The second-largest fossil forest of itsage in North America, if not th - PDF document

URVEY The second-largest fossil forest of itsage in North America, if not the entireworld, is in the Circle Cliffs portion ofGrand Staircase…Escalante NationalMonument in southern Utah. How-ever, because of its remote location,very few people know of its existence,much less have ever visited it. Fur-thermore, this forest has never beenscientifically documented. Neverthe-less, the Bureau of Land Managementrecognized its significance long agoand designated six sections of land asthe Wolverine Petrified Wood NaturalArea. The forest itself does not havean official name, but is sometimescalled the Wolverine Petrified Forest.It lies about 15 miles southeast of thetown of Boulder and 20 miles east ofEscalante. Although the WolverinePetrified Wood Natural Area includesabout 3,800 acres, most of the exposedpetrified wood and the largest logsare on the floor of an unnamed drywash that drains into WolverineCreek. Here the logs are mostly pre-served in place and exposed by ero-sion; it is this deposit that is called theWolverine Petrified Forest. Elsewherein the Natural Area the wood consistsmainly of small isolated blocks thatwere transported relatively longerdistances after exposure. In someplaces, the gentle slopes are denselylittered with blocks of all sizes. Chinle FormationThe fossil wood preserved in theWolverine Petrified Wood NaturalArea originates from the Late TriassicChinle Formation, which forms thegenerally reddish-colored slopesbelow striking vertical cliffs of tansandstone here and throughout theCircle Cliffs region. This formationwas deposited about 225 million yearsago, at the beginning of the Age ofand streams on the floor of a broadbasin on the western edge of theancient supercontinent called Pan-gaea. At that time, Utah was geo-graphically situated farther souththan today, probably near the latitudeof present-day Cuba. Streams in theChinle basin were generally flowingin a north to northwesterly directiontoward the sea, which occupied whatis now eastern Nevada and Califor-nia. The now-fossilized wood foundin the Wolverine Petrified Wood Areacame from trees that grew along thebanks of one of these streams andpossibly from adjacent highlands. Although the Chinle Formation isreported to be about 550 feet thick inthis area, most of the petrified wood itcontains seems to be near the middleof the formation in a distinctive bedof pinkish sandstone about 10 feetthick. This bed is in the Petrified For-est Member and probably correlateswith the Black Forest Bed in theupper part of the same member inPetrified Forest National Park, Ari-zona. Evidence for this correlation isbased on bedding similarity and thepresence of black fossil wood, some ofwhich is restricted to the Black ForestBed and equivalent rock units. The bed of pinkish sandstone wasdeposited by only one of the manynorth-flowing streams that formedthe Chinle Formation. The orienta-tion of the logs seems to be random,although about one-quarter of themare oriented generally northeast. Thismay indicate that stream flow fol-lowed the same general trend. Thereare places in the bed of sandstonewhere as many as three logs overlapeach other, suggesting that logs prob-ably were carried into the area notjust once but at several times during arelatively short interval during theLate Triassic. Characteristics of the Fossil WoodUnlike most of the fossil wood in theChinle Formation elsewhere in Utahand in the more famous and muchlarger Petrified Forest National Park,the wood in the Wolverine PetrifiedWood Natural Area is typically blackon fresh surfaces. The original darkThe WThe W Reconstruction of the living tree. URVEY color of the specimens has beendesert varnishŽ that commonlydevelops on exposed rock surfaces inthe southwestern United States and inother arid regions of the world. Thesurfaces of many of the logs arebrownish when they are first exposedby erosion, but weathering soonremoves the brown coloring andreveals the characteristic black colorof the wood.Most of the fossil wood in the Wolver-ine Petrified Wood Natural Arearesults from the petrification processcalled permineralization. In thisprocess, empty spaces in the originalwood have been filled with quartzand the original cell structure is visi-ble with a microscope. The remainingorganic matter in some of the woodwas subsequently completelydestroyed and then replaced by thesame mineral; as a result, these fossilslack cell structure.These fossils are clearly the remains oftrees, because they look like treetrunks or fragments of tree trunks thathave lost their bark as well as theirlimbs and roots. This similarity wasnot lost on the local Native Americanswho traditionally considered them tobe the spent arrow shafts of one oftheir gods. The stumps of limbs androots are still attached to some of thetrunks in the Wolverine Petrified For-est. Loss of the limbs and roots prob-ably occurred mostly as the treesfloated downstream. Many of thetrees probably fell into the streamswhen the riverbanks they were grow-ing on were undercut by channel ero-At least some of the trees in theWolverine Petrified Forest wereThe evidence for this consists of nar-row,transverse ridges around thetrunks that resemble structurestrees. Such girdling undoubtedlycaused the deaths of some of theseancient trees just as it results in thedeath of modern trees. Eventually,the ancient trees that had beenattacked would have fallen over afterthey died. Then, because of naturaldecay, they lost their limbs and rootson the forest floor before beingwashed into streams that formed thisThe longest and most complete logsin the Wolverine Petrified Wood Nat-ural Area occur in the Wolverine Pet-rified Forest. Here, the logs are up tonearly 100 feet long and range fromabout 1.5 - 3 feet in diameter. Becausethey are still partially buried, the logsevidently have not been moved byerosion since they were exposedmany years ago, although parts ofthem have broken off and disap-peared. Arecent search of theWolverine Petrified Forest revealednearly 100 such in-place logs. Scat-tered between these more-or-less General view of the strata exposed near the Wolverine Petrified Forest. Compare with the chartabove. The strata in the steep cliffs are assigned to the Wingate Sandstone and all of the stratabelow the cliffs are assigned to the Chinle Formation. The arrow indicates the bed of pinkishsandstone that contains the petrified wood exposed in the forest and vicinity. units exposed in the vicinity of WolverinePetrified Forest and the approximate positionof the petrified wood preserved in it. Location of the Wolverine Petrified Forest in the northeastern corner of Grand Staircase-Escalante National Monument, Utah. The index map shows the location of the area in relationto the Colorado Plateau and of Petrified Forest National Park in Arizona. URVEY intact logs are hundreds of small blocks of petrified woodthat were transported some distance after exposure andbreaking off the large logs. This process is ongoing and thefossil wood continues to be broken into still smaller andsmaller pieces. In many places along the walls of thewash, more logs and small fragments of logs are just nowbeing slowly exposed to view by erosion. Extinct Trees of the Forest Most of the fossil wood appears to represent , a species of fossil wood common throughoutthe Chinle Formation at many localities in Utah, Arizona,and New Mexico. Asmall amount of a second species ofWoodworthia arizonicais also present inWolverine Petrified Forest. It has a much more limited dis-tribution and occurs mostly in Petrified Forest NationalPark in a bed that probably was deposited at the same timeas the petrified wood-bearing bed in the Wolverine Petri-fied Wood Natural Area. Both species are extinct, and wecan only surmise what they looked like when living. A. View of an ancient log jamŽ in the process of being exposed by ero-sion in the Wolverine Petrified Forest. These logs still retain the browncoloring that is found on the exterior of the wood before it has been sub-jected to weathering after exposure. B. Close-up of a log showing the stump of a large lateral branch. Thesurface of the log is still brown in color because it has just been exposedby erosion. Black wood is exposed where the end of the lateral branchhas been broken off. Note the pick for scale. C. Alarge trunk of the tree showing alateral root protruding upward in front of the figure (arrow). D. Transverse ridges on a log of by bark beetles between the bark and the wood of the tree. The ridgesare about 1/2 inch wide. E. Slightly expanded root structure on the base of the trunk of antree which is overlapped by a secondtrunk in the Wolverine Petrified Forest. AABBCCDDEE URVEY However, a recent study of the trunks identified as carioxylon arizonicumthat are exposed in Petrified ForestNational Park showed that the mature tree had a diameterof nearly 10 feet about 5 feet above the base and stood asmuch as 180 feet tall. The trunk of the tree tapered evenlyfrom a slightly expanded base to the top. Aring of steeplyinclined roots grew into the ground at steep angles and along, stout taproot held the tree upright. Lateral brancheshad no systematic arrangement on the trunk and divergedfrom it at steep upward angles. Presumably, the branchesswept downward and outward so as to present a largearea of foliage to solar radiation. The apices of the branch-es may have turned upwards slightly like certain moderntrees. Most of the branches likely carried viable foliageuntil the tree died. The bark was relatively thin and mod-ern looking. As is typical of modern trees growing in thetropics, the wood did not contain annual growth rings,although it did show evidence of occasional growth inter-ruptions. Unfortunately, the leaves and reproductivestructures are unknown. The taxonomic relationships ofthe tree are unknown beyond saying it is an extinct coniferof some type.Unlike the smooth surfaces of the trunks of the oxylon arizonicumtrees, the fossil trunks of the Woodworthiatree are covered with distinctive small, shallowholes about 1/4 inch in diameter. These holes apparentlyrepresent small branches that never fully developed. Thetree was about half the height of the Araucarioxylon arizon-and also had lateral branches irregularly arrangedalong the trunk. Like the wood of the Araucarioxylon ari-trees, the wood of the Woodworthia arizonicatreedoes not have annual growth rings and its leaves andreproductive structures are unknown. Thus, it can onlybe treated as another type of extinct conifer.No fossil leaves, cones, or seeds have been found in thebed of sandstone that contains the Wolverine Petrified For-est, but other beds in the Chinle Formation in the area docontain such fossils. These fossils indicate that a variety offerns, cycadophytes, conifers, and other small plantsinhabited this part of Utah at other times during the LateTriassic. Similar plants probably grew beside the treesnow represented by the fossil trunks in the Wolverine Pet-rified Forest but for some unknown reasons were not pre-The 225-million-year-old Wolverine Petrified Forest ofsouthern Utah documents one brief episode in the longgeologic history of the state. The forest contains the fos-silized remains of several elements of an ancient terrestrialecosystem that existed in southern Utah at the beginningof the Age of Dinosaurs. These fossils and the rocks thatcontain them indicate that the climate has changed overthe years and that this area was not always a desert. Infact, this was a moist tropical environment in which largetrees and probably other plants grew along the banks ofmany streams and rivers. About the author:Dr. Sidney Ash is an adjunct professor of Earth and Plane-tary Sciences at the University of New Mexico in Albu-querque. Prior to retirement a few years ago he was Pro-fessor of Geosciences at Weber State University in Ogden,Utah. Ash specializes in the study of the plants of the Ageof Dinosaurs and has published many articles about themin professional journals. He first visited the Wolverine Pet-rified Wood Area in 1972 while searching for plant fossilsin the Chinle Formation. A. Cross section of the wood of the treeshowing the cells as seen through the microscope. This is an exampleof permineralization mode of preservation. Scale bar equals 150microns. B. Surface of the trunk of a Woodworthia arizonicatreeshowing the small branch scars characteristic of this fossil. Scale bar The Wolverine Petrified Forest can be reached from the town of Boulder by driving about 18 miles east on the Burr Trail, which this section, to a rough unpaved side road that travels south. The forest is about 8 miles south of this junction on the southside of the roadin the area where the road crosses the usually dry Wolverine Wash. Asmall parking area is provided here and the forest is brieon a nearby sign. For peace of mind on visits to the area, a high-clearance or four-wheel-drive vehicle is recommended, especiVisitors to the Wolverine Petrified Wood Natural Area are reminded to leave the petrified wood for others to enjoy. In fact, cthing, including petrified wood, in Grand Staircase … Escalante National Monument is now illegal to all but permitted researchers. Quali-fied researchers can obtain the exact location of the Wolverine Petrified Forest from representatives of the Bureau of Land Man ABB