William J. Fritz

Tectonics of the Yellowstone hotspot wake in, southwestern Montana

Deposits off a Neogene paleovalley provide a geologic datum in southwestern Montana that brackets the age of faulting associated with the Yellowstone hotspot. Ages of paleovalley deposits on top of the modern Blacktail, Ruby, Tendoy, and Centennial ranges and in adjacent valleys range from 16 to 2 Ma. At least 2 km of offset has occurred since emplacement of the Timber Hill basalt (6.0 ±0.1 Ma); much may have occurred after deposition of the 2.0 Ma Huckleberry Ridge Tuff, which is now found in isolated outcrops as far north as the Ruby Range. Gravel clasts within the paleovalley have a northern provenance, whereas volcanic and volcaniclastic rocks were derived from the Snake River Plain to the south. We propose that uplift associated with passage of the hotspot caused the drainage reversal.

Transported trees from the 1982 Mount St. Helens sediment flows: Their use as paleocurrent indicators

Sediment flows resulting from the 1980 and 1982 eruptions of Mount St. Helens, Washington, transported and oriented numerous stumps and logs. Counts taken soon after the 1982 eruption on unaltered portions of the sediment flow provide meaningful statistics for comparison with fossil wood deposits. Of the transported stump and log population, 4–13% was deposited as upright stumps and 78–94% as horizontal logs with a few diagonal and upside-down stumps. The horizontal logs were oriented parallel to the river channel, showing that trees can be good paleocurrent indicators. Features useful in differentiating transported stumps from in-situ ones are a wide root system (trunk/root ratio <1), short broken trunks, broken large roots, and a low percentage of upright stumps in relation to horizontal logs. The oriented horizontal logs, transported vertical stumps mixed with trees buried in place, and sedimentology of the flows aid in interpreting fossil wood deposits such as the Yellowstone “fossil forests”.

A shallow marine volcaniclastic facies model: an example from sedimentary rocks bounding the subaqueously welded Ordovician Garth Tuff, North Wales, U.K.

Abstract Volcaniclastic sedimentary rocks bounding the Ordovician Garth Tuff in North Wales were deposited in a shallow marine setting adjacent to a magmatic arc. The volcaniclastic sedimentary rocks are primarily of angular to euhedral quartz, angular euhedral feldspar, and volcanic rock fragments. These grains are of a pyroclastic origin, but have been reworked to various degrees. Sedimentary rock fragments, rounded quartz, muscovite, biotite, iron-rich chlorite and various clay minerals occur in lesser amounts. The sedimentary rocks can be divided into a proximal offshore to foreshore facies association (POFFA) and an offshore to lower shoreface facies association (OLSFA). The sand-dominated POFFA, exposed near Capel Curig, is characterized by large wave ripples, hummocky cross-stratification, co-sets of trough cross-beds, reactivation surfaces, plane beds, and graded turbidite layers in suspension-deposited mudstone. Water depths calculated from bedding plane exposure of large wave ripples and determined by the presence of hummocky cross-stratification and by facies association vary from a few meters to around 30 m (45–50 m theoretical maximum) in a 45 m-thick stratigraphic section beneath the tuff. Eight kilometers to the southeast, the Garth Tuff is bounded by a thick sequence of suspension-deposited laminated mudstone with thin graded beds of siltstone and fine-grained sandstone. This facies association (OLSFA) represents the introduction of material by distal turbidites and by pelagic sedimentation. The OLSFA was deposited in water depths well below storm wave base, possibly in excess of 200 m. No evidence of storm waves or surface-generated currents occur until over 40 m above the tuff. This study documents marine sedimentary rocks, deposited in water depths ranging from foreshore to offshore, as the bounding facies of the Garth Tuff. It is, thus, reasonable to conclude that the Garth Tuff was emplaced and welded in water depths greater than the thickness of the tuff in the area of this study.

A subaqueous welded tuff from the Ordovician of County Waterford, Ireland

Abstract The Metal Man Tuff (MMT) from the Ordovician of County Waterford, Ireland was emplaced and welded in water depths greater than the thickness of the pyroclastic flow. The MMT is the basal member of the Middle Tramore Volcanic Formation (MTVF) of the 5-km-thick Tramore Group. The MMT consists of a 10-m-thick basal graded zone that represents a pyroclastic flow consisting of angular clasts of black mudstone, pumice, gray flow banded rhyolite, and pink massive rhyolite set in a matrix of non-deformed ash shards and pumice. Maximum grain size grades from large cobbles and small boulders to pebbles. The basal 10–30 cm is depleted with respect to the largest boulders resulting in an inversely graded basal layer. The basal graded zone passes upward into a transition zone with a strong eutaxitic foliation defined by elongated fiamme of mudstone and flattened pumice. Overlying this is an upper welded zone with a pronounced eutaxitic foliation, columnar jointing, flattened ash shards and shards deformed around phenocrysts and spheroids. The presence of these features indicate that the deposit is welded, was hot, and was in motion as the shards deformed. The MMT represents a pyroclastic flow that was a hot primary product of an eruption rather than re-mobilized cold pyroclastic debris. The MMT is bounded by suspension deposited fine-grained tuff, tuffaceous mudstone and terrigenous mudstone deposited below storm wave base. Many of the mudstone horizons contain brachipod faunas from shelf-depth water. Nowhere in the 5-km-thick Tramore Group is there terrigenous sandstone, evidence of unidirectional flowing water, nor any indication of shoreline, alluvial environments, or subaerial exposure. It thus seems reasonable to conclude that the MMT was emplaced and welded subaqueously. The geochemistry of the MMT is typical of other high-silica (70–78 wt.% SiO2) rhyolite from the Ordovician of Ireland. The chemistry of the MMT is consistent from top to bottom allowing it to be distinguished from associated syndepositional intrusive rock and fine-grained tuffs.

Cross-identification of ring signatures in Eocene trees (Sequoia magnifica) from the Specimen Ridge locality of the Yellowstone fossil forests

Abstract We have found a characteristics ring signature in Eocene trees ( Sequoia magnifica ) from the 15–25 m interval of the Specimen Ridge section of the Lamar River Formation in Yellowstone National park U.S.A. This signature indicates that these stumps were contemporaries growing in the same forest. This interpretation supports the theory that trees on the Specimen Ridge section were preserved where they grew in low energy valley facies far from the volcanic source peaks that produced the volcanic sediment.

Volcaniclastic sedimentation in and around an Ordovician subaqueous caldera, Lower Rhyolitic Tuff Formation, North Wales

The Lower Rhyolitic Tuff Formation of Ordovician age in North Wales records the collapse, infilling, and subsequent resurgence of a volcanic caldera with an original diameter of about 15 km. This volcanic center controlled patterns of volcaniclastic sedimentation, providing enough topographic relief for both a shallow-lagoon depositional basin within the caldera and a source of sediment derived from the rim. Within the caldera, sediment consists of tuffaceous laminated siltstone and immature, coarse-grained, volcaniclastic sandstone containing plane beds, ripple cross-laminations, symmetrical wave ripples, and hummocky cross-stratification. Coarse-grained, matrix-supported, conglomerate layers and layers of ash-flow tuffs are also present. These sediments accumulated in shallow water above fair-weather wave base. Conglomerate units represent debris flows from the caldera rim, a nearby shoreline, and elevated areas associated with resurgent domes. Sedimentation outside of the caldera consisted of deposition of background suspension and volcanic-ash suspension, and turbidite deposition on a pyroclastic apron. The outer margin of the apron was dominated by fine-grained suspension and turbidite deposition, whereas the inner margin of the apron contains hummocky cross-stratification and other evidence of reworking by episodic storm waves. Local highs with associated shallow-water sedimentation existed outside the caldera. Even though deposited in high-energy marine environments, all sedimentary rocks are both texturally and mineralogically very immature. This textural immaturity differs from the typical very mature marine sediments and was caused by rapid depositional rates and a local volcanic sediment source.

Armored Mud Balls from Cabretta and Sapelo Barrier Islands, Georgia

ABSTRACT Armored mud balls similar to ones previously studied on marine and lacustrine shorelines are now forming on Sapelo and Cabretta Islands, Georgia. This report constitutes the first study of armored-mud-ball formation in a harrier-island environment. Small, 0.3-1.5-m-high sea cliffs of Pleistocene marsh deposits in the lower foreshore erode to form the mud-ball cores. During transport to the upper foreshore, beach sediment partially armors the surface of the mud cores. Predominance of sand-size armor and rootbound cores set these samples apart from earlier studies. Intermittent rolling in the swash zone on the gently sloping beach flattens the core and concentrates armor on opposing sides. No evenly armored, very highly spherical mud halls, common to fluvial environments, were found in his study area. Because several armored mud balls were found buried in sand on the backshore, it is likely that some will be preserved. Such lithified, armored mud balls may be especially useful in determining seaward facies of ancient barrier-island deposits.

Theoretical wave modeling of large wave ripples in volcaniclastic sediments, Ordovician Llewelyn Volcanic Group, North Wales

Abstract Volcaniclastic sedimentary rocks of Ordovician age underlying the Garth Tuff member of the Capel Curig Volcanic Formation, North Wales were deposited on a shallow marine shelf. Deposition at the Capel Curig anticline 2 km southwest of Capel Curig was on a high-energy proximal offshore to foreshore. Analysis of large wave ripples and associated sedimentary structures suggests a shallow marine depositional environment, above storm wave base and in water depths of 10–30 m. This estimate of water depth from wave calculations is similar to that estimated by facies analysis.

Giant Armored Mud Boulder from the 1982 Mount St. Helens Mudflows

ABSTRACT A giant armored mud boulder 71 by 70.5 by 60 cm was formed and transported on the surface of recent Mount St. Helens mudflows. This armored mud ball is the largest recorded in the geologic literature and the first from volcaniclastic terrane.

Sedimentary Sequence from 1980 and 1982 Mount St. Helens Sediment Flows: A Model for Older Volcaniclastic Deposits: ABSTRACT

Sediment flows resulting from May 18, 1980, and March 19, 1982, eruptions of Mount St. Helens produced a three-unit sequence that provides a model for interpreting similar deposits in the rock record. After the 1982 eruption, rivers rapidly reestablished pre-eruption channel levels and downcut through the new flows, allowing examination of the internal structures before extensive modification by reconstruction projects. The depositional sequence consists of a basal graded to massive layer (Unit 1) of large clasts in grain-to-grain contact overlain by a distinctly finer-grained stratified unit (Unit 2) of similar thickness. The top unit (Unit 3) contains very large matrix-supported clasts and transported log debris. Grain size and total thickness of the sequence varies from 3.5 m (11 ft) thick, 40 cm (16 in.) boulders, and coarse sand in proximal flows above Camp Baker to 0.5 m (1.5 ft) thick, coarse sand, and silt in distal flows on the lower Cowlitz River. This sequence resembles some fluvial deposits, but clearly formed during extremely rapid deposition related to mudflows. We have noted similar Tertiary volcaniclastic sediments and believe Mount St. Helens sediment flows provide a model for interp eting volcaniclastic deposits often considered dominated by fluvial processes. Figure End_of_Article - Last_Page 464------------