Fall 2008 Field Trip

THE FALL 2007 FIELD TRIP OF THE PHYSICAL GEOLOGY CLASS
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With around 30 geology students, we qualified to take the college bus for the first time in the history of our field trips. This was also the first excursion to the Albert Pike area in southern Arkansas in a number of years. The area is characterized geologically by folded and twisted rocks, riddled with faults. We were looking at the deep roots of an ancient mountain chain, the ancestral Ouchita Mountains, formed in the late Paleozoic during a near collision between North and South America (due to continental drift). The rocks here are Early Paleozoic and abundant graptolite fossils are found in the dark shales of Ordovician age.

GRAPPLING WITH GRAPTOLITES. STUDENTS EXAMINE A RARE OUTCROP OF POLK CREEK SHALE (WHICH, DUE TO ITS SUSCEPTIBILITY TO EROSION, IS USUALLY COVERED WITH SOIL OR ALLUVIUM) AND SEARCH FOR GRAPTOLITE FOSSILS. THE POLK CREEK SHALE IS VERY BLACK DUE TO ITS HIGH ORGANIC CONTENT. THE GRAPTOLITE FOSSILS ARE PIECES OF GRAPTOLITE COLONIES THAT FLOATED ON THE SURFACE OF A DEEP SEA THAT COVERED WHAT IS NOW THE SOUTHERN PART OF NORTH AMERICA IN THE ORDOVICIAN PERIOD (AROUND 450 MILLION YEARS AGO). AT THE TOP OF THE OUTCROP IS STEPHEN MILLER. FROM LEFT TO RIGHT AT THE BOTTOM ARE JAMIE HILTS, LAURIE EARNEST, SHERON HENDRIX, AND PATRICIA CROUCH.

THE GRAPTOLITE HUNTERS. FROM LEFT TO RIGHT, LAURIE EARNEST, SHERON HENDRIX, JAMIE HILTS, PATRICIA CROUCH, RONALD MOODY, KRISTIE HILLIARD, STEPHEN MILLER, BRUCE BORMANN, AND TWO UNIDENTIFIED STUDENTS.

A FIND! LAURIE EARNEST'S DAD EXAMINES HIS GRAPTOLITE FOSSILS WITH A MAGNIFYING GLASS AS JAMIE HILTS AND SHERON HENDRIX LOOK ON.

MORE FINDS! LAURIE EARNEST, HOLDING THE ROCK HAMMER, DISPLAYS A HANDFUL OF FOSSILS. HER DAD HOLDS ANOTHER SPECIMEN. SHERON HENDRIX, IN THE BACKGROUND, CONTINUES TO SEARCH.

A FIND? BRUCE BORMANN SHOWS WHAT HE HAS FOUND - A HOLE IN A ROCK. ACTUALLY, THIS HOLE IS INTERESTING IN THAT IT RECORDS SOME ROUNDISH SOMETHING THAT ONCE WAS IN THE SHALE AND WAS LATER WEATHERED OUT. IT COULD HAVE BEEN, FOR EXAMPLE, A CLOD OF CLAY ROUNDED BY ROLLING ALONG THE BOTTOM OF THE SEA. EVERYONE WHO LOOKED FOR FOSSILS FOUND PLENTY OF SPECIMENS TO TAKE HOME.

GEOLOGIC MAP OF THE ALBERT PIKE FIELD TRIP. THE BOX INDICATES THE AREA OF THE ROADCUT OUTCROP THAT WE INVESTIGATED. WE BEGAN AT THE LOWER RIGHT IN THE PINK Opc REGION OF THE POLK CREEK SHALE. WALKING THE OUTCROP UPHILL, WE CAME UPON THE LIGHT BROWN Obf AREA WHERE THE BIG FORK CHERT CROPS OUT. BOTH UNITS ARE ORDOVICIAN IN AGE. NOTE THE THICK RED LINES INDICATING THE LOCATION OF THRUST FAULTS, WHERE ONE SLAB OF ROCK HAS BEEN THRUST OVER ANOTHER. THIS HAPPENED DURING THE LATE PALEOZOIC MOUNTAIN BUILDING EPISODE MENTIONED PREVIOUSLY. (THIS MAP WAS ADAPTED FROM A MAP DISPLAYED ON THE ARKANSAS GEOLOGICAL COMMISSION WEB SITE.)

BIG FORK CHERT. THE SEDIMENTARY BEDS ALONG THE OUTCROP DIP STEEPLY, GENERALLY TO THE SOUTH. WE ARE ACTUALLY ON THE SOUTH SIDE OF AN ANTICLINE, A FOLD WHERE BEDS FORM AN ARCH-LIKE STRUCTURE. TO THE NORTH, THE BEDS DIP IN THE OPPOSITE DIRECTION (NORTH). BRUCE BORMANN SUPPLIES SCALE AS WE LOOK AT THE STEEPLY DIPPING BIG FORK CHERT BEDS. CHERT IS A ROCK WITH A COMPOSITION THE SAME AS QUARTZ (SILICON DIOXIDE) BUT WITH A MICROCRYSTALLINE STRUCTURE.

HOW IS THIS POSSIBLE! STEPHEN COLLINS STANDS IN FRONT OF A STRUCTURE THAT IS DIFFICULT TO UNDERSTAND. HOW CAN SEDIMENTARY BEDS BE AT RIGHT ANGLES AS SEEN HERE? ABOVE STEPHEN, THE BEDS TREND A BIT DOWN TO THE LEFT. HOWEVER, TO HIS RIGHT THE BEDS TREND NEARLY VERTICALLY. WHAT'S UP WITH THIS? I SUBMIT A POSSIBLE EXPLANATION IN THE FOLLOWING IMAGE.

A FAULTED FOLD HERE IS A POSSIBLE EXPLANATION FOR THE STRUCTURE SEEN IN THE IMAGE ABOVE. THE AREA WE ARE LOOKING AT IS RIDDLED WITH TIGHT FOLDS AND FAULTS. IMAGINE AN ANTICLINE AS INDICATED IN THE IMAGE FAULTED IN THE VICINITY OF ITS AXIAL PLANE (THE PLANE THAT DIVIDES THE FOLD IN HALF). IF THE MOVEMENT ALONG THE FAULT IS AS DEPICTED, YOU COULD EASILY WIND UP WITH THE STRUCTURE SEEN IN THE ABOVE IMAGE.

A FAULT IN THE BIG FORK CHERT. THE ZONE OF BROKEN UP ROCK TRENDING FROM THE LOWER RIGHT TO THE UPPER LEFT MARKS THE LOCATION OF A FAULT THAT HAS DISPLACED THE ROCK ABOVE WITH RESPECT TO THAT BELOW. THE BROKEN UP ROCK IN THE FAULT ZONE IS CALLED GOUGE.

TIGHT FOLDS IN THE BIG FORK CHERT. NOTE HOW THE BEDS DIP DOWN TO THE RIGHT ON THE LEFT SIDE OF THE IMAGE, THEN UP AGAIN TOWARD THE MIDDLE, THEN BACK DOWN AGAIN ON THE RIGHT. THE V-SHAPED FOLD ON THE LEFT IS CALLED A SYNCLINE, WHEREAS THE ARCH-SHAPED FOLD TO ITS RIGHT IS AN ANTICLINE. THESE FOLDS CAN BE ON ALMOST ANY SCALE. THE WHOLE OF THE OUACHITA MOUNTAIN UPLIFT, EXTENDING FROM WHERE WE ARE TO A HUNDRED MILES TO THE NORTH IS A LARGE ANTICLINE. WE ARE ON THE SOUTH SIDE OF A SMALLER ANTICLINE IMBEDDED IN THE OUACHITAS THAT EXTENDS LESS THAN A MILE TO THE NORTH. AND HERE YOU SEE FOLDS THAT ARE ONLY A FEW FEET ACROSS. YOU GET THE SAME SORT OF STRUCTURE WHEN YOU BUNCH UP A BLANKET ON A BED.

	GRAPPLING WITH GRAPTOLITES. STUDENTS EXAMINE A RARE OUTCROP OF POLK CREEK SHALE (WHICH, DUE TO ITS SUSCEPTIBILITY TO EROSION, IS USUALLY COVERED WITH SOIL OR ALLUVIUM) AND SEARCH FOR GRAPTOLITE FOSSILS. THE POLK CREEK SHALE IS VERY BLACK DUE TO ITS HIGH ORGANIC CONTENT. THE GRAPTOLITE FOSSILS ARE PIECES OF GRAPTOLITE COLONIES THAT FLOATED ON THE SURFACE OF A DEEP SEA THAT COVERED WHAT IS NOW THE SOUTHERN PART OF NORTH AMERICA IN THE ORDOVICIAN PERIOD (AROUND 450 MILLION YEARS AGO). AT THE TOP OF THE OUTCROP IS STEPHEN MILLER. FROM LEFT TO RIGHT AT THE BOTTOM ARE JAMIE HILTS, LAURIE EARNEST, SHERON HENDRIX, AND PATRICIA CROUCH.

	THE GRAPTOLITE HUNTERS. FROM LEFT TO RIGHT, LAURIE EARNEST, SHERON HENDRIX, JAMIE HILTS, PATRICIA CROUCH, RONALD MOODY, KRISTIE HILLIARD, STEPHEN MILLER, BRUCE BORMANN, AND TWO UNIDENTIFIED STUDENTS.

A FIND! LAURIE EARNEST'S DAD EXAMINES HIS GRAPTOLITE FOSSILS WITH A MAGNIFYING GLASS AS JAMIE HILTS AND SHERON HENDRIX LOOK ON.

MORE FINDS! LAURIE EARNEST, HOLDING THE ROCK HAMMER, DISPLAYS A HANDFUL OF FOSSILS. HER DAD HOLDS ANOTHER SPECIMEN. SHERON HENDRIX, IN THE BACKGROUND, CONTINUES TO SEARCH.

	A FIND? BRUCE BORMANN SHOWS WHAT HE HAS FOUND - A HOLE IN A ROCK. ACTUALLY, THIS HOLE IS INTERESTING IN THAT IT RECORDS SOME ROUNDISH SOMETHING THAT ONCE WAS IN THE SHALE AND WAS LATER WEATHERED OUT. IT COULD HAVE BEEN, FOR EXAMPLE, A CLOD OF CLAY ROUNDED BY ROLLING ALONG THE BOTTOM OF THE SEA. EVERYONE WHO LOOKED FOR FOSSILS FOUND PLENTY OF SPECIMENS TO TAKE HOME.

	GEOLOGIC MAP OF THE ALBERT PIKE FIELD TRIP. THE BOX INDICATES THE AREA OF THE ROADCUT OUTCROP THAT WE INVESTIGATED. WE BEGAN AT THE LOWER RIGHT IN THE PINK Opc REGION OF THE POLK CREEK SHALE. WALKING THE OUTCROP UPHILL, WE CAME UPON THE LIGHT BROWN Obf AREA WHERE THE BIG FORK CHERT CROPS OUT. BOTH UNITS ARE ORDOVICIAN IN AGE. NOTE THE THICK RED LINES INDICATING THE LOCATION OF THRUST FAULTS, WHERE ONE SLAB OF ROCK HAS BEEN THRUST OVER ANOTHER. THIS HAPPENED DURING THE LATE PALEOZOIC MOUNTAIN BUILDING EPISODE MENTIONED PREVIOUSLY. (THIS MAP WAS ADAPTED FROM A MAP DISPLAYED ON THE ARKANSAS GEOLOGICAL COMMISSION WEB SITE.)

BIG FORK CHERT. THE SEDIMENTARY BEDS ALONG THE OUTCROP DIP STEEPLY, GENERALLY TO THE SOUTH. WE ARE ACTUALLY ON THE SOUTH SIDE OF AN ANTICLINE, A FOLD WHERE BEDS FORM AN ARCH-LIKE STRUCTURE. TO THE NORTH, THE BEDS DIP IN THE OPPOSITE DIRECTION (NORTH). BRUCE BORMANN SUPPLIES SCALE AS WE LOOK AT THE STEEPLY DIPPING BIG FORK CHERT BEDS. CHERT IS A ROCK WITH A COMPOSITION THE SAME AS QUARTZ (SILICON DIOXIDE) BUT WITH A MICROCRYSTALLINE STRUCTURE.

HOW IS THIS POSSIBLE! STEPHEN COLLINS STANDS IN FRONT OF A STRUCTURE THAT IS DIFFICULT TO UNDERSTAND. HOW CAN SEDIMENTARY BEDS BE AT RIGHT ANGLES AS SEEN HERE? ABOVE STEPHEN, THE BEDS TREND A BIT DOWN TO THE LEFT. HOWEVER, TO HIS RIGHT THE BEDS TREND NEARLY VERTICALLY. WHAT'S UP WITH THIS? I SUBMIT A POSSIBLE EXPLANATION IN THE FOLLOWING IMAGE.

	A FAULTED FOLD HERE IS A POSSIBLE EXPLANATION FOR THE STRUCTURE SEEN IN THE IMAGE ABOVE. THE AREA WE ARE LOOKING AT IS RIDDLED WITH TIGHT FOLDS AND FAULTS. IMAGINE AN ANTICLINE AS INDICATED IN THE IMAGE FAULTED IN THE VICINITY OF ITS AXIAL PLANE (THE PLANE THAT DIVIDES THE FOLD IN HALF). IF THE MOVEMENT ALONG THE FAULT IS AS DEPICTED, YOU COULD EASILY WIND UP WITH THE STRUCTURE SEEN IN THE ABOVE IMAGE.

	A FAULT IN THE BIG FORK CHERT. THE ZONE OF BROKEN UP ROCK TRENDING FROM THE LOWER RIGHT TO THE UPPER LEFT MARKS THE LOCATION OF A FAULT THAT HAS DISPLACED THE ROCK ABOVE WITH RESPECT TO THAT BELOW. THE BROKEN UP ROCK IN THE FAULT ZONE IS CALLED GOUGE.

TIGHT FOLDS IN THE BIG FORK CHERT. NOTE HOW THE BEDS DIP DOWN TO THE RIGHT ON THE LEFT SIDE OF THE IMAGE, THEN UP AGAIN TOWARD THE MIDDLE, THEN BACK DOWN AGAIN ON THE RIGHT. THE V-SHAPED FOLD ON THE LEFT IS CALLED A SYNCLINE, WHEREAS THE ARCH-SHAPED FOLD TO ITS RIGHT IS AN ANTICLINE. THESE FOLDS CAN BE ON ALMOST ANY SCALE. THE WHOLE OF THE OUACHITA MOUNTAIN UPLIFT, EXTENDING FROM WHERE WE ARE TO A HUNDRED MILES TO THE NORTH IS A LARGE ANTICLINE. WE ARE ON THE SOUTH SIDE OF A SMALLER ANTICLINE IMBEDDED IN THE OUACHITAS THAT EXTENDS LESS THAN A MILE TO THE NORTH. AND HERE YOU SEE FOLDS THAT ARE ONLY A FEW FEET ACROSS. YOU GET THE SAME SORT OF STRUCTURE WHEN YOU BUNCH UP A BLANKET ON A BED.