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Learn about metamorphic facies and how these categories of certain minerals show the grade of metamorphism in a body of rock.

Correlation of Strata The need to classify and organize rock layers according to relative age led to the geologic discipline of stratigraphy. Rocks at different locations on Earth give different "snapshots" of the geologic time column.  At a particular location, the rocks never fully represent the entire geologic rock column due to extensive erosion or periods of non-deposition or erosion. The thickness of a particular rock layer (representing a particular time period) will vary from one location to another or even disappear altogether. The process that stratigraphers use to understand these relationships between strata at different localities is known as "correlation". For example, rocks named Juras (for the Juras Mountains) in France and Switzerland were traced northward and found to overlie a group of rocks in Germany namedTrias.  The Trias rocks in turn, were found to underlie rocks named Cretaceous in England (the chalky “White Cliffs of Dover”). Based on these relationships, is the Juras older or younger than the Cretaceous?  What are the two possible scenarios? The location where a particular rock layer was discovered is called a "type locality".  Most of the “type localities” of the geologic time column are located in Europe because this is where the science of stratigraphic correlation started. The Sedgwick/Murchison Debate In 1835, Adam Sedgwick (Britain) and Roderick Murchison (Scotland) decided to name the entire succession of sedimentary rocks exposed throughout Europe.  They were geology colleagues and friends, but they had a famous argument over the division between the Cambrian and Silurian in Wales.  Sedgwick’s topmost Cambrian overlapped with Murchison’s lowermost Silurian.  Eventually the disputed rock layers were assigned the age “Ordovician”. Rocks Divisions versus Time Divisions It is important to remember that the rock record is an incomplete representation of real geologic time due to the presence of unconformities. Therefore, geologists are careful to distinguish geologic time from the rocks that represent snapshots of geologic time: TIME DIVISIONS CORRESPONDING ROCK DIVISIONS (AND ROCK UNITS) Eon Examples: Precambrian/Phanerozoic Eonothem           Era           Examples: Paleozoic/Cenozoic/Mesozoic           Erathem                Period                Examples: Cambrian/Ordovician/Silurian                System               Groups                     Formations (The main stratigraphic unit)                          Members Rock divisions, such as the Cambrian System, can be correlated worldwide based on fossils.  In contrast, rock units such as groups, formations, and members are localized subsets of systems.  Rock units depend on the environment of deposition, which varies from one location to another. Stratigraphic Rock Units The rock divisions (Eonothem, Erathem, and System) simply divide rocks into the appropriate time eon, era, or period.  Obviously, all Cambrian System rocks are from the Cambrian regardless of their location on Earth's surface. In contrast, the rock units (Groups, Formations, Members) are localized features (of limited regional extent) that depend on the local environment of deposition.  The main rock unit of stratigraphy is the formation, a localized and distinctive (easily recognizable) geologic feature (i.e., the Chinle Formation of Late Triassic lake and river deposits in Arizona, Nevada, Utah, and New Mexico). Different formations are distinguished and correlated based upon lithology (overall rock characteristics), which includes: 1) Composition of mineral grains 2) Color 3) Texture (grain size, sedimentary structures) 4) Fossils Formations are “clumped” into groups and divided into members. Datum- In correlation, a datum is a line of equivalent age. The ideal datum is a stratigraphic marker that is both geographically extensive and represents an instantaneous moment in geologic time.  A good example is a volcanic ash layer that formed by a specific volcanic eruption followed by worldwide dispersal by atmospheric currrents. Using Fossils for Strata Correlation Sedimentary rocks that date from the same age can be correlated over long distances with the help of fossils. Principle of Fossil Correlation- Strata containing similar collections of fossils (called fossil assemblages) are of similar age.  Also, fossils at the bottom of the strata are older than fossils closer to the top of the strata. Index Fossils- Index fossils are the main type of fossil used in correlation.  To be an index fossil, a fossil species must be: 1) Easily recognized (unique). 2) Widespread in occurrence from one location to another. 3) Restricted to a limited thickness of strata (limited in age range). The limited life-spans of these organisms allows us to easily constrain the age of rocks in which they occur. The best index fossils are those that are free floating and independent of a particular sedimentary environment.  For example, organisms that are attached to one particular type of sediment are going to have limited geographic extent and will not be found in many rock types.   By contrast, organisms that are “free floaters” or “swimmers” will have a wider geographic extent and be found in many different rock types (i.e., trilobites). A fossil zone is an interval of strata characterized by a distinctive index fossil. Fossil zones typically represent packets of 500,000 to 2,000,000 years.  Fossil zones boundaries do not have to correlate with rock formation boundaries.  Fossil zones may be restricted to a small portion of a formation or they may span more than one formation. A fundamental assumption in fossil correlation is that once a species goes extinct, it will never reappear in the rock record at a later time. Fossil types that are generally restricted to just one type of sediment are called facies fossils.  They are not very useful in correlation, but are extremely useful for reconstructing paleoenvironments.   What is a Fossil? Some examples of fossils are: 1) The preservation of entire organisms or body parts.  This includes the preservation of actual body parts (mammoths in tundra), as well as morphological preservation via the replacement of biological matter by minerals (petrified wood). A petrified log in Petrified Forest National Park, Arizona, U.S.A.-impressions 2) Casts or impressions of organisms. Eocene fossil fish Priscacara liops from Green River Formation of Utah 3) Tracks. Trackways from ''Climactichnites'' (probably a slug-like animal), in the Late Cambrian of central Wisconsin. 4) Burrows. Thalassinoides, burrows produced by crustaceans, from the Middle Jurassic of southern Israel. 5) Fecal matter (called coprolites). Carnivorous dinosaur dung found in southwestern Saskatchewan,  USGS Image. Theories on The Origin of Fossils At one time, fossils were considered to be younger than the rocks in which they occurred.  People speculated that fossils formed when animals crawled into preexisting rock, died, and became preserved in stone. Some people interpreted the widespread occurrence of fossilized marine organisms on land as support for a world-wide flood as described in scripture. Leonardo da Vinci’s (1452 - 1519) Interpretation of Fossils Self-portrait of Leonardo da Vinci, circa 1512-1515. Regarding fossils that occur in strata many miles from the sea, da Vinci argued that: 1) The fossils could not have been washed in during a "Great Deluge" because they could not have traveled hundreds of miles in just 40 days. 2) The unbroken nature of the fossils suggest that they were not transported by violent water; instead the fossils represent formerly living communities of organisms that were preserved in situ. 3) The presence of fossil-rich strata separated by fossil-poor strata suggests that the fossils were not the result of a single worldwide flood, but formed during many separate events. Lateral Variations in Formations Historically, geologists initially believed that the layer-cake sequence of sedimentary rocks existed worldwide (i.e., that the layers extended indefinitely without change). By the late 1700’s people began to realize that formations had a limited extent both vertically (up and down) and laterally (horizontally across Earth's surface). People also began to realize that lithologic variations (changes in texture, color, fossils, etc) can occur laterally within formations themselves. Today we interpret such variations in the context of modern depositional environments.  For example: ENVIRONMENT OF DEPOSITION EXPECTED LITHOLOGY Near shore marine- The energy is high due to rough waters at the water-land interface. Coarse sediments, and fossils of robust organisms that can withstand high energy environments. Deep ocean- The energy is low due to the general calmness of water away from land. Fine sediments, and fossils of more fragile organisms. Note that the two different lithologies can be deposited simultaneously (representing the same moment in geological time) so long as they are deposited at different locations. Different lithologies grade laterally into one another in a manner called intertonging.  An example is the way that the Old Red Sandstone of Wales (a terrestrial deposit) grades laterally into marine sediments of Devonshire to the south (both are Devonian). Intertonging reflects the changes in depositional environments that occur over space and time (lateral and temporal variations).  Often these changes in environment are linked to shoreline migrations resulting from sea-level changes over time.  Depositional Environments and Sedimentary Facies Depositonal environments are characterized initially by the sediments that accumulate within them, and ultimately by the sedimentary rock types that form.  For example, a reef environment is characterized by carbonate reef-building organisms.  Ultimately, the sediments become lithified to form fossiliferous limestone. A sedimentary facies is a three-dimensional body of sediment (or rock) that contains lithologies representative of a particular depositional environment.  For example, FACIES LITHOLOGIES Floodplain Mudstone and shale with interbedded sandstone. Ocean basin Laminated pelagic clays, cherts, and possible limestone. Delta Well-sorted, well-rounded, and possibly cross-bedded sandstone. Analysis of sedimentary facies helps geologists to reconstruct past geologic environments and paleogeography. Transgressions vs. Regressions The sea-level has fluctuated throughout geologic history, and these changes have a profound effect on the geologic rock record. A transgression is an advance of the sea over land. A regression is a retreat of the sea from land area. A transgressive facies pattern is characterized by: 1. The movement of marine facies landward over terrestrial facies. 2. A fining-upward sequence (the new marine environment is lower energy than the prior terrestrial environment). 3. A basal, erosional unconformity (erosion was more profound before the seas advanced). A regressive facies pattern is characterized by: 1. The movement of terrestrial facies seaward and over marine facies. 2. A coarsening-upward sequence. 3. An erosional unconformity at the top. Walther’s Law- Over time, the lateral changes in sedimentary facies due to transgressions and regressions will also produce vertical changes in sedimentary facies: 1. A transgressive facies sequence fines in the direction of the transgression, and also fines upward. 2. A regressive facies sequence coarsens in the direction of the regression, and also coarsens upward. What causes transgressions and regressions? 1. Worldwide rises and falls in sea level (eustatic changes), perhaps related to climatic change. 2. Tectonic uplift, isostatic rebound, or crustal subsidence. 3. Rapid sedimentation. It is often difficult or impossible to determine the exact cause of a transgression or regression seen in the geologic record.  The cause may be worldwide or local.  The fact that there is a transgression or regression indicates an “apparent” sea-level change.  The Stratigraphy of Unconformities Recall that unconformities represent missing time due to: 1)      Periods of non-deposition. 2)      Periods of erosion. The main types of unconformities are: 1. Disconformity 2. Angular unconformity 3. Nonconformity 4. Paraconformity Unconformities vary from one location to another (just like rock formations and sedimentary facies).  In other words, some locations along the unconformity surface will represent more missing geologic time than others. Unconformities may eventually disappear laterally and transition into a conformable sequence of strata. Oil companies use large scale, unconformity bounded rock units called sequences to correlate rocks in a process called sequence stratigraphy. Six major unconformity-bounded sequences are recognized worldwide in the Phanerozoic.  These sequences are not restricted to period or era boundaries. The major sequences are believed to represent worldwide fluctuations in sea-level.

En el último post expliqué la lógica que subyace a la clasificación que hizo F. Bordes de las herramientas de piedra musterienses. Es decir...

The actor known for comic roles in Blackadder, Mr Bean and Johnny English will take on the role of dour French detective Jules Maigret, according to reports.

Lond-based photographer Brock Elbank is on a journey to photograph 150 freckled people for his next exhibition in 2017. He wants to celebrate beauty and, since mid-2015, he has succeeded with 90 striking portraits. If you want to be featured, you can still contact him!

#yangchaoyue

Pair of 17th century hand colored, copper plate engraved Baroque celestial maps by Johann or Johannes Zahn (1641-1707), "Facies Hemisphaerii Coelestis Inferior Austalis" and "Facies …

Hoje tem 5 ideias incríveis usando potes de requeijão. São ideias de lembrancinhas facies e baratinhas reutilizando materiais.#decoração #festainfantil #lemb...

Flysch is a sequence of sedimentary rocks that is deposited in a deep marine facies in the foreland basin of a developing orogen. Flysch is typically

Chuck Norris's cousin, Vic Norris, digs hidden kimberlite under grassy vegetation anomaly discovered by the WGS  in the Iron Mountain kimberlite district in Wyoming. Kimberlite is very difficult to recognize. It is basically a potassic peridotite and comes in a variety of colors and textures. Most notable is green kimberlite due to abundant serpentinized olivine.  It typically erupts from a feeder dike complex at depth and rises to a pipe-like structure known as a diatreme and blows out at the surface like a canon under great pressure.  Hypabyssal facies kimberlites samples fro the Iron Mountain district, Wyoming. Note the large, rounded mineral grains -  these are hematite -serpentine pseudomorphs after olivine. This type of kimberlite forms in dikes and at the 'blow' of the  the kimberlite pipe. Hypabyssal facies kimberlite, Masontown, Pennsylvania. This kimberlite dike is enclosed by black shale. Almost looks like basalt, but this is a sample of Ison Creek kimberlite I collected in  Kentucky, known as basaltic kimberlite. Snap Lake hypabyssal facies kimberlite, Fort Smith, Canada. Diamond-bearing diatreme facies kimberlite breccia from Lake Ellen, UP, Michigan. You probably would never have guessed this to be kimberlite. This bleached, tuffaceous, crater facies kimberlite from the  Iron Mountain district, Wyoming has some pyrope garnet and picroilmenite and looks more like scoria than kimberlite IG3 Kimberlite from Iron Mountain. Another tuffaceous kimberlite. The Ferris 2 kimberlite from Wyoming. Large fractured chromian diopside (chrome diopside gemstone) megacryst in Sloan 2 kimberlite from Colorado. Gemstones like this are typically not recovered from diamond mines even though they are as beautiful as any emerald. Kimberlite from the Victor pipe in Canada.

Verläufe der A. coronaria sinistra. Entspringt der Aorta und teilt sich nach wenigen cm bereits in den R. interventricularis anterior (der im Sulcus interventricularis anterior verläuft. Verläuft auf "Unterseite" (Facies diaphragmatica) des Herzens Anastomosiert mit Ramus interventricularis posterior der rechten Koronararterie) und den R. circumflexus (der im Sulcus coronarius nach die Rückseite verläuft) auf. Versorgungsgebiet (gilt für Normalversorgertyp).

Pressure-Temperature-time paths This post is in the middle of a series on metamorphism. Concepts such as metamorphic facies or grade all allow us to link a metamorphic rock to a particular set of conditions, under which it was metamorphosed. This is a simplification, of course. Hold a piece of schist in your hand: we know

PRICES MAY VARY. COQUINA | Composed of bone and shell fragments, coquina is nearly 100% fossil matter. Held together by a carbonate cement, this skeletal mass forms in low-energy environments such as lagoons and reefs GREAT FOR GEOLOGY CLASSROOMS | Ideal for identification & classification exercises. Great for lessons in sedimentary facies, paleontology, or stratigraphy. This set of six samples is excellent for group study and examination GEOLOGIST SELECTED | Each specimen is selected and hand p

Types of Metamorphic Rocks  Coming up with a way to classify and name the great variety of metamorphic rocks on Earth hasn't been easy. After decades of debate, geologists have found it most convenient to divide metamorphic rocks into two fundamental classes: foliated rocks and non-foliated rocks. Each class contains several rock types. We distinguish foliated rocks from each other partly by their component minerals and partly by the nature of their foliation, whereas we distinguish non-foliated rocks from each other primarily by their component minerals.  Foliated Metamorphic Rocks  To understand this class of rocks, we first need to discuss the nature of foliation in more detail. The word comes from the Latin folium, for leaf. Geologists use foliation to refer to the parallel surfaces and/or layers that can occur in a metamorphic rock. Foliation can give metamorphic rocks a striped or streaked appearance in an outcrop, and/or can give them the ability to split into thin sheets. A foliated metamorphic rock has foliation either because it contains inequant mineral crystals that are aligned parallel to one another, defining preferred mineral orientation, and/or because the rock has alternating dark-coloured and light-coloured layers. Foliated metamorphic rocks can be distinguished from one another according to their composition, their grain size, and the nature of their foliation. The most common types include  Slate is a foliated metamorphic rock that forms at relatively low temperature and pressure. Slate: The finest-grained foliated metamorphic rock, slate, forms by metamorphism of shale or mudstone (rocks composed dominantly of clay) under relatively low pressures and temperatures. Slate contains a type of foliation called slaty cleavage, which allows it to split into thin sheets that make excellent roofing shingles (figure above a). Slaty cleavage develops when pressure solution removes portions of clay flakes that are not perpendicular to the compression direction, while clay flakes that are perpendicular to the compression direction grow. During the process, some flakes passively rotate into parallelism with the cleavage plane, pushed into the new orientation by compression. For example, end-on compression of a sequence of horizontal shale beds produces vertical slaty cleavage (figure above b). Commonly, such compression also causes the layers to bend into curves called folds.  Examples of foliated metamorphic rocks formed at high temperatures and pressures. Phyllite: Phyllite is a fine-grained metamorphic rock with a foliation caused by the preferred orientation of very fine grained white mica. The word comes from the Greek word phyllon, meaning leaf, as does the word phyllo, the flaky dough in Greek pastry. The parallelism of translucent fine-grained mica gives phyllite a silky sheen known as phyllitic luster (figure above a). Phyllite forms by the metamorphism of slate at a temperature high enough to cause neocrystallization of white mica.  Metaconglomerate: Under the metamorphic conditions that produce slate or phyllite, a protolith of conglomerate becomes metaconglomerate. Specifically, pressure solution and plastic deformation flatten pebbles and cobbles into pancake-like shapes. The alignment of inequant clasts defines a foliation (figure above b).  Schist: Schist is a medium- to coarse-grained metamorphic rock that possesses a type of foliation, called schistosity, defined by the preferred orientation of large mica flakes (muscovite and/or biotite; figure above c). Schist forms at a higher temperature than does phyllite.  The formation of gneiss, which takes place at very high temperatures and pressures. Gneiss: Gneiss is a compositionally layered metamorphic rock, typically composed of alternating dark-coloured and light-coloured layers that range in thickness from millimetres to meters. This compositional layering, or gneissic banding, gives gneiss a striped appearance (figure above a). How does the banding in gneiss form? Some evolved directly from the original bedding in a rock. For example, metamorphism of a protolith consisting of alternating beds of sandstone and shale produces a gneiss consisting of alternating beds of quartzite and mica. Gneissic banding can also form when the protolith undergoes an extreme amount of shearing under conditions in which the rock can flow like soft plastic  (figure above b). Such flow stretches, folds, and smears out pre-existing compositional contrasts in the rock and transforms them into aligned sheets. Finally, banding in some gneisses can develop by an incompletely understood process called metamorphic differentiation. During differentiation, chemical reactions segregate different minerals into different layers (figure above c).  Migmatite: Under certain conditions, gneiss may begin to melt, producing felsic magma and residual, still solid, mafic rock. If the melt freezes again before flowing out of the source area, a mixture of igneous rock and relict metamorphic rock forms. This mixture is called migmatite. In effect, a migmatite is part metamorphic and part igneous. Nonfoliated Metamorphic Rocks  Nonfoliated metamorphic rocks contain minerals that recrystallized or grew during metamorphism, but have no foliation. The lack of foliation means either that metamorphism occurred in the absence of compression and shear, or that most of the new crystals can only grow in an equant form. We list below some of the rock types that can occur without foliation. Examples of quartzite and marble typically, but not always, these are non-foliated. Hornfels: Hornfels is a fine-grained nonfoliated rock that contains a variety of metamorphic minerals. The specific mineral assemblage in a hornfels depends on the composition of the protolith and on the temperature and pressure of metamorphism.  Quartzite: Quartzite forms by the metamorphism of pure quartz sandstone. During metamorphism, pre-existing quartz grains recrystallize, creating new, larger grains. In the process, the distinction between cement and grains disappears, open pore space disappears, and the grains become interlocking. When quartzite cracks, the fracture cuts across grain boundaries in contrast, fractures in sandstone curve around grains. Quartzite looks glassier than sandstone and does not have the grainy, sandpaper-like surface characteristic of sandstone (figure above a). Depending on the impurities it contains, quartzite can vary in colour from white to gray, purple, or green.  Marble: The metamorphism of limestone yields marble. During the formation of marble, calcite composing the protolith recrystallizes, so fossil shells, pore space, and the distinction between grains and cement disappear. Thus, marble typically consists of a fairly uniform mass of interlocking calcite crystals.  Sculptors love to work with marble because the rock is relatively soft and has a uniform texture that gives it the cohesiveness and homogeneity needed to fashion large, smooth, highly detailed sculptures. Marble comes in a variety of colours white, pink, green, and black depending on the impurities it contains. Michelangelo, one of the great  Italian Renaissance artists, sought large, unbroken blocks of creamy white marble from quarries in the Italian Alps for his masterpieces (figure above b).  Not all marble is non-foliated. If the original protolith contained layers with different impurities, and shear caused the marble to flow plastically, the resulting marble has colour banding that makes it a prized decorative stone (figure above c). Defining Metamorphic Intensity  Intensity of metamorphism is indicated by metamorphic grade Not all metamorphism takes place under the same physical conditions. For example, rocks carried to a great depth beneath a mountain range undergo more intense metamorphism than do rocks closer to the surface. Geologists use the term metamorphic grade in a somewhat informal way to indicate the intensity of metamorphism, meaning the amount or degree of metamorphic change. (To provide a more complete indication of the intensity of metamorphism, geologists use the concept of metamorphic facies; see Metamorphic Facies) Classification of metamorphic grade depends primarily on temperature, because temperature plays the dominant role in determining the extent of recrystallization and neocrystallization during metamorphism. Metamorphic rocks that form at relatively low temperatures (between about 250C and 400C) are lowgrade rocks, and metamorphic rocks that form at relatively high temperatures (over about 600C) are high-grade rocks.  Intermediate-grade rocks form at temperatures between these two extremes (figure above a).  Different grades of metamorphism yield different metamorphic mineral assemblages. As grade increases, recrystallization and neocrystallization tend to produce coarser grains and new mineral assemblages that are stable at higher temperatures and pressures (figure above b).  Geologists discovered that the presence of certain minerals, known as index minerals, in a rock indicates the approximate metamorphic grade of the rock. The line on a map along which an index mineral first appears is called an isograd (from the Greek iso, meaning equal). All points along an isograd have approximately the same metamorphic grade. Metamorphic zones are regions between two isograds; zones are named after an index mineral that was not present in the previous, lower-grade zone. To compare rocks of different grades, you could take a hike from central New York State eastward into central Massachusetts in the eastern United States. Your path starts in a region where rocks were not metamorphosed, and it takes you into the internal part of the Appalachian Mountain belt, where rocks were intensely metamorphosed. As a consequence, you cross several metamorphic zones (figure above c). Metamorphic Facies In the early years of the 20th century, geologists working in Scandinavia, where erosion by glaciers has left beautiful, nearly unweathered exposures of rocks once buried very deeply in the crust, came to realize that metamorphic rocks, in general, do not consist of a hodgepodge of minerals formed at different times and in different places, but rather consist of a distinct set of minerals that grew in association with each other at a certain pressure and temperature. It seemed that such mineral assemblages more or less represent a condition of chemical equilibrium, meaning that the chemicals making up the rock had organized into a group of mineral grains that were to anthropomorphize a bit comfortable with each other and their surroundings, and thus did not feel the need to change further. The geologists also determined that the specific mineral assemblage in a rock depends on pressure and temperature conditions, and on the composition of the protolith.  This discovery led the geologists to  propose the concept of metamorphic facies. A metamorphic facies is a set of metamorphic mineral assemblages indicative of a certain range of pressure and temperature. Each specific assemblage in a facies reflects the original protolith composition. According to this definition, a given metamorphic facies includes several different kinds of rocks that differ from each other in terms of chemical composition and, therefore, mineral content but all the rocks of a given facies formed under roughly the same temperature and pressure conditions. Geologists recognize several facies, of which the major ones are zeolite, hornfels, greenschist, amphibolite, blueschist, eclogite, and granulite. The names of the different facies are based on a distinctive feature or mineral found in some of the rocks of the facies.  The common metamorphic facies. The boundaries between the facies are depicted as wide bands because they are gradational and approximate. Note that some amphibolite-facies rocks and all granulite-facies rocks form only if the protolith is dry. The relatively rare P-P (”prehnite-pumpellyite”) facies, is named for two metamorphic minerals. We can represent the approximate conditions under which metamorphic facies formed by using a pressure temperature graph (figure above). Each area on the graph, labeled with a facies name, represents the approximate range of temperatures and pressures in which mineral assemblages characteristic of that particular facies form. For example, a rock subjected to the pressure and temperature at Point A (4.5 kbar and 400C) develops a mineral assemblage characteristic of the greenschist facies. As the graph implies, the pressure and temperature conditions defining boundaries between facies cannot be precisely determined, and the transitions between facies are gradual. We can also portray the geothermal gradients of different crustal regions on the graph. Beneath mountain ranges, for example, the geothermal gradient passes through the zeolite, greenschist, amphibolite, and granulite facies. In contrast, in the accretionary prism that forms at a subduction zone, temperature increases slowly with increasing depth, so blueschist assemblages can form. Credit: Stephen Marshak (Essentials of Geology)

The unprecedented occurrence of Isistius triangulus, with such a high density in Cookiecutter Creek relative to other sites in Florida, may indicate that Cookie cutter Creek preserves a unique deep-water facies within the Tamiami Formation, according to local sea-level fluctuations during the Pliocene to Pleistocene. In October 2016, an email inquiring about the geologic age of a creek in Sarasota County led to the serendipitous ‘discovery’ of fossil teeth of the sharks Isistius triangulus and Squatina sp. in Florida. Up to that point, no fossils of Isistius or Squatina from Florida were present in the vertebrate paleontology collections at the Florida Museum of Natural History, nor had they been listed in any scientific publication. However, public inquiries revealed that the presence of both genera in the Florida fossil record has been known among the amateur paleontology community for well over a decade. Once co-author Marks was made aware that the Isistius teeth represented something novel, more rigorous collecting was conducted to procure more specimens. A total of 94.5 kg of matrix from four gravel deposits within the creek, informally known as Cookiecutter Creek, were screenwashed and picked to produce 186 whole or partial lower teeth of Isistius triangulus and four teeth belonging to Squatina. In addition, a diverse vertebrate fauna was recovered that ranged from Pliocene to Pleistocene in age. Screenwashing of 7.5 kg of matrix from the in situ Pleistocene Caloosahatchee Formation revealed no specimens of Isistius or Squatina, which implies their actual source is the underlying Pliocene Tamiami Formation. The unprecedented occurrence of Isistius triangulus, with such a high density in Cookiecutter Creek relative to other sites in Florida, may indicate that Cookiecutter Creek preserves a unique deep-water facies within the Tamiami Formation. The associated vertebrate fauna includes taxa from marine, brackish, freshwater, and fully terrestrial paleoenvironments; providing evidence of local sea-level fluctuations during the Pliocene to Pleistocene. The temporally mixed accumulation in these gravel deposits inhibits a more accurate reconstruction of these sea-level fluctuations.

Fallen Roof ruin We stopped at another Cedar Mesa Icon - Fallen Roof House. It was still cloudy but dry. The drive back to the parking area at the trail-head was much longer and the hike in was also longer and a little more difficult. The canyon was deeper and was filled with more debris. The trail takes you down into a side canyon first and then into the main canyon after a half hour walk. There are no technical sections, just a little scrambling. The ruins themselves are high on the northern cliff face and you have to keep a looking as you walk down the main canyon or you can miss them. It is a little scramble up to get to them, but again, if you take your time and plot out your route , you can reach them without much difficulty. Both Fallen Roof and Flaming House are built in a more massive unit of the Cedar Mesa Sandstone. They are both mega dunes facies from large aeolian sand dunes. The "flames" at Flaming House are the bedding planes that have erroded out of the slip face of the preserved dune. At Fallen Roof, the underside of the dune is spalling off and falling down as water works its way through the rock. The water precipitates minerals and salts that it has dissolved and this helps weaken the rock and leaves the nice patterns on the roof where a slab has fallen. These slabs have been falling before the dwellings were built because you can see the slabs used in the walls. The precipitated salts on the roof leave patterns that remind me of the early cave painting in France. Finally, the slabs that have fallen are still visible where they fell. Our walk out got a little more interesting when lightning started hitting the top of the mesa. We made it back to the car before the rain. Our drive back out to the highway was in a light rain that fortunately didn't make the dirt road too slick. That would not have been the case a couple of hours later.... MG_0740

Porphyritic metadacite to porphyritic meta-andesite from the Precambrian of Egypt. (photo provided by Karen van Opstal) “Imperial Porphyry” is a beautiful, important, historically-valuable decorative stone. It was initially quarried during the Egyptian Ptolemaic Dynasty and was also used in the Roman Empire. In later centuries, it was reused in southern and southeastern Europe. This rock type comes from quarries at Mons Porphyrites in eastern Egypt. The locality name is the basis for the petrologic term “porphyritic”, which refers to a mix of large and small crystals in an igneous rock. Imperial Porphyry rocks are dark reddish or dark purplish with light-colored feldspar phenocrysts. The red and purple colors are the result of alteration of the original rock, which is dark gray-colored. These rocks are part of the Dokhan Volcanics, a greater-than-1 kilometer thick succession of late Precambrian-aged, terrestrial, intermediate to felsic volcanic rocks (= lava flows, volcanic tuffs, and volcanic agglomerates). The nature, age, mineralogy, geochemistry, and paleotectonic setting of the Dokhan Volcanics indicate that Imperial Porphyry rocks are lava flows that accompanied subduction zone volcanism during the Pan-African Orogeny. Subduction was followed by a collision event along the Mozambique Belt in the late Precambrian, during which the ancient small supercontinent Gondwana formed (www.researchgate.net/figure/Map-of-the-Gondwana-supercont...). Gondwana was part of a larger supercontinent called Pannotia, which rifted apart in the latest Precambrian (upload.wikimedia.org/wikipedia/commons/d/d7/Pannotia.svg). Geochemical analysis of Imperial Porphyry rocks has shown that they are 62.2 to 64.4% silica, which makes them porphyritic quartz andesites and porphyritic dacites. A detailed mineral analysis of Imperial Porphyry is given in Makovicky et al. (2016). The mineralogy shows that the rocks have been subjected to fluid alteration and greenschist-facies metamorphism, possibly related to the Pan-African Orogeny and/or burial metamorphism and/or Red Sea rifting orogenesis. The reddish to purplish coloration is from partial hematitization of mafic minerals. Because the rocks are slightly metamorphosed, they are better referred to as "meta-andesite" and "metadacite". Stratigraphy: upper Dokhan Volcanics, Ediacaran, upper Neoproterozoic, ~593-602 Ma Locality: old Roman quarry at Mons Porphyrites, above Wadi Abu Maamel, Red Sea Mountains, Eastern Desert, eastern Egypt --------------------- Geologic info. mostly synthesized from: Makovicky et al. (2016) - Imperial Porphyry from Gebel Abu Dokhan, the Red Sea Mountains, Egypt, part I. mineralogy, petrology and occurrence. Neues Jahrbuch für Mineralogie Abhandlungen [Journal of Mineralogy and Geochemistry] 193: 1-27.