation Jointing and Tectonic Jointing and Fracturing (aka Brittle Deformation).
• Root Wedging.
• Abrasion by wind or water.
• Impact Fracturing (aka Percussion) in streams or landslide events.  (May include human-caused impacts.)

Shown below are examples of Thermal Fracturing and Exfoliation Jointing. 

 Figure 1.

Figure 2.

 Figure 3.

Figure 4.

Figure 5.

In the early portion of the video, the clast-size scale discussed is known as the "Wentworth Scale".  

Clays, Claystones, Mudstones, and Shales (<0.004 mm on the Wentworth Scale) are the most common Clastic sediments/sedimentary rocks, though they owe part of their genesis to Chemical Weathering.  [The property of "fissility" (as shown above) and its origin is addressed first at about 3:06 in the video above.]

The tiny plates of the Clay Minerals are the residuum of Chemical Weathering of Silicate Minerals, largely by way of Hydrolysis and Oxidation.  (Remember that the Chemical Weathering susceptibility of the "rock-forming" Silicates is related to their position on the Bowen Reaction Series.  Olivine and Calcium Plagioclases are "first", while K Feldspar and Muscovite are "last".  See the "note" about Quartz in the next paragraph.)

Within the remainder of the Wentworth Scale realm (>0.004 mm), Quartz is the most common component and under surface conditions, Quartz is usually just "broken into smaller components" by Physical Weathering, while remaining chemically stable.

[For the sake of brevity, more related photos will be shown in another post, soon.]  

24 - Carbonate platforms

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

31) Sedimentary Rocks Overview

Modes of Chemical Weathering

This post was "dropped in" to help clarify the terms in past and upcoming posts in regard to chemical weathering of minerals.

To revisit Physical Weathering (aka Mechanical Weathering) briefly:

Modes of Physical Weathering include:

• Ice Wedging (water held within fractures and micro-fractures expands during freezing).
• Thermal Expansion and Contraction - Diurnal and Seasonal.  [There is (or has been) some disagreement over the relative importance of Diurnal Expansion and Contraction.]
• Exfoliation Jointing and Tectonic Jointing and Fracturing (aka Brittle Deformation).
• Root Wedging.
• Abrasion by wind or water.
• Impact Fracturing (aka Percussion) in streams or landslide events.  (May include human-caused impacts.)

A common "result" of these Physical Weathering Modes is that the rock's chemistry does not change, it is just broken into smaller pieces, yielding more surface area.  Chemical Weathering results in actual changes in chemistry due to Oxidation, Dissolution, and Hydrolysis.  In some cases, depending on the water chemistry, any two or all three of the processes may occur simultaneously. 

The stability of minerals - as solid, naturally-occurring chemicals - is largely related to the strength of their internal chemical bonds, whether Ionic, Covalent or a combination of the two.  Some minerals have both.  When a compound is composed of a metallic cation and a polyatomic anion, the compound is considered to be ionic, though there might be covalent bonds within the polyatomic anion.

The Polarity of Water molecules (one side positively-charged, the other side negatively-charged) is responsible for many of Water's interesting properties, including water acting as a "carrier" of ions (positive and negative).  

Under natural conditions, water can combine with atmospheric gases producing natural acids such as Carbonic, Nitrous, and Sulfurous (some of these may be enhanced by gases within soils as rainwater percolates downward).  Human activities can add additional acids to the mix.

Figure 1.

Figure 2.

The most ubiquitous form of Oxidation that we see is Iron Oxidation, whether it is of iron-bearing Oxides and Silicates in Diabase (Basalt), of the iron-bearing steel of old cars or old beer cans, or of iron-bearing Sufides, e.g., Pyrite.

Dissolution is the "dissolving" of certain minerals, most easily seen with the halides which have weak Ionic Bonds (NaCl - halite, table salt easily dissolves in water).  In the case of calcium carbonate (Calcite in Limestone) and calcium sulfate (Gypsum in Evaporites), the ionic bond between the Calcium and the polyatomic anion is fairly-easily broken by the combination of water and naturally-occuring acids. 

 Figure 3.

Figure 4.

When we think of caverns, we often think about structures such as stalagmites, stalactites, columns, and such.  These are actually secondary depositional structures, not Dissolution Features.  As pictured above, Sculpted Ceilings are a good subsurface example of Dissolution.  

In the early stages of Karst development, the caverns begin as exposed joints, fractures, and bedding planes in limestone and marble (and rarely gypsum) - below the Water Table.  Dissolution slowly takes place as the water replenished by more-acidic percolated groundwater, following surface precipitation events.  

Where the Water Table is "tied" to exposed surface features, e.g., nearby creek and river valleys, as down-cutting occurs, the underground Water Table is "dragged down", exposing the cavern ceiling, while the cavern floor dissolution continues.  

(It is after this subterranean exposure that the stalactites begin to grow.  But that's another story.)

Figure 5.

The hand-sized sample of Potassium Feldspar above shows the effects of Hydrolysis (here termed Kaolinization), as seen by the chalky appearance.  As you can see traces of the original Feldspar Cleavage, the Kaolinization is not yet complete.

Except for Quartz, every Silicate Mineral in the Bowen Reaction Series is susceptible to Hydrolysis.  As related previously, the minerals near the "top" of the Bowen Reaction Series (as with the presumed aphanitic Olivine, Pyroxene, and Plagiolcase in the Diabase) are more susceptible to the effects of Chemical Weathering than is the K Feldspar (near the bottom). 

[Below the surface, Quartz is soluble in certain circumstances, but "that's another story", too.]

30) Large Scale Igneous Features

29) Extrusive Igneous Rocks

Geology: Introduction to Volcanology!

28) Intrusive Igneous Rocks