Stocks and sills of the Early Mesozoic Intrusive SUite are comagrnatic with the Happy Creek and King lear volcanic facies. The unconformable(?) overlying lower member of the Bliss canyon Fonnation in this area has Middle Triassic fossils several hundreds of meters higher in the section. A cessation of proximal volcanism is indicated in the upper part of the member by the disappearance of flow facies.
The thickening and increasing occurrence of the carronate beds in the upper member also supports formation. Fonnasie (see earlier discussion) or from the uppennost Bliss canyon Fonnasie than from the base of the Bliss canyon. This latter occurrence is unusual for the lower member of Bliss Canyon on the east side of the range.
Because these volcaniclastic rocks are still only a minor component of the section, this section remained within the Bliss Canyon Formation. Its limestone represents the middle member of the Bliss Canyon fonnation and appears to thin northward (especially near Jackson Creek, this may be due to lateral variation of sedimentation or tectonics). Discussion of the Happy Creek Formation in each area will include: (1) a description of field and petrographic observations of lithology, volcanic and sedimentary features and relationships, and provenance and paleoflow data (if any) by area; (2) age data, if available, both biostratigraphic and geochronological; and (3).
Map of the Jackson Mountains, NW Nevada showing geographic areas. discussed in the text on the Formation of the Happy Stream.
LITHOLOGIC SYMBOLS USED IN STRATIGRAPHIC COLUMNS
Interl:>ed with the flCMS are flow breccias with angular clasts of basaltic andesite; interstices between clasts have either a volcanic matrix or coarse gray calcite cement. Andesites and carbonates exhibit a peperitic relationship, i.e., sedimentary layers that are irreparably underlain by a flow are observed to be coarsely folded and ductilely deflated prior to lithification of the sediment. The andesite porphyry augite outcrops within the carbonate layers have similar characteristics associated with them.
Green andesite dikes, very similar in lithology to andesite flows, are found in the upper part of the section and may have been part of the flow feeder system. Agg Augite porphyry flows identical to those in the basal part of the section from the SW Pinewood area a few miles to the west (see Chapter 11) were dated by Rb/Sr isochrons at m.y. Depositional Environment A fair amount can be inferred from the nature of member A augite porphyry flows.
Such hyaloclastite particles are formed when lava flOYJS encounter a body of water, which spatters and fragments glassy particles as the surface of the flOYJ is quenched. Note the span support, rounding and sorting of the pebbles, the sandy and well-sorted clean matrix, and very low-angle imbrrication. Ihe enlargement and decrease in rounding.vards at the top of the section was probably due to slight regression or progradation, caused by relative sea-level fall or by an increased influx of volcanic flows and debris, or even by a decrease in energy.
The top of the section – the gray plagioclase ± hornblende andesites of member E – represents subaerial flCMS. To summarize: The section records subaerial to marine and flCMS in the lower part. The outcrop area includes strata in several thrust sheets south of the mouth of Jackson Creek and north of Bliss Canyon.
Almost all of the original textures and stratigraphic relationships within the northern extent of these structural parts have been obscured and suppressed by later events. The rate of metamo:rphic decreases rapidly in the south, so that in the southern extension of the nappe more original features are preserved. The Bliss Stream Fonnation in contact with these Bliss Canyon metasediments in the northern part of the area consists of metamorphosed green volcanic arenites and is structurally overlain by dark green mafic phyllite and schist.
2 MOUTH OF JACKSON CREEK
In the southern part of the area, amygdular, soft green metamorphic volcanic flows of Member A directly overlie the upper part of Bliss Canyon. The best estimate for the structural thickness of the A member in the northern part of the area is about 240 m. The area described in this section extends only from north of Jackson Creek to the south (restricted area~ 4), northward to the southern extent of the Happy Creek pluton.
Locally in the upper half of the member, the flows are oxidized and reddened at their tops in zones up to several m thick. The texture of the dacite flow is granitic (subhedral granular) in the interior, to the more common bimodal porphyritic fabric with a fine-grained intergranular matrix of feldspar arrl. Some of the flows are also lead-actic to trachytic, reflecting varying degrees of flow-alignment of the phenocrysts.
Men1ber H of the Happy Creek Formation fills and even overlaps some minor graben structures ('which cut Member G) in the outcrop of the Happy, Mary Sloan, and Jackson. Streams and was deposited on the floor of the large fault basin in the northern half of Area 3. Member H is 180 m thick in the main basin in the northern part of the area and may be 300 m thick or more in some of the smaller basins.
The base of the section (members A, C, and E) is intruded by a monzonitic pluton (Harrison Grove pluton) with a U/Pb zircon age of 187±2 m.y. In summary, the best estimate for the Happy Creek area in this area is Lower Jurassic or older (members A through F) to late Middle Jurassic (members G and H). Deosition environments Augite-phyre flows and flCM breccias are overlain by sedimentary lithologies at the base of Member A.
This is supported by the fact that the upper part of Bliss Canyon is very shallow, as is Member A to the north in area 1. The cornagmatic andesite dikes in the underlying upper part of Bliss Canyon are feeder dikes for the member. A. A braided stream system is preferable to a meandering river because of the likely very high sediment load in the system.
4 HOBO CANYON
A field view of the overall clast imbrication of pebbles in the lens indicated a source to the east. In addition, there is a horizon of light gray volcanics in the middle part of the section at about 300 m. In the northern part of the area, meinber F is directly overlain by the King leer Fonnation, and the dacites, (which are plagioclase-rich with a felt texture) are stained red in the hundred meters or so only belCM the unconformity (the weathering) and oxidation which preferentially affects the matrix).
In fact, one of these flows is interbedded with the sediments of the basal King learn Fonnation (red volcanic arenite and conglomerate, and green chert-pebble-conglomerate and chert-arenite). At the base of the valley fill sequence of element F is another fluvial channel deposit, overlain by several dacite flow facies. Age Russell (1981) reported Rb/Sr geochronological data from four flCMS from the top of Member G of Happy Creek near King lear Peak.
At the base of the sequence, some beds have xenoliths of pale gray micritic limestone. Age The underlying Bliss canyon Fonnation is youngest to earliest Carnian in the middle of the section in this area (Russell, 1981; and see earlier discussion, chapter 3). Member G is quite distinctive because of the very high degree of vesiculation of the flow.
However, the hornblende, while also abundant in volume, is unbalanced in the lower part of the piece. Formations in the thrust belt (including the diorite sill) are unconfon:nn.::lbly overlain by syn-orogenic King Lear sediments as well. The same hydrothermal alteration was quite early, as epidotic clasts are present in the conglomerates.
Elsewhere, within the thrust domain, it lies on a highly weathered and uplifted part of Happy Creek. In the northern and central parts of the area it is surrounded and folded by the western, eastern thrust system. In the southern part of the nappe area, it has been deformed by the eastern, western thrust belt.
This section covers a large area, including the Happy Creek drainage and the extreme northeastern extension of the range. The strata of the King Lear Fonnation are located within the Happy Creek drainage in accordance with paragraph G of the Happy Creek Fonnation.
1 HAPPY CREEK
JACKSON
TUFF
1 HAPPY CREEK (CONTINUED)
