The Ohanapecosh Formation consists of eight very thickly to extremely thickly bedded (>1 m thick in general), and eight very thinly to thickly bedded (<1 m thick) volcaniclastic facies, and three coherent facies. In addition to bed thickness, facies have been distinguished by depositional structures (grading, lamination), clast type, clast proportion and mineralogy (Tables 3.1, 3.2). The descriptions of the best preserved examples include bed thickness, grading, clast colour, clast shape, clast types and their approximate volume, average and maximum grain size and volumetric proportion of matrix. With the exception of some poorly preserved beds, all the beds included in the stratigraphic sections belong to a described facies. Miocene intrusions are described, but not shown on the stratigraphic logs for simplicity; covered stratigraphy is also indicated.
2 cm
b
4 cm
tree
rings
al
al
al
al
a
Fig. 3.14 Clasts in the Ohanapecosh Formation. a) Silicified wood from fine sandstone and mudstone beds (facies 8) in the lower Chinook Pass section. Some outer tree rings are made of anthracite and pyrite, sample WA-01; b) Red-altered, rim-type accretionary lapilli (al) and armoured lapilli in bed of fine sandstone and mudstone (facies 8) within a sequence of mafic volcanic breccia (facies 10) at Ohanapecosh Campground. Cores of armoured lapilli are made of mafic scoria clasts. Note that some accretionary lapilli are broken.
Name
Major components
Minor components
Thickness Sedimentary textures
Location (Member)
Initiation and sedimentary processes
1
Normally graded fiamme-andesite breccia Fiamme/pumice clast, feldspar crystal fragment, andesite clast, matrix
- >20 m Tabular, normally graded, basal andes- ite breccia
Basal dense clast-supported breccia
Cayuse Pass, Chinook Pass, Cougar Lake
(Chinook Pass)
Subaqueous volcaniclastic density current Derived from subaerial pyroclastic flow
2
Fiamme-andesite breccia Fiamme/pumice clast, feldspar crystal fragment, andesite clast, matrix- >20 m Tabular, normally graded, basal andes- ite breccia
Basal dense clast-supported breccia
Cayuse Pass
(Chinook Pass) Subaqueous volcaniclastic density currentDerived from subaerial pyroclastic flow
3
Normally graded fiammebreccia Fiamme/pumice clast, feldspar crystal fragment, matrix Dense clast 20 m Tabular, normally gradedBasal dense clast-supported breccia Chinook Pass, Cougar Lake, ?Backbone Ridge(Chinook Pass and ?White Pass) Subaqueous volcaniclastic density currentDerived from subaerial pyroclastic flow
4
Reversely graded fiammebreccia Fiamme/pumice clast, feldspar crystal fragment, matrix Dense clast 40-50 m Tabular, reversely graded Chinook Pass, Backbone Ridge(Chinook Pass and White Pass) Subaqueous volcaniclastic density currentDerived from subaerial pyroclastic flow
5
Graded or massive volcanicbreccia Fiamme/pumice clast, dense clast, matrix Feldspar crystal fragment 0.5–15 m Tabular, normally graded, massive or reversely graded Cayuse Pass, Chinook Pass, Cougar Lake, White Pass, Ohana-pecosh Campground, Backbone Ridge, south Packwood (Chinook Pass, White Pass and Johnson Creek)
Subaqueous volcaniclastic density current
Derived from subaqueous remobilisation, subaerial pyroclastic flow or sub- aqueous explosive eruption
6
Coarse volcanic breccia Fiamme/pumice clast, dense clast,matrix Feldspar crystal fragment 1–20 m Tabular, normally graded White Pass, Ohanapecosh Campground, Backbone Ridge(White Pass) Subaqueous volcaniclastic density currentDerived from subaqueous remobilisation, subaerial pyroclastic flow or sub- aqueous explosive eruption
7
Clast-supported polymicticbreccia-conglomerate Dense clast Matrix 3 m Normally graded Chinook Pass(Chinook Pass) Subaqueous volcaniclastic density currentRemobilisation from upper wave-base source
8
Fine sandstone and mudstone Matrix Fiamme, dense clasts, feldspar crystal fragment, wood frag- ment, accretionary lapilli<1 mm–
2 m Tabular, massive or normally graded or laminated, planar or cross-bedded Cayuse Pass, Chinook Pass, Cougar Lake, White Pass, Ohana-pecosh Campground, Backbone Ridge, south Packwood (Chinook, White Pass and Johnson Creek)
Turbidity current, suspension settling or background sedimentation Derived from subaerial fallout, remobilisation or non-volcanogenic processes
9
Fine mafic sandstone Vesicular basaltic clast zeolitic cement 0.1–2 m Tabular, massive or normally graded White Pass(White Pass) Subaqueous volcaniclastic density current, or suspension settlingDerived from subaerial plume fallout or subaqueous remobilisation
10
Mafic volcanic breccia Vesicular basaltic clast accretionary lapilli,zeolitic cement 3 cm–1 m Tabular, normally graded White Pass, Ohanapecosh Campground, Backbone Ridge(White Pass) Grain flow or subaqueous suspensionDerived from eruption-fed fallout or from remobilisation
11
Normally graded andesite bre-ccia to fiamme breccia Fiamme, dense clast, feldspar crys-tal fragment, matrix - 60 cm Tabular, normally graded, basal dense clast breccia Chinook Pass, Cougar Lake(Chinook Pass) Subaqueous volcaniclastic density current or rock fallProbably derived from remobilisation
12
Reversely to normally gradedpumice breccia Pumice clasts, mudstone matrix Fiamme, accretionary lapilli, wood fragment 1–1.5 m Tabular, reversely to normally graded, numerous mudstone interbeds Basal dense clast-supported breccia
Chinook Pass
(Chinook Pass) Suspension settlingDerived from abrasion and progressive waterlogging of pumice clasts in rafts, derived from subaerial fallout or dilute pyroclastic flow over water
13
Crystal-rich sandstone Feldspar crystal fragment, matrix Dense clast, fiamme, wood, ac-cretionary lapilli <1m Tabular, massive or normally graded Chinook Pass, Cayuse Pass, Ohanapecosh Campground, Back-bone Ridge, south Packwood (Chinook Pass, White Pass and Johnson Creek)
Subaqueous volcaniclastic density current or turbidity current Derived from eruption-fed or remobilisation processes
14
Poorly porphyritic andesitebreccia Dense andesite clast Fiamme, matrix 0.1–2 m Tabular, normally graded Chinook Pass, Cougar Lake, Backbone Ridge, south Packwood(Chinook Pass, White Pass and Johnson Creek) Subaqueous volcaniclastic density current, or suspension settlingDerived from remobilisation of parts of dome or lava
15
Fiamme sandstone and fi-amme mudstone Fiamme/pumice clast, matrix Accretionary lapilli, dense clast, wood fragment 0.1–2 m Tabular, normally or reversely graded, rare cross-beds Cougar Lake, Ohanapecosh Campground, Backbone Ridge, south Packwood (Chinook Pass, White Pass and Johnson Creek)
Subaqueous turbidity current or subaqueous volcaniclastic density current Derived from eruption-fed or remobilisation processes
The abundance of pumice and dense clasts versus matrix was documented by image analysis and functional stereology (chapter 2) in various bed of the Chinook Pass Member. These data quantify the volume and grain size distribution of pumice and dense clasts and are used as complement to the facies analysis description.