RESEARCH· ·1970·

Linwstone and conglO'Ine?·ate.-A limestone formation
and its basal conglomerate rest unconfonnably on
both the N emo and Estes Systems. The basal conglomerate
is exceedingly varied in composition and
thickness frmn one place to another. In the southwest
part of section 33, it rests on quartzite and ironformation
and is composed mainly of highly 1nagnetic
iron-formation pebbles. The iron-formation between
the older quartzite and the conglomerate is a thin bed
of banded 1nagnetite-quartz rock which n1ay represent
fine-grained debris from the underlying N emo Systen1
iron-formation. On the south side of Estes Creek, in
section 3, only a. thin quartz-pebble conglmnerate
marks the base of the limestone; in section 34 1nost
exposures show no conglomerate, but along one short
stretch the basal limestone bed contains quartzite
boulders as much as 3 feet in diameter. Slate and slaty
conglomerate occur below the limestone in section 4,
but do not continue on strike to the southeast.
The limestone itself is fairly uniforn1 in appearance
in 1nost places. It is covered by a buff to brown
weathering rind that can easily be recognized from a
distance. I-Iowever, the fresh rock is gray, fine grained,
siliceous, and very hard. Acid tests indicate that some
of it is dolomitic. Bedding planes are indistinct. A few
beds of limestone-pebble conglomerate occur in this
unit but not in the Estes Creek area. In two places, one
in section 4 and one in section 34, the unit is badly
sheared and converted to white 1narble. The thickness
of the formation is about 200 feet. The overlying rocks,
shown only in the syncline in sees. 34 and 35, are fissile,
gray, sericitic schist.
STRUCTURE
A workable tentative interpretation of the structure
of the Estes Creek area is shown on figure 2. Broadly
speaking, the general area is anticlinal. The anticlinal
core reveals a structural cmnplex involving folded
Ncmo and Estes rocks, which long predates the anticline
itself. To bo more precise, the complex rests on the
west flank of a. basement gneiss anticline, which lies
under Paleozoic rocks east of the Estes Creek area.
The complex thus exposes the oldest sedimentary formations
deposited on the gneiss, and these older formations,
Runner's ( 1934) N emo and Estes Systmns,
show two periods of deformation that predate the
deposition o:f the limestone of the Estes Creek area.
The deformation after the limestone deposition reoriented
most older structures and further complicated
the general structure to the extent that much of the
older record has been obliterated.
The oldest fold structures involve only the Nen1o
System. Refolding has largely obscured the nattwe of
BAYLEY
B99
these old folds, but remnants of folds throughout the
N emo district suggest that the axial trend of the fold
structures was northwesterly, roughly parallel to the
present west edge of the underlying gneiss. The 1nain
iron-formation exposure, in the SE:i4 sec. 33 (fig. 2),
appears to be the keel of a syncline which was downfolded
into the quartzite well below the erosion surface
upon which conglomerates of the Estes System
lie. The other iron-formation exposures in sections 33
and 4, also appear to be parts of a syncline, but no
satisfactory reconstruction seen1s possible. A northwest-trending
septmn of vertically dipping bedded
quartzite lies between the two n1ain exposures of ironformation
and is assumed to be an anticline. The conclusion
that the quartzite is older than the iron-formation,
implicit in the above interpretation, is based on
a district-wide analysis. Reference to figure 2 will show
that the iron-formation beds are vertical and that the
plunge of minor plications is about 75° N,V, For a
syncline, this is a reversed plunge but is easily explained
by slight rotation during a subsequent folding.
Erosion of the N emo Systen1 after folding resulted
in a rough hill-and-basin foreland topography which
gave rise to the overlying Estes System. ~iost of that
systen1 is probably fanglomerate and continental, but
the finer grained facies, the quartzite, quartz-1nica
schist, chlorite schist, and n1eta-arkose, were deposited
in a marine or lacustrine environment.
The second fold systen1 involves both the Nemo and
Estes Systems. The extent and 1nagnitude of this folding
is not at all clear. The great conglmnerate reentrant
in sections 33 and 34 would semn to be a syncline
from this period ; the flanking areas of N emo
quartzite and iron-for1nation are probably anticlines,
but there is considerable uncertainty about this interpretation.
However, the fact that the overlying linlestone
rests unconformably on the N emo rocks and the
Estes conglomerate indicates that it blanketed a
diverse terrane, though, as indicated by the small
amount of basal conglmnerate beneath the lilnestone,
not a very rough one. The most obvious effect of the
post-Estes folding is the defonnation of the post-N en1o
erosion surface. This surface, where it crosses n1ain
iron deposits, must have warped clown southeastward,
thereby roti1ting the iron deposit en n1asse. The present
attitude of the old surface is impossible to cleter1nine
exactly because it is difficult to see any bedding in the
coarse basal part of the conglomerate, but all beds
away frmn the base are vertical or clip 75° to 80° N.,
and the elongated boulders in the conglomerate plunge
about 80° N.; thus, there is a strong indication that the
erosion surface is overturned and eli ps north. This
conclusion about the attitude of the post-Nemo erosion