Hipótesis funcionales

The bed thickness nomenclature follows Ingram [1954], with the addition of “extremely thick” for beds >10 m thick. Breccia is used as a non-genetic term to describe any clastic facies composed of angular clasts coarser than 2 mm [Fisher, 1961a]. Similarly, sandstone (1/16–2 mm) and mudstone (<1/16 mm) are used to designate grain size classes [Wentworth, 1922; Fisher, 1961a]. I have used the term “facies” instead of “facies association”, for simplicity.

Most components of the Ohanapecosh Formation are volcanogenic. Primary volcaniclastic facies are generated directly by volcanic eruptions, and are not remobilized [White and Houghton, 2006], and may include juvenile and non-juvenile clasts. The term juvenile refers the products of volcanic eruptions that were at magmatic temperature when erupted [White and Houghton, 2006], i.e. on first contact with the atmosphere or the hydrosphere. Pumice clast refers to a highly vesicular (>60 vol.%) volcanic fragment that is intermediate to felsic in composition, whereas a scoria clast is less vesicular (<60 vol.%) and mafic in composition. Where compacted by diagenesis or welding, pumice clasts lose porosity and are transformed into fiamme [Bull and McPhie, 2007]. Dense clasts are not now and never were vesicular.

The proportions of clasts and matrix of representative samples of facies were estimated in the field, and on polished rock slabs and thin sections in the laboratory. Grain size distribution was determined following the technique described in Chapter 2. The best examples were also studied by image analysis from scans and field photographs at 1,200 dpi. The detection limit of fine-grained particles is generally around 2 mm, due to rock alteration, and this limit was used as a boundary between clasts and matrix. Thus, the term matrix is used broadly for interstitial fine clasts <2 mm. Alteration on feldspar and ferromagnesian crystals prevented making precise estimates of their volumes. Volumetric data are presented as pie diagrams.

The average diameter of clasts measured in the field corresponds to the mean diameter population, i.e. most common long-axis dimension of clasts. Maximum clast diameter corresponds to long-axis dimension of the coarsest clast. However, these values are in fact an apparent diameter on the rock face (chapter 2, Appendix B).

4.2. Pumice clasts and fiamme

Pumice clasts (Fig. 3.13a) are ubiquitous throughout the Ohanapecosh Formation. They are pale to dark green to black, and the formerly glassy groundmass is entirely devitrified and composed of secondary minerals. They span 1–300 mm in length and their aspect ratios (clast length/thickness) are 1–2.5 (max 5), reflecting the minimal effect of regional compaction. Coarse fiamme (up to 200 mm in length) are also present and have their long axes oriented parallel to bedding (Fig. 3.13b); they are considered to be former pumice clasts, now compacted because of their relic vesicular texture, their content in largely euhedral phenocrysts, and their similar alteration colour compared to pumice clasts. The pumice clasts range from aphyric to phenocryst-rich. Plagioclase and minor ferromagnesian (including pyroxene) phenocrysts are commonly partially to fully replaced by secondary minerals and can reach 10-20 vol.% of the pumice clast volume. No quartz as a phenocryst phase in pumice clasts and fiamme was found. In the Chinook Pass Member, most pumice clasts have 25-30 vol.% euhedral to subhedral plagioclase phenocrysts (2 mm, up to 5 mm). The pumice clasts are vesicular (Fig. 3.13a) and fiamme show moderate collapse of the vesicles parallel to the bedding. In contrast, small pumice clasts (<5 mm) are angular, blocky and equant, and are generally too small to contain phenocrysts (Fig. 3.13b). Preservation of vesicular texture in the small pumice clasts is rare (Fig. 3.13a). In the White Pass Member, pumice clasts are poorly to moderately (<30 vol.%) porphyritic. The pumice clasts can be tube pumice, and few specimens of “woody” type [Kato, 1987; Allen et al., 2010] were found, in which well- preserved vesicles are exceptionally elongate (aspect ratio >>100).

4.3. Free broken crystals

Fragments of plagioclase crystals (average 1-2 mm) are abundant in many facies (Figs 3.13b, 3.13d), and reflect the mineralogy of the coarser pumice clasts and fiamme. Relic ferromagnesian minerals are commonly too altered to identify. Very rare volcanic quartz crystals were found in the volcaniclastic facies (<3 grains over tens of thin sections).

4.4. Scoria clasts

Scoria clasts are present in the White Pass Member, and interpreted to be basaltic in composition by their mineralogy, vesicularity and colour (Fig. 3.13c). They are sub- angular to very angular, 2–10 mm in average size, range from red or dark grey to black, and are poorly to moderately vesicular (<40 vol.% of vesicles). They contain feldspar microlites arranged in a trachytic texture, and rare plagioclase phenocrysts (<1 vol.%). Ferromagnesian crystals occur in many scoria clasts, but are commonly altered and

d

xl

D

D

D

D

F

1 mm

b

xl

_F

F

a

ves

ves

ves

sc

sc

sc

sc

sc

sc

sc

xl

cem

c

Fig. 3.13 Clasts in the Ohanapecosh Formation. a) Highly-vesicular pumice clast in normally graded or massive volcanic breccia (facies 5; unit 30, sample WA-03), White Pass section; b) Fiamme, broken feldspar crystals and fine (<2 mm) matrix in the middle sub-facies of normally graded fiamme-andesite breccia (facies 1) in the Cayuse Pass section, sample WA-22; c) Clast-supported scoria clasts in mafic volcanic breccia (facies 10) at White Pass (unit 137, sample WA-09). A few broken feldspar crystals and zeolite cement are present; d) Dense clasts and fiamme in a finer matrix with broken feldspar crystals in coarse volcanic breccia (facies 6, sample WA-07). Dense clast (D), scoria clast (Sc), fiamme (F), feldspar crystal (xl), cement (cem), vesicles (ves).

difficult to distinguish from the ubiquitous altered groundmass. Vesicles are round to highly contorted in shape, commonly <0.1–1 mm across and filled with zeolites and other secondary minerals.

4.5. Dense clasts

Numerous types of dense clasts occur in the Ohanapecosh Formation. The mineralogy of the dense clasts reflects mafic to intermediate compositions and they lack quartz crystals. They are aphyric to moderately porphyritic with up to 50 vol.% feldspar crystals and minor amounts of relic ferromagnesian crystals. Dense clasts span <1–1,000 mm in size and show different types of alteration (Fig. 3.13d). The White Pass Member is rich in red, dark red, dark green, and dark brown dense clasts, whereas the Chinook Pass Member abounds with white, green or dark green to dark brown aphyric dense clasts, and lacks red dense clasts, except where in contact with Miocene Tatoosh sills. Dense clasts are mostly angular to very angular. Sub-rounded dense clasts are minor, and only occur in five facies, and they are restricted to basal dense clast breccia in three of them. Rare perlitic textures occur in some equant clasts and indicate that they were formerly glassy.

4.6. Plant fossils

Fossils and casts of leaves and silicified tree fragments were found at various places, but in minor quantities. An isolated trunk fragment was found at lower Cayuse Pass (Fig. 3.14a); it is silicified, >60 cm long, and contains minor pyrite and possible anthracite. The growth rings suggest a former trunk diameter of >60 cm.

4.7 Accretionary lapilli

Rim-type accretionary and armoured lapilli [Schumacher and Schmincke, 1991] were found in few places, and can reach 20 mm across. They commonly show multiple rims and their cores are up to 10 mm (American Lake, Ohanapecosh Campground; Fig. 3.14b) or absent (e.g. Backbone Ridge, Ohanapecosh Campground, White Pass). Accretionary and armoured lapilli are commonly spread throughout the thickness of thin beds, and are in places concentrated in layers within very thin beds. They are absent in the thick to extremely thick beds. Intact and broken accretionary lapilli are found together in most beds (Fig. 3.14b).

4.8. Matrix

Although probably entirely composed of fine particles originally, matrix now includes secondary crystals formed during diagenesis. The matrix is ubiquitously altered, but its similar colour and texture compared to the preserved clasts strongly suggest that the original components were all volcanic, and had the same bulk composition (Figs 3.13, 3.14). The grain size is dependant on the preservation state of the rock, and is commonly <2 mm.