George E. Mustoe

The origin of honeycomb weathering

Honeycomb weathering occurs throughout the world, but the origin remains a matter of controversy. Wind erosion, exfoliation, frost shattering, and salt weathering have been proposed as explanations, although few attempts have been made to substantiate these hypotheses with chemical or mineralogical studies. Chemical analyses and field observations indicate that honeycomb weathering in coastal exposures of arkosic sandstone near Bellingham, Washington, results from evaporation of salt water deposited by wave splash. Microscopic examination of weathered surfaces show that erosion results from disaggregation of mineral grains rather than from chemical decomposition. Thin walls separating adjacent cavities seem to be due to protective effects of organic coatings produced by microscopic algae inhabiting the rock surface. Cavity walls are not reinforced by precipitation of elements released by weathering, as has often been suggested at other locations. Honeycomb weathering develops rapidly and can be observed on surfaces that were planar less than a century ago.

Plagiogranite and keratophyre in ophiolite on Fidalgo Island, Washington

A sequence of Jurassic rocks on Fidalgo Island, Washington, is interpreted to be ophiolite. The order of rock types, from the base upward, is serpentinite, layered gabbro, a dike complex made up mostly of plagiogranite, volcanic rocks that are dominantly keratophyre, coarse breccia with clasts of keratophyre and plagiogranite, pelagic argillite, and siltstone-sandstone turbidites. The plagiogranites and keratophyres have identical chemical compositions and are mutually gradational in field setting and textures, all of which suggests that they are cogenetic. These rocks are distinguished from calc-alkalic rock types by their very low content of K 2 O (where SiO 2 = 70%, K 2 O = 0.2% to 0.7%). Metasomatic alteration of the rocks appears to be insignificant, judging from (1) well-preserved primary igneous textures, (2) well-preserved primary intrusive and extrusive contacts, and (3) uniformity of chemical composition across igneous units. An oceanic origin of the ophiolite is suggested by the capping of pelagic sediments. Their fine grain size, abundance of radiolaria, and enrichment in Mn and other metals are virtually identical to those of modern Pacific pelagic sediments and unlike that of arc or epicontinental sediments. This interpretation conflicts with the apparent paucity of plagiogranite and keratophyre on the present-day sea floor. Field relations and chemical trends indicate that the plagiogranite-keratophyre magma is not the product of fractionation of the same melt that crystallized layered gabbro. High water content of the plagiogranite-keratophyre magma is indicated by hydrothermal alteration of the gabbro near plagiogranite intrusions and the occurrence of hornblende instead of pyroxene in mafic varieties. We suggest that this water is from the sea and that the anomalously low K 2 O content of these magmas is due to exchange with sea water.

Bacterial oxidation of manganese and iron in a modern cold spring

A marshy cold spring occurs in late Pleistocene glacial outwash exposed on the floor of the Squalicum Creek valley near Bellingham, Washington. A 5-m by 25-m zone of black soil with an average depth of 30 cm surrounds the spring. This soil contains about 43% MnO 2 and 20% to 30% iron oxide calculated as Fe 2 O 3 ; the oxidized material appears amorphous when analyzed by X-ray diffraction. Field observations and laboratory studies indicate Mn and Fe are accumulated due to bacterial oxidation of trace amounts of these metals supplied by ground water. Two strains of pseudomonad bacteria isolated from the black soil rapidly oxidize Fe and Mn when grown on a culture medium containing soil organic matter as a nutrient. Both bacteria can be cultured on tryptone-glycerol agar but are unable to oxidize Mn or Fe added to this synthetic medium. Although the composition of the culture medium is important in controlling whether oxidation occurs, the reaction is not merely due to the catalytic effect of hydroxy acids contained within the nutrient mixture. Optical microscopy and X-ray fluorescence analysis using a scanning electron microscope equipped with an energy-dispersive detector reveal that Fe and Mn oxides precipitated by the microbes occur as extra-cellular deposits, and these metals are not accumulated within the bacterial cells. Although microbiologists have shown that several genera of bacteria are able to oxidize Mn in laboratory cultures, these studies are of uncertain value when used to explain the role microbes play in sedimentary Mn-oxide deposits, because most studies have involved microbes isolated from ordinary soil rather than from Mn-rich environments. This locality demonstrates the ability of soil bacteria to accumulate high concentrations of Fe and Mn under natural conditions.

Eocene Bird, Reptile, and Mammal Tracks from the Chuckanut Formation, Northwest Washington

Abstract The diversity and abundance of bird and animal tracks preserved in Eocene strata of the Chuckanut Formation in Washington contrasts to the scarcity of body fossils. These ichnofossils were made by vertebrates that inhabited river margins, the only depositional environment favorable for track preservation. Three of the four localities described herein contain tracks from at least two different types of animals. Site SM-6 contains approximately 200 shallow circular plantigrade footprints, perhaps made by a type of archaic mammal of the Orders Pantodonta or Dinocerata. Site RU-1 yielded footprints from a small shorebird and tracks from an early equid or tapiroid. The same type of perissodactyl tracks were preserved at Site KC-1, along with a single webbed bird track, and trackways from a large heron-like bird and a turtle. Site SM-9.5 contained multiple bird tracks of a type not found at the other localities. The discovery of tracks only at Chuckanut Formation sites that expose large bedding planes indicates the importance of considering outcrop architecture during the search for vertebrate ichnofossils, and inspires the hope that similar fossils may eventually be found in correlative formations in the Pacific Northwest.

Eocene amber from the Pacific Coast of North America

Fossil plant resins are common in Late Cretaceous rocks of North America and in Oligocene and younger sediments of equatorial and Southern Hemisphere locations. The scarcity of amber in early Tertiary rocks, however, poses puzzling geological questions. Detailed examination of mid-Eocene amber-bearing sediments from Coalmont, British Columbia, and Seattle, Washington, indicates that resins were produced by taxodiaceous conifers, with Metasequoia occidentalis being the most likely source. Laboratory heating experiments and paleotemperature analysis of these sediments based on coal rank and vitrinite reflectance suggest that burial temperatures may significantly affect infrared spectra of amber, a phenomenon not previously recognized by researchers who have long used spectral characteristics to speculate on the botanical origin of fossil resins. The occurrence of amber in mid-Eocene rocks of the Pacific Northwest, Arkansas, and southern California suggests that resin production ended as; the climate of North America cooled during the mid-Tertiary and that late Tertiary amber was produced by flowering tropical plants not present in the Eocene forests. This evolutionary record appears somewhat different from that of Western Europe, where progressive southward migration of coniferous amber forests occurred during mid-Tertiary time.

Mudflow disturbance in latest Miocene forests in Lewis County, Washington

ABSTRACT The lower part of the Wilkes Formation (uppermost Miocene) exposed along lower Salmon Creek in Lewis County, Washington, consists of volcaniclastic-dominated deposits. The section contains a stacked series of volcanic runout mudstone beds overlain by more distal runout mudstone beds, interlayered with carbonaceous mudstone and lignite-woodmat beds that are in turn overlain by poorly sorted volcaniclastic mudstone and sandstone. The section contains a record of forested lowlands inundated by volcanic mudflows, followed sequentially by rising water level associated first with deposition in swamp and lake-margin environments and later by deposition in a lake environment. The lakebed sediments contain common siderite concretions of varied form, including coprolite-shaped concretions that are confined to lakebed deposits. The volcanic mudflow deposits are similar to deposits of mudflows-lahars of modern Cascades stratovolcanoes. Two volcanic ash fall beds contained within lignites in the middle of the section yield 40Ar/39Ar radiometric dates. After separation of plagioclase crystals into populations of cloudy appearance (inherited) and clear appearance (newly crystallized), a best age of 6.13 ± 0.08 Ma is determined for the lower ash bed. Sediments contain abundant and well-preserved pollen and spores that document botanical changes progressing from Nyssa-dominated to Taxodium-dominated to mixed forest assemblages. The existence of Taxodium and other warm-climate taxa in the Wilkes Formation indicates the presence of a wet, warm temperate climate in the Puget lowlands during the latest Miocene.

Enigmatic origin of ferruginous “coprolites”: Evidence from the Miocene Wilkes Formation, southwestern Washington

Excrement-shaped masses of siderite and limonite have been reported from clay-rich sedimentary rocks that range in age from Late Permian to Holocene. These objects have been widely accepted as being coprolites, but the ferruginous composition, absence of internal inclusions, and scarcity of associated vertebrate remains suggest that they may instead be pseudofossils created by mechanical deformation of plastic sediment. Possible abiotic mechanisms include coseismic liquefaction, intrusion of sediment into hollow logs, or expulsion of sediment in response to gravity. Alternatively, carbon stable isotope ratios indicate that sediment extrusion may have been related to emission of biogenic methane during early stages of diagenesis.

Density and loss on ignition as indicators of the fossilization of silicified wood

Measuring density of silicified wood and determining weight loss after 450°C heating provides useful data for interpreting the process of permineralization. These simple gravimetric methods do not replace X-ray diffraction, electron microscopy, polarized light microscopy, Raman spectroscopy, and other specialized techniques for studying fossil wood, but they can be performed rapidly, and require minimal laboratory facilities. Woods mineralized with opal have densities of 1.9–2.1 g/cm3, compared to 2.3–2.6 g/cm3 for wood mineralized with chalcedony or quartz. Weight loss after 450°C heating, commonly described as “loss on ignition” can be used to roughly estimate the % of original organic matter that remains in chalcedony or quartz-mineralized wood, using the density of extant taxa for comparison. For opalized wood, 450°C weight loss mostly represents dehydration of the hydrous silica. Data from specimens from 20 localities reveal two characteristics: 1) silicified woods typically consist either of opal or chalcedony/quartz, not an intermediate mixture of the two silica polymorphs; 2) the percentage of organic matter that remains after petrifaction is usually very small.

Crocodylian Tracks from Lower Oligocene Flysch deposits of the Barail Group, Manipur, India

A marine tidal delta siltstone from Gelmon locality in Northeast India preserved three crocodylian footprints and an elongate depression that appears to be a tail drag mark. Similar drag marks occur in nearby bedding surfaces. The discovery of crocodylian tracks from the basal part of Laisong Formation, Barail Group (Late Eocene–Early Oligocene age) of Manipur, India is noteworthy because of the age and the geographic location. Crocodylian tracks are rare in Cenozoic formations and they have not previously been reported from Asia. The footprints are herein named as a new ichnogenus and ichnospecies, Indosuchipes manipurensis.

Mammal and Bird Tracks from the Eocene Puget Group, Northwest Washington, USA

In 1997, coal extraction at the John Henry Mine in western King County, Washington, USA, exposed bedding planes in Eocene sandstone that contained numerous bird and mammal tracks. By the time scientists arrived at the site several months later, the track-bearing surfaces had mostly been obliterated by landslides. Several track specimens were collected but not curated, described, or studied. In 2011, the specimens were found in a storage room at the University of Washington Burke Museum of History and Culture, triggering an investigation that yielded many photographs of the fossil site taken at the time of the 1997 discovery. Perissodactyl mammal tracks are named herein as Oplidcatylapes eocenica ichnogenus and ichnospecies nov. Photographs also show a trackway containing eight footprints that have prominent claw impressions. These tracks are inferred to have been made by a creodont, but because of the absence of specimens or track casts to serve as holotypes, ichnotaxonomic names have not been assigned.