Appendix I: Sample Descriptions

We made initial sample evaluations and descriptions using the petrographic microscope. We examined the sieved but otherwise untreated sample in an optic oil having an index of refraction of 1.515. Subsequent sample descriptions were made during processing, after the samples had been sized, treated with acids, and ultrasonically cleaned (see Methods section). Some rarer phases became visible only in the magnetic fractions after separation in the Frantz separator.

Because most if not all of the tephra layers sampled in the Owens Lake cores are at least partly reworked, they contain both the comagmatic pyroclastic airfall material of the tephra layers, plus any accidental xenocrystic or xenolithic fragments from the walls of the magma chamber and the vents, as well as detrital-clastic, authigenic and biogenic material derived from within the Lake Owens drainage basin. The minerals described below include all three types.

Primary comagmatic minerals of the tephra layers can be usually distinguished when they are encased within glass shards, or when they have some glass adhearing to their surfaces. When the glass coatings are absent, it may be difficult or impossible to determine whether a mineral crystal or fragment is derived from the tephra, or whether it is part of the clastic or organic load, unless the mineral grains are readily identifiable as of plutonic, metamorphic, sedimentary, or organic origin.

Abbreviations used for glass shard morphology in sample descriptions given below are:

bw: bubble-wall shards or platy shards. Glass shards that are clear, angular, and have a large radius of curvature which may not be readily apparent under the microscope. These are fragments of shattered glass bubbles, generally formed explosively during eruption. They look like very small pieces of a thin, shattered pane of window glass.
bwj: bubble-wall junction shards, are junctions of two or more bubbles, with parts of the bubbles adhering. These are either y-shaped junctions between spherical or oval bubbles, or curvilinear to straight, ribbed shards, junctions between elongated vesicles.
eig/sob: equant to irregulatly-shaped shards, containing sherical or ovoid vesicles. These may be poorly or well vesiculated. Some types may be very thin-walled and fragile, having the appearance of a fishing net. We refer to such types as "webby".

Glass shards with elongated vesicles are usually:

s - spindle shaped (stubby or drawn-out);
t - tubular; drawn-out into long cylinders or capillaries;
c - conical

The vesicles may be hydrated or not. Usually, the spindle-shaped grains show hydration most readily. The hydration is a function of time and temperature, and probably also of other factors of the tephra sample's history (see Sarna-Wojcicki and Davis, 1991).

Colors used in the initial descriptions of tephra samples are those in the Munsell color chart (Goddard and others, 1979). Colors were noted on untreated hand samples.

OLB-92-S
OL92-1
OL92-1001
OL92-1003
OL92-1015
OL92-1016
OL92-1019
OL92-1020
OL92-1021
OL92-1022
OL92-1023
OL92-1024
OL92-1025
OL92-1026
OL92-1027
OL92-1028
OL92-1029
OL92-1030

OLB-92-S

Unconsolidated, floury, very fine-grained ash, pinkish gray (5 YR 8/1), containing ~80% highly vesiculated, hydrated, webby or ribbed pumiceous glass shards. Shards with bubble-wall (bw) and bubble-wall junctions (bwj) are also common. Platy, blocky shards were rare. Vesicle shapes are mostly elongate spindles (s); tubular (t) and conical (c); the sample also contain equant- to irregularly-shaped grains with spherical to ovoid bubbles (eig/sob) in smaller amounts. Crystalline grains present included calcite (~5%), biotite (~10%), feldspars and quartz (~4%), and ortho- and clino-pyroxenes (~1%). Brown shards comprise <1% of the glass.

After acid treatment, the proportion of glass to crystalline material increased to 90/10. The sample also contained compound glass/crystalline grains and murky, dirty grains of indeterminate composition; grains of zircon and hornblende were also noted.

OL92-1

Light olive-gray sample (5Y 5/2), containing ~70% glass shards, and ~30% of mineral grains. Shards are mostly subangular, ribbed, and/or bw and bwj. Approximately 20% of the shards are blocky. The shards are often vesiculated and slightly to moderately hydrated. Vesicle shapes are mostly s and c; eig/sob are also common. Crystalline minerals present were feldspars (10-12% plagioclase; 1-2% microcline), biotite (5%), hornblende (1-2%), and quartz. The remaining miscellaneous grains are brown shards and altered grains. After acid treatment, the percentage of glass shards increased slightly, and about 5% of microlitic shards became visible. Biotite, hornblende, orthopyroxene, compound grains, and brown glass shards were concentrated in the magnetic (0.6A fraction).

OL92-1001

Very fine-grained, loosely consolidated to unconsolidated sample, yellowish gray (5Y 8/1) in color, consisting of 75-80% glass shards. These were mostly ribbed, hydrated, vesiculated pumiceous shards; 10-12% biotite, ~5% feldspar and quartz; <1% hornblende, <1% zircon, <1% clinopyroxene, <1% brown glass, 2-3% altered or devitrified material. Vesicles types are mostly s; t, c, and eig/sob types are also present. Sample had a slight surficial coating. After acid treatment, ~10% of compound grains became visible.

OL92-1003

Sandy, unconsolidated, small sample with some large glass grains, pale to moderate yellowish brown (10YR 6/2 to 10YR 5/4). Washed residue contained ~40-50% moderately to well vesiculated, hydrated, ribbed and webby shards. The remainder are heavily coated grains and mineral grains. The latter include feldspars, biotite, pink zircon, quartz, apatite, calcite, and magnetite. Brown shards were also present. After acid treatment, proportion of glass shards increased to ~50-55%. About 2-3% of glass fraction are brown shards. Compound grains are common. Vesicles are mostly eig/sob types, but slightly elongate tubes, cones, and spindles make-up a subordinate but significant fraction. A large number of vesicles are not hydrated, and are present with hydrated vesicles within the same shards. Also present are hornblende, pyroxene, and an unidentified isotropic mineral.

OL92-1015

Diatomaceous and calcareous, very fine-grained light olive gray (5Y 6/1) sample. About 5 to 6 % of this sample contains glass shards, and is thus an ashy sediment. Minerals present are zircon, feldspars, greenish biotite, clinopyroxene, orthopyroxene, hornblende, apatite, with a calcite cement. Contains ostracode fragments and mesoscleric sponge spicules. Diatoms present are alkaline and fresh-water to brackish types including: Stephanodiscus cf. nigrans, Surrirella sp., Campylodiscus sp., Epithemia sp., Melosira moniliformis, and Navicula sp.

After acid treatment, most of the biogenic material was eliminated. Sample still contained only ~5% glass shards.

OL92-1016

Very clayey, very fine-grained sample, light olive gray (5Y 5/2) containing ~55% angular, poorly to moderately vesiculated, slightly hydrated shards. These are mostly ribbed, bw/bwj shards. Platy or webby shards make-up a small fraction of the sample. Vesicles are mostly eig/sob types, but t, c, or s types are common. About 35% of the sample is composed of mineral fragments and other materials. Mineral grains are olive green biotite (~15%), feldspars and quartz (~15%), pyroxene (~3-4%), and hornblende (1%). Biogenic components include ~2-3% diatoms (mostly Campylodiscus and Stephanodiscus), ostracode fragments, and plant debris. The remainder are heavily coated and/or devitrified grains that tend to have a slight to moderate coating of carbonate and clay.

After acid treatment, glass fraction increased to 70% due to elimination of biogenic component and clean-up of heavily coated grains. Magnetic fraction at 0.6A and +8 degree tilt contained biotite, hornblende, clino- and ortho- poyroxenes, oxyhornblende, magnetite, zircon (?), brown shards, and compound grains.

OL92-1019

Diatomaceous clay, light olive gray in color (5Y 6/1), containing mostly fresh-water centric flora and mineral grains. Only ~1-2 % glass shards are visible, although many grains are obscured by a heavy coating. Acids removed biogenic fraction and altered/dirty material. Glass fraction increasd to 25% in residue; it is composed of mostly subangular bw/bwj shards and some ribbed, poorly vesiculated, slightly hydrated grains. Most of the heavy minerals separated in the magnetic fraction (0.6A; +8 degree): biotite, hornblende, pyroxene, etc.

OL92-1020

Coarse, sand-sized tephra, yellowish gray (5Y 7/2) in color. Contains ~90% good, clear, subangular bw and bwj shards and ~10% crystalline material. About 5% of the sample consists of compund, microlitic glass, ~15% of vesiculated, hydrated glass. Vesicles are mostly of the eig/sob type. Also present were t, c, and s shards, the latter with slight elongation. Minerals present are clinopyroxene, feldspars, quartz, biotite, hornblende, and apatite (in decreasing order of abundance). A slight coating on many grains is present.

After acid treatment, more orthopyroxene was observed in sample. The pyroxenes are almost all etched on crystal terminations. A few percent of brown glass shards are present.

OL92-1021 through OL92-1030

The samples from a depth of 303.94 to 320.01 m in the core are described together, inasmuch as they have essentially the same petrographic characteristics and chemical composition, and define a more-or-less uninterrupted, tuffaceous interval containing chemically and petrographically homogenous tephra.

The samples are composed of dominantly coarse- to medium-sand sized tephra, with minor intervals of medium- to very-fine-grained tephra (see table 3), 50 to 95% of which are glass shards.

OL92-1021

Coarse, sand-sized tephra, yellowish-gray (5Y 7/2). Glass shards comprise 70% of the sample. They are bw/bwj, chunky, subangular; 15% are moderately to strongly ribbed, hydrated, and vesicular. Vesicles are of the s, c, and eig.sob types. A few percent of the glass shards are microlitic or brown. There is a slight pitting and a slight to mederate coating on the shards. Minerals include microcline and plagioclase, quartz, hornblende, and pyroxene. After acids, 5% more of compound grains were observed. These are mostly pumiceous, highly vesiculated shards. Magnetic fraction (0.6A +8 deg.) contained brown shards, compound grains, hornblende, oxyhornblende, orthopyroxene, biotite, apatite, magnetite, and some other dirty, opague to translucent nondescript grains.

OL92-1022

Coarse, sand-sized tephra, yellowish gray (5Y 7/2), consisting of ~85% subangular, chunky, mostly bw and bwj shards that are slightly vesiculated, hydrated, and pitted. Approximately 17% of glass shards are moderately ribbed, and about 2-3% are webby. The remaining 15% are composed of feldspar and quartz (~13%) (including sanidine, plagioclase, microcline; many have glass coatings), biotite, hornblende, and orthopyroxene (~2%). Brown shards make-up <1% of sample. Shards are slightly to moderately coated.

After acid treatment and magnetic separation, brown and compound shards, hornblende, oxyhornblende, biotite, pyroxene, and magnetite were concentrated in the 0.6A =8 deg. fraction.

OL92-1023

Coarse, sand-sized tephra, very pale yellowish brown (10YR 6/2), with similar shard morphology to OL92-1021 and 1022, consisting of ~75% glass shards. This sample has a higher proportion (~33%) of finely ribbed, pumiceous shards. Also contains pyrogenic (?) sanidine and plagioclase, as well as microcline, quartz, hornblende, orthopyroxene, biotite, and <1% calcite. A light coating of iron oxides covers many of the grains.

OL92-1024

Coarse, sand-sized tephra, yellowish gray (5Y 8/1), containing 85% glass shards. About 75% of these are bw and bwj, subangular, slightly vesiculated and often hydrated. About 25% are moderately to strongly ribbed/webby pumiceous shards. Vesicle shapes are usually elongated s, t, and c's, and eig/sob's as well. Minerals are anhedral to subhedral feldspars (with glass coatings), quarts, hornblende, and biotite. A slight coating covers many grains. Minor pitting is also observed.

OL92-1025

Coarse to fine grained tephra, yellowish gray (5Y 8/1) on weathered surfaces, and very light gray (N8) on fresh inner surfaces, composed of ~90% glass shards. Dominantly frothy glass, strongly ribbed and webby, hydrated and vesiculated pumice. About 5% of sample consists of mineral grains, mostly feldspars and biotite, and 5% is microphenocrystic. There is a slight coating on grains.

OL92-1026

Coarse to medium-sand-sized tephra, 95% subangular glass shards which are slightly coated with iron oxides and carbonate. About 50% of the shards are ribbed and moderately to strongly vesiculated; ~40% are platy, and about 10% are bw and bwj. Vesicle shapes are mostly ovoid and irregular, but cylindrical, spindle, and conical shapes are commonly observed. Most vesicles are partly hydrated. A few percent of brown shards are also present. Minerals are ~2-3% biotite, ~2% feldspar (sanidine and plagioclase), quartz, and pyroxene.

OL92-1027

Coarse to medium sand-sized tephra, yellowish gray (5Y 7/2), containing 90% glass shards that are slightly to moderately coated with iron oxides and carbonate. About 70% of the shards are ribbed bw and bwj; ~20% finely ribbed and pumiceous; ~10 % are platy shards. Vesicles are eig/sob's, with some s and c types. Most vesicles are hydrated. Minerals are biotite (~3), feldspars (<1%), quartz (<1%), calcite (<1%), and compound grains (~5%).

OL92-1028

Coarse, sand-sized tephra, light olive gray (5Y 5/2), composed of 65% slightly- to moderately-carbonate-coated, shards. These are mostly ribbed bw and bwj shards. About 1/3 of these are blocky, solid (unvesiculated). Vesicles are usually eig/sob's, with some s, t, and c types. They are often hydrated. Minerals observed are feldspars (sanidine and plagioclase), commonly with glass coats, and quartz (~combined 30%), biotite (~2%), calcite (~2%), hypersthene, apatite, hornblende (<1% each), and rare garnet (?) grains. A few ostracode fragments are present. After acid and ultrasonic clean-up, zircon and clinopyroxene are visible, and allanite (?).

OL92-1029

Very fine-grained tephra, light olive gray (5Y 5/2) containing ~85% glass shards, most of which are ribbed, slightly curved, with subordinate platy, bw and bwj. Vesiculated shards are common. There is a slight to to moderate coating of carbonate on many of the grains. Biogenic fragments, diatoms, pollen, octracodes, and sponge spicules, are a minor proportion of the sample. Minerals present are feldspars (plagioclase), quartz, biotite, and ilmenite (?). The magnetic (0.6A +8 deg.) fraction contained a very small amount of sample that included hornblende, pyroxene, biotite, epidote(?), and brown glass shards, as well as some compund grains.

OL92-1030

Basal tephra sample, consisting of coarse, pumiceous, sand-sized tephra, olive gray (5Y 3/2), containing ~55% subrounded, subangular, chunky, moderately-vesiculated, hydrated, mostly ribbed bwj and pumiceous shards. About 20% of the sample is pyrogenic biotite, ~20% feldspars plus quartz, ~1-2% hornblende, and <1% isotropic, altered mineral. Many of the grains are obscured by an iron oxide coating.

After acid and ultrasonic cleaning, and magnetic separation, the magnetic fraction (0.6A +8 deg.) contained grains of minerals such as olivine (?) and epidote (?) (or pinkish thulite?), allanite, and zircon.

U.S. Department of Interior, U.S. Geological Survey
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