Blue-Gray Limestone, Crinoids, and Large Quartz Crystals in Western Arkansas

     Yes, this is Maizie and me licking our lips (or nose, in her case) at the striking roadcuts of the Ozark Mountains (actually plateaus) of northern Arkansas and the Ouachita Mountains of west-central Arkansas.

       The western part of Arkansas includes, from north to south: the Ozark Plateaus or Mountains, the Arkansas River Valley, the Ouachita Mountains, and the West Gulf Coastal Plain. The Ouachita Mountains are one of the few east-west-trending ranges in the U.S. (The Uinta Mountains in Utah and Mt. Tom/Mt. Holyoke Range in Massachusetts are two of the other east-west-trending features.)

      The geologic units are outlined below, including shades of blue for blue-gray Mississippian rocks. (NB: unless one is a geomorphologist, most geologists just gloss over anything coloured yellow for the Tertiary and Quaternary.)

       The Mississippian Pitkin Limestone is 360 to 320 years old and is chucky-jam full of fossils, including crinoids and bryozoans. The crinoid stems (which resemble rolls of Smarties candies) are parts of animals called sea-lilies that attached to the seafloor. In this rock sample, which may or may not have travelled from northern Arkansas to Colorado, shows the unweathered fossils.

        Here are some weathered out crinoid stems.

        And here is the stem and the less-often preserved top of a crinoid animal.

       The photo below shows weathered-out screw-shaped bryzoans, commonly referred to as Archimedes screws (in the right bottom part of the photo.) The upper left part of the photo shows the crinoid stem pieces. The crinoids are so plentiful in the Pitkin Limestone that some people apparently fling them on the ground like pop-rocks. We did not partake of that practice.

      The Pitkin Limestone overlies the Fayetteville Shale as seen in this roadcut. The "tight" shale is the source of much gas development in Arkansas via hydraulic fracking.

      One of the most spectacular parts of our Natural State of Arkansas journey was traveling south along state route 7 from Russellville to Ouachita Hot Springs National Park, right through the heart of the Ouachitas.

         Here's the view (with Maizie) toward the Arkansas Valley:

        Ouachita Hot Springs National Park:

       Pictured below are some large quartz crystals from Blue Springs, Arkansas, which are now at the Crystal Bridges American Art Museum grounds in Bentonville.

       . . .And a few images incorporating geology and art from Crystal Bridges:

      And lastly, a beautiful morning with cool mist in the Ozarks somewhere between Mountain Home and Eureka Springs:

Have you explored Arkansas? Hoping you had/have a traveling companion and navigator as amazing as Maizie. . .

Steph

The Universe Truly IS in a Grain of Sand

          You may have noticed I've been a bit sand-obsessed since my trip to Great Sand Dunes National Park in Colorado in October. Discovering these individual sand grain images magnified 250 times has blown in a fresh look at sand. The sea urchin spines in the right part of the image are particularly striking in this calcium carbonate-rich sand:

      The three-pronged sponge spicule in this image from a Maui beach is but one sand grain; "sand" is defined as a size of sedimentary particle ranging from 1/16 to 2 mm, rather than composition (I.e., quartz).

     Sand grains may also be glacially deposited as these grains of garnet, agate, epidote, quartz, magnetite, and hematite, in Lake Winnibigoshish, WI.

      Sand grains of gypsum from the White Sands area in New Mexico, are some of the most uniform in color, though they are quite soft (hardness of 2 on Moh's Hardness Scale):

      And the hydraulic fracturing sands in western and southern Wisconsin are quite uniform, hard (hardness of 7 on Moh's Hardness Scale) quartz grains:

      "Puffy stars," calcium carbonate forams on Okinawa beaches are quite uniform in size and have a distinctive shape:

     Note the rounded, smooth shells, foraminifera, piece of coral, and the volcanic fragment (in the lower right.)

     Check out these colorful, luminescent, rounded bits of foraminifera, shells, and quartz in this sand mix.

And, to tie things back to where we started this week, here are rounded, smoothed, sea urchin sand fragments from Hawai'ian sand; these are essentially cross sections of the long, green spines seen in the first image.

Take this all with a grain of calcium car- bonate or quartz or gypsum salt or. . .

Steph

There are GLOSTA lovers in Colorado!

Migmatites and Ptygmatic Folding

         Let's examine ptygmatic folding in migmatites (huh?). Take a look first:








                                                         Migmatites are a mixture of metamorphic and igneous rock. The pink plagioclase feldspar in the rock below formed a ptygmatic fold after remelting of the lighter colored minerals:

         
           Thus, this remelting creates a mixture of the unmelted metamorphic part with the recrystallized igneous part. Migmatites tend to occur at very high-pressure and high-temperature zones and in very old rocks (such as Precambrian).

           The word ptygmatic was introduced into the geological literature by Jakob J. Sederholm in 1907 and originates from the ancient Greek word for “fold” or “anything folded.” The term “ptygmatic fold” is somewhat redundant, like saying “a folded fold.” However, the term “ptygmatic” in the geologic literature generally refers to tight folds that form when the folded material has a greater viscosity than the surrounding medium. In migmatites, ptygmatic folds generally form in the more viscous lighter layers.   

      A look at Bowen's Reaction Series:

              

          shows why the darker minerals such as olivine, pyroxene, amphibole, and biotite tend not to remelt as it takes higher temperatures for this to happen. The lighter-colored K-spar, muscovite and quartz melt at much lower temperatures forming those ptygmatic folds of newly igneous material.

      Sometimes, differentiating between migmatites and metamorphic gneisses can be difficult--but we'll leave that for a later, gneisser time.

Migmatitely,

Steph