HAPPY CREEK FORMATION BIOSPARITE OOSPARITE
MICRITE SILTSTONE ARGILLITE SANDSTONE
FLAT-PEBBLE CONGLOMERATE CONGLOMERATE
BRECCIA
META- ARENITE
PHYLLITE AND PELITIC TO PSAMMITIC SCHIST MARBLE AND RECRYSTALLIZED Ll MESTONE MAFIC (VOLCANIC) SCHIST
BASALTIC ANDESITE ANDESITE
DACITE
DEBRIS FLOWS AND ANDESITE Sl LICIC TUFF
RHYOLITE MONZONITE TONALITE
DIORITE GABBRO
0
CLAST OR XENOLITH OF DATED INTRUSIVE FACIES L:7 RB/SR AGE DETERMINATION0
U/PB Zl RCON AGE DETERMINATION fORMATION CONTACTMEMBER CONTACT
[NO UPPER CONTACT SEEN IN THIS AREA]
1930
1150
10 30 950
800 770 720 620 600 540 530 510
0 METERS
---
·--- --- --- --- --- ---
E
E)
A
:l~-~-~-~-~-~-~-~-~-~-~-1:-~-~::::::E)---
-F-=. _;....=_
-=·-;...;;:_-_...;::;._
.=_ .=_. =_. -=· -=. _T=-"'L---A - - -
...
·-·-·-·-·-·-·-·-···-·-
--- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- --- ---
E)
c:::7 204.7 :t 13.6 M.V.
A
[ NO LOWER CONTACT SEEN IN THIS AREA ]
FIG.I9
88
consists of dark green augite
±
plagioclase phyric basaltic andesite, with abundant augite phenocrysts conunonly up to 1 em in size in a fine- grained groundmass. The flCMS are highly amygdular, with cavities up to 1 em or m:>re in size. Interl:>edded with the flCMS are flow breccias with angular clasts of the basaltic andesite; the interstices between the clasts have either a volcanic matrix or grey coarse calcite spar cement.'Ihe matrix to the augite porphyry flCMS contains nun scale plagioclase laths in an intersertal texture; the groundmass has been recrystallized to randomly oriented microlites of plagioclase, quartz, chlorite and mafic granules, and was originally quite fine-grained to glassy. The plagioclase phenocrysts have been extensively replaced by plates of a white mica (sericite?). The augite crystals are large and equant and display a disequilibrium relationship with the groundmass, and locally have reaction rims of very-fine grained brown material (these coronae are probably composed of hornblende).
OVerlying the augite porphyry flCMS is light grey, thickly bedded limestone. This facies consists of 20 m of recrystallized oosparite at the bottom and about 60 m of biosparite at the top; there is a 10 m thick augite porphyry flow in between. 'Ihe biosparite is a crinoidal grainstone, with round to pentagonal and star-shaped crinoid columnals, well-sorted, abraded and disaggregated, and can be cross-bedded. No micrite matrix was seen in these lithologies. The skeletal grains in the biosparite are from crinoids and calcareous algae with coarse granular calcite spar cement and sutured boundaries where grains touch, and in thin-section can be sheared and broken with micritic alteration along the fractures. 'Ihin green siltstone layers are found between the thick
limestone beds. The andesites and carbonates display a peperitic relationship, i.e., the sedimentary strata inunediately underneath a flow are observed to be corwolutely folded and ductilely defonned before lithification of the sediment. 'lhey also have angular volcanic glass debris and shards and microlites (highly chloritized and devitrified, and with reaction rims) intllna.tely intermixed with the carbonate grains in a zone adjacent to the flow. This zone also exhibits pillow fo:nns of flow rock encased in ductilel y defonned carbonate strata. Some of the individual carbonate grains were defonned during this event. Also, patches of carbonate sediment were caught up by and defonned and baked within and between the flows. Dikes of the augite porphyry andesite within the carbonate strata have similar characteristics associated with them.
The caroonate strata are overlain by a 20 m interval of thinly laminated to cross-laminated arenaceous micrite, micrite and black sandstone (angular black grains in a light, calcareous natrix). Thin stromatolitic nats are present, characterized by crinkly, laterally continuous very thin laminae. 'Ihis is overlain by 100 m of silicified grey fine-grained sandstone, siltstone, and argillite, thinly and evenly bedded to cross-stratified (loo-argle). '!here are also thin beds of green volcanic lithic arenite. 'lhe siltstone (which can have herring- bone cross-lamination - i.e., bimcxlal) is entirely composed of very argular, very fine-grained and. poorly sorted volcanic rock fragments, devitrified glass and igneous microlites in a muddy natrix. 'lhe microlites include quartz, feldspar and. a nafic phase. The provenance is entirely volcanic; no terrigenous carrp:>nent is present. Bioturbation is
also present in these facies.
OVerlying this sequence is about 50 m of augite-phyric andesite flow rock, and then more arenaceous micrites and siltstones (30 m) , then 150 m of augite porphyry identical to that below. Next in the stratigraphic sequence is 80 m of green to grey laminated to thickly bedded silicified
argillite, siltstone and fine-grained sandstone. 'Ibis coarsens ~
to a thick (120 m) interval of intraclastic conglomerate, with a sandy matrix and well-sorted and rounded ellipsoidal flat pebbles up to 10 em in size. The sand matrix is clean and well sorted, and there is clast- support with imbricate fabric (the pebbles are all parallel to one another and imbricated at a very low angle or parallel to bedding) . The pebbles are green, aphanitic and quite unifo:nn in nature and are composed of very fine-grained volcanic rock. This facies coarsens ~ at the top to a poorly sorted and subangular conglomerate with the same clast type. OVerlying these conglomerates are grey plagioclase
±
hornblende andesite porphyry flov,r rocks, at least 780 m thick.Green andesite dikes very similar in lithology to the andesite flows are found in the upper part of the section, and may have been part of a feeder system to the flows. Corrnnonly they have soft-sediment defonnation and peperite textures associated with them, hence intrusion probably tCXJk place while the sediments were unlithified. '!here are also cross-cutting granitic dikes from the pluton to the south, truncating all the other litholc:qies and the high angle faults.
'!he basal augite porphyry flow sequence is placed in member A. '!he interfingering sedimentary facies (biosparites, siltstones and conglomerates) are put in member B. '!he plagioclase
±
hornblendeandesite flows above are assigned to member E.
Agg Augite porphyry flows identical to those in the basal part of the section from the southwestern Pine Forest Range several miles due west (see chapter 11) were dated by Rb/Sr isochrons at 204.7±13.6 m.y. (R.
Kistler, pers. comrn., 1988). The two sections correlate well enough that extrapolation of that age data to this area is highly reasonable.
The limestones were sampled for conodonts, with no success (D. Clark, written communication, 1986). In addition, two large cross-cutting monzonite (the Happy Creek pluton; IGd on the state map - Stewart and carlson, 1978) and diorite (the Parrot Peak pluton; Mzgr on the state map) plutons to the south are colinear on an isochron at 173.3±14.3 on a Rb/Sr plot (R. Kistler, written communication, 1988). A maximum age of between latest Triassic to Early Jurassic (Norian to Pliensbachian) and the Middle Jurassic to early late Jurassic (Aalenian to Oxfordian) is inferred for this section.
Depositional Envirornnent A fair amount can be inferred from the nature of the augite porphyry flows of member A. The presence of abundant and large arnygdules indicates extrusion subaerially or subaqueously at depths of several hundred m or less (Moore, 1965; Jones, 1969). The textures (porphyry with a very fine-grained to glassy intersertal to hyalopilitic groundrnass) are characteristic of extruded lava flows. The associated autobreccias, which have either a volcanic matrix or calcite cement, are also flow-related, and could have formed in flow-front, top- of-flow and flow-foot settings. The calcite cement - bladed or equant
mosaic and coarse and sparry - is :rrost characteristic of the freshwater to mixed phreatic zone (belOYJ the water table), which agrees with the syntaxial calcite overgrowths on the echinodenn fragments (Scoffin, 1987). The nature of these flOYJS is :rrost compatible with subaerial to shallow submarine origin as thick, blocky, somewhat viscous aa or blocky flOYJS. less viscous pahoehoe flOYJS (not present in the section) would be thinner and might have associated lava tubes and pillOYJS, while thick, colurnnar-j ointed horizons would be more characteristic of massive, flood occurrences.
The finer-grained clastic sediment in the lower occurrence of member B is highly indicative of a very shallow subtidal to intertidal marine envirornnent. The bimodal herringbone cross-lamination in the siltstones, for example, is characteristic of rapid sedimentation in a high-energy setting with frequent and regular reversal of current - i.e. , a tidal flat setting (Reineck and Singh, 1980). The stromatolitic mat (laterally linked hemispheroid) was fonned in a supratidal to intertidal envirornnent, while the presence of bioturbation would indicate normal marine waters. The siltstones and samstones, with good sorting but angular particles and compositionally bmnature (as they consist entirely of glassy to very fine-grained volcanic debris) are probably hyaloclastite debris reworked locally in the tidal to subtidal envirornnents. SUch hyaloclastite particles are fonned as lava flOYJS encounter a body of water, spalling and fragmenting off glassy particles as the surface of the flOYJ is quenched. The micritic and marly facies probably also represent intertidal to subtidal facies. The overall setting for this set of facies could very well have been in a lagoon
between a carbonate barrier bar and a basal tic andesite stratovolcano (see next paragraph).
'Ihe thick-bedded carbonate biohenns, COII1fX'Sed of crinoidal and calcareous algal skeletal particles, were deposited in a high-energy envirornnent (good sorting, cross-bedding, with abraded and broken grains and no micrite mud). An offshore barrier bar or very shallow submarine dune field to shoreface or even subaerial dune field origin would be possible. However, given the relatively coarse grain-size (making aeolian reworking difficult), the evidence that the sediments were quite wet (peperi te textures) and the overall geometry, an offshore bar is most likely. 'Ihe carbonate facies exhibits substantial lateral facies variation within the outcrop area, and thins and lenses out into the augite porphyries and the intertidal sediments locally.
In the upper strata of member B, the flat-pebble conglomerate is extremely diagnostic of a very high-energy beach or supratidal setting.
Note the clast support, rounding and sorting of the pebbles, the sandy and well-sorted clean matrix, and very low-angle imbrication. 'Ihe pebbles were derived from nearby flows and reworked and deposited in the surf zone as the flows interacted with the marine realm, and perhaps redeposited by storm events in the supratidal zone. 'Ihe coarsening and decrease in rounding up.vards at the top of the section was probably due to slight regression or progradation, caused by relative sea-level drop or by an increased influx of volcanic flows and debris, or even by a decrease in energy.
'lbe peperi te textures seen throughout the middle part of the section record the contemporaneity of deposition of the very shallow to
supratidal sed.ime.ntary facies and the basaltic to andesitic volcanism of members A (the flCMS) and E (the dikes that also cross-cut and deform the rocks of member B before lithification) . 'Ihe carbonate and clastic facies were being deposited in a very active volcanic region, along the shoreline interface of marine- and volcanically-dominated facies domains.
The top of the section - the grey plagioclase± hornblende andesites of member E - represent subaerial flCMS. No interbedded sed.iments were seen, and few diagnostic features. PillCMS or other indicators of marine extnlsion are lacking, although flo;v-related autobreccias are present.
To surrrrnarize: The section records subaerial to shallo;v marine aa flCMS in the lower part. These are overlain by a carbonate barrier bar and lagoonal facies (the barrier bar probably lay to the present-day north of the lagoon), with occasional interruption by additional flows.
The lagoon was strongly tidally dominated, with a proxinal sed.ime.nt source in the volcanic flCMS (including hyaloclastite debris - the flCMS were interacting with the marine realm) and some contribution from the carbonate facies further offshore (the marls and micrites). Higher in the section, the beach to supratidal flat-pebble conglomerates record overall regression or progradation, with the overlying andesite flCMS probably forming in a subaerial locale. Aburrlant peperitic relationships docmnent synchronicity of volcanism and very shallo;v marine sedimentation.
(2) Mouth of Jackson Creek
Of all the outcrop areas, this one has been the most extensively affected .by contact metamorphism related to several large adjacent and
inferred subjacent plutons. The outcrop area includes the strata in several thrust sheets south of the mouth of Jackson creek and north of Bliss canyon. Virtually all of the original textures and stratigraphic relationships within the northern extent of these structural slices have been obscured and overprinted by later events. The metamo:rphic grade decreases rapidly to the south so that in the southern extent of the nappes more of the original features are preserved. See fig. 20.
Stratigraphy The depositional contact with the underlying Bliss canyon Fonnation is identifiable in this area, though it may have been tectonically transposed. Green plagioclase porphycy andesite dikes of Happy creek-affinity intrude and locally metasomatize the Bliss canyon, which consists of marble, meta-quartzite and meta-conglomerate in this area. The Happy creek Fonnation in contact with these metasediments of the Bliss canyon in the northern part of the area consists of metamorphosed green volcanic arenites and is structurally overlain by dark green mafic phyllite and schist. The meta-arenites, where depositional textures are preserved, have thin bedding (1 to 2 em) with lCM-angle cross-lamination to planar lamination and appear to have been moderately well-sorted. In thin-section there was no preservation of depositional texture, with the exception of some c:orrpositional banding inferred to be relict bedding, and of quartzose clots inferred to be siliceous pebble ghosts. The mafic schists are totally recrystallized;
the only remnants of the original texture seen were large relict plagioclase grains. These latter may be preserved relatively intact, or in other cases are recrystallized arourrl the edges with some replacement