1. San Juan Volcanic Field - Mapping Study
Nested Caldera Study: San Juan and Silverton Caldera Complex Analysis
June 2015 – Volcanism Focus
Johanna Vaughan

2. The focus of this study encompasses the San Juan Volcanic Field (SJVF) analyzing the
volcanology, hydrothermal geology, and ore deposit geology of the San Juan Mountains. These
mountains are composed of eroded mid-Cenozoic intermediate-felsic deposits associated with the SJVF.
This area represents a structurally complex and intensely altered region of several remnant volcanic
eruptive centers with unique geographical distribution. The SJVF is associated with Tertiary volcanism of
the Cenozoic Era (spanning the course of approximately ~5 Mya.), unique from other types of worldwide
arc volcanism as large scale subduction occurs far west of the SJVF. The mechanisms and modes of
deposition in association with volcanic activity encompass the deposition of several volcanic and
subvolcanic intrusive rocks, involving a range of unique deposits within a concentrated area. This study
is focused on the nested caldera complex of the Silverton caldera forming within the older San Juan
Caldera near Silverton, Colorado. Filed mapping has allowed for the production of a geologic map
(spanning 2,630 meters in length) displaying the various deposited, as well as the location of major fault
clusters. The geologic map is rich in orientation measurements, incorporated into quantitative stereonet
analysis in support of interpretation. Qualitative data, such as field observances from outcrop stops over
the course of this study support the cross sectional figure showing the projection of various units to
depth, giving insight into spatial distribution.
This study focuses on the unique ignimbrite flare up during the Tertiary, producing high volumes
of intermediate and felsic volcanic rocks (Figure 1 A and B.). The total volume of erupted material has
been approximated at 40,000 km^3, however erosional effects over geologic time have shaped the
landscape through mechanisms such as mass wasting. The tectonic control of the deep mantle source is
still a topic of controversy, but the deposition of unique and closely spaced units allows for
interpretation of recent explosively related volcanic activity. The Silverton caldera is interpreted as
experiencing a major collapse episode with eruption occurring during periods of caldera filling. This
sequence has resulted in “trap-door” regional subsidence to the southeast section of the caldera. The
caldera spans a vast region, and the study area is focused around the western portion of the complex
where significant faulting and various deposits occur. The larger scale “trap- door” subsidence is not
encompassed within the study area, and as such mechanics of faulting as described as “piecemeal” due
to interpretation based on observations.
This study area is important as there is the occurrence of intensive mineralization and
hydrothermal alteration (Figure 2.) occurring as a separate and later phase of deformation post-
volcanism. Mineralization throughout the area involves the recurrent intrusion and extrusion of magma
along areas of weakness such as ring fracture zones and accommodational faulting zones. The high
magnitude of normally faulted planes with down drop towards the southeast (Figure 3.) is associated
with the geometry of the larger scale Silverton caldera complex. Subsidence and eruption of this
complex has produced a heavily faulted area related to the western rim of the Silverton caldera. The
caldera structure has provided fractured and faulted areas that have guided the emplacement of
igneous intrusions following the deposition of volcanic rock units (Figure 1 A and B.). This progression
has produced the rich concentrations of ore deposits (Silver, Lead, Zinc, Copper, and Gold) located
within fractures of the heavily fractured zones located in association with the main rim faults (Figure 3.).
Here we examine the intricate pattern of fracture related structures, the wide distribution of volcanic
deposits, and the post volcanism alteration associated with the nested caldera complex surrounding
Silverton, Colorado.

3. The identification of deposit units, and significant structures within the study areas has been
performed through qualitative and quantitative analysis encompassing field mapping done through
group effort. There are cumulatively six identified units deposited within the study area (Table 1.) which
have been analyzed to interpret the characteristics of the Silverton caldera volcanic activity of the
Tertiary. From youngest to oldest there is the occurrence of; Quaternary Deposits, Porphyritic
Rhyodacite Intrusions, Welded Vitric Lapilli Tuff, Weakly Welded Propylized Tuff, Heavily Altered
Andesitic Tuff, and Andesitic Lava Flow. The characteristics of each of these deposits has been
incorporated into Table 1., and the distribution of units can be seen clearly through the geologic map
construction (Figure 1. A, B.) All structural measurements giving insight into the tectonic history
associated with volcanism have been incorporated into stereonents (Figure 4. A, B, C, D, E, F, G), and
give insight into bedding and fault orientation. The alteration and mineralization observed throughout
the study area (Figure 2.) gives support for ore deposit occurrences.
The interpretation of this study places geologic sequence in the following order; large scale trap-
door subsidence and collapse of regional nested caldera complex associated with the deposit of several
volcanic units (Welded Vitric Lapilli Tuff, Softly Welded Propylized Tuff, Pervasively Altered Andesitic
Tuff, and Andesitic Lava Flows). Sub volcanic intrusive rocks (Porphyritic Rhyodacite Intrusions) are seen
to encompass two separate stages within the study area, with the older intrusion occurring
superimposed above the younger (which displays abundant and larger phenocryst content). There is the
unique occurrence of sequential faulting units displaying an overall oblique sense of movement
incorporating both a normal component with down drop to the east, and a strike slip motion in
association with accommodational fault mechanics. The most intensely faulted region of the map
(Figure 3. – where there is the occurrence of two fault groups within close proximity forming an oblique
angle between dips) is interpreted as being associated with rim faulting mechanics. The blue set of faults
(Figure 1 B. and Figure 3.) are interpreted as accommodation in association with regional collapse
mechanics. The faulting throughout the region has helped to facilitate the intrusive subvolcanic deposits
(Porphyritic Rhyodacite Intrusions) which occur after the explosive eruption cycle associated with the
nested caldera collapse. These intrusions are associated with uplift mechanics following subsidence as
compensation of isostatic balance within the area. The intrusions and associated volatile components
have helped to form the rich mineralization ore deposit regions throughout the region, now exposed
and once widely mined for ore content. Hydrothermal alteration occurs later in geologic time, effectively
altering the volcanic and subvolcanic intrusions mainly through fault and fracture pathways. The most
intense alteration of units is interpreted to occur in association with Tuff deposits, with the oldest
(Green heavily altered Andesitic Tuff – Figure 3.) unit experiencing pervasive alteration in association
with weak welding and high porosity of character of the matrix.
Stereonet interpretation fits well with the larger scale tectonic picture (Figure 3.), as the major
fault clusters (Figure 4 C.) are seen to trend generally north/northeast (Figure 4 D.). This interpretation
fits well with the geographic position of the study area, on the western edge of the Silverton caldera rim.
There are three main fault clusters (Figure 4 C.) roughly identifiable through stereonet analysis, and all
of the faults measured correlate well with north-northeast trending strikes. (Figure 4 D.) Although
faulting is very prevalent throughout the study area (Figure 4 B.), there is no evidence for significant
folding, as the poles to bedding planes show a circular trend concentrated towards the center of the
stereonet projection. This occurrence supports the lack of folding within the area, and works well with

4. the horizontal deposition of layers, remaining largely intact (following the counters of topography)
despite immense erosional mass wasting. The poles to joints (Figure 4 F.) display a wide array of
orientations and support the observance of intense fracturing and stress related deformation along
major fault zones. Slickenline measurements are sparse within the area (Figure 4 G.), but support the
regional oblique faulting trends observed. Strike slip motion is supported through Slickenline stereonet
analysis, and the occurrence of normally faulted areas in the western region of the map (Figure 1 A.)
together support the oblique sense of displacement the study area has experienced.
Interpretation of units support the regional “trap-door” subsidence model, as intrusive outcrops
are exposed within the northwest section of the map (Figure 1 A. and Figure 3.) where erosional effects
have exposed the intrusive units that were emplaced higher when the southeast section of the Silverton
caldera collapsed. However, the study area shows support of piecemeal subsidence mechanics in
conjunction with the repetitive sequences of normal faulting observed through the three faulting units
(Red, Yellow, and Blue Fault lines – Figure 1. A). As a result of faulting and down drop towards the
southeast, there is the observed repetition of units across cross sectional view (Figure 3.), with Andesitic
Lava Flow units representing the thickest sections (~15 meters). There is an observed increase in the
thickness of units as we move further east, towards the central Silverton caldera.
The conclusion of this study shows the unit distribution and structural mechanics associated
with a typical nested caldera complex, with the occurrence of deposition of differing units in association
with subsidence and uplift during Tertiary volcanism of the SJVF. The volcanic collapse mechanism is
piecemeal in nature, characterized by an oblique sense of offset created through rim faulting
manifestation. Snow cover during the course of field work prevented the first person observation of
potentially helpful outcrops. The occurrence of quaternary deposits and displacement of large boulders
related to glaciation of the area are the manifestation of erosional effect in present geologic time. The
volcanic character of the study area is concluded as explosive and significant (accounting for ~40,000
km^2 of deposited materials) in deposition volume. Quantitative analysis through stereonet use
correlates well with qualitative in field data and quantitative orientation measurements. The
mechanisms fueling volcanic activity at depth are uncertain, as well as the depth to which certain units
are orientated. Further study of the area could give insight into the nature of explosive eruptive volcanic
complexes, and the larger scale SJVF occurrence.
*Figures one through three consist of handmade maps and figures and have not been included in
this online submission

5. Description of Study Area Units
Quaternary
Deposits
Description
There are various types of quaternary deposits located throughout the
study area including:
Fluvial Sediments Deposits are well sorted with normal grading and rounded cklasts (sand-
boulder sized). These deposits are associated with transport and
deposition through water related mechanics
Colluvium This deposit type is observed as poorly sorted in nature, consisting of
unconsolidated sub-angular to angular material mostly composed of
andesitic lava flow fragments.
Talus Slope
Deposits
Deposits are large and widespread in distribution in association with
structural history and erosional mechanics. Deposits are angular in
character.
Man-Made Human developed infrastructure and associated mining activity scattered
throughout the study area.
Landslide Material There is the occurrence of large clasts suspended within finer grained
matrix in association with high energy eruption mechanics.
Porphyritic
RhyoDacite
Intrusion
Description
Fresh Rich grey/greenish grey
Weathered Light pink to grey weathered surface
Appearance There are two observed closely situated intrusions of differing ages.
Magmatic Event A Densely distributed phenocrysts displaying euhedral grains and litics (1
mm to 4 mm). There is the prominent occurrence of plagioclase and k-
feldspar (~0.35-2 cm)
Magmatic Event B There is the occurrence of abundant, and very large (0.5-2.5 cm)
plagioclase phenocrysts content (~20-30% of the unit composition). There
is the occurrence of alteration from plagioclase to epidote appearing
green in alteration zones.
Quartz content is vitreous (~0.5 cm) and unfragmented in nature. Pyrite
grains are seen, but are small in size (~0.5-1 mm).

6. Welded Vitric
Lapilli Tuff
Description
Fresh Light grey/Greenish grey
Weathered Rusty brown/grey
Appearance There is the occurrence of vitreous quart, fragmental in nature, and
abundant throughout the matrix.
Unlike softer tuffs of the study area, this deposit is moderately to well
welded and majorly clast dominated.
There is the occurrence of grey bombs (~30 cm in diameter with polymict
nature.
There is a minimal percentage of mafic component contained within
phenoclast content.
Softly Welded
Propylized Tuff
Description
Fresh Light green
Weathered Dark grey
Appearance This unit is weakly welded making way for pervasive hydrothermal
alteration through the ash dominated matrix.
Flow banding is observed in this unit (8-10 cm in thickness). This
interpretation observes the indication of deposition as pyroclastic flow.
Calcite content is observable as phenocrysts ranging in size from 0.5 to 1
cm in diameter. This is associated with hydrothermal activity in relation to
Propylitic and Phyllic (quartz - sericite - pyrite) alteration.
Phenocrysts are fragmental in habitat indicative of explosive eruption
style.
There is the observance of heavy faulting and jointing with the association
of an important fault zone present associated with heavy Phyllic
alteration.

7. Pervasively
Altered Andesitic
Tuff
Description
Fresh Light green/yellow-tan
Weathered Dark grey - brown
Appearance This united is in association with prominent pyrite content (~0.25 mm - 1
cm) seen to weather slightly through local alteration zones within the
unit. There seems to be the presence of chlorite and epidote content
within the matrix having a light green-blue color.
Alteration within this unit is seen to occur abundantly in association with
fractures and joints associated with insitu units.
There is the occurrence of sericite, produced from the weathering of
plagioclase.
Andesitic Lava
Flow
Description
Fresh Dark and blue/grey in color
Weathered Dark brown
Appearance Massive and resistant to erosional effects often associated with
topographic highs
There is the presence of dense phenocryst content. Phenocryst content is
observed as quartz, k-feldspar, and plagioclase rich with euhedral habit
Locally non-pervasive alteration zones occur in association with
weaknesses such as columnar jointing and fracturing as a result of
regional faulting mechanics and deformation
Upper 5 m of unit outcrop displays phyric texture
Devitrification in association with quartz and chert is seen through
lithophyse content

8. Fracture zones within the rock are consistent with alteration zones (Figure
3.). The distribution of heavy alteration ranges from 20 cm - 2 m
Mafic component of lava flow is rich in pyroxene mineral content;
appearing dark and vitreous with a tabular habit
Significant occurrence of quartz content
Table 1. Volcanic and associated subvolcanic intrusive unit descriptions. Each of the units have been observed throughout the
nested caldera region of the San Juan and Silverton caldera complexes.
Figure 1.
A. Finalized field map interpretation of the study area with structural and geology related
significances indicated through color and symbolism use. The key for symbolism and color
used has been incorporated into the group study submission.
B. Field map as drawn and formed through field work surrounding the nested caldera study
region. This figure represents the earliest form of the finalized geologic map (Figure 1 A.),
and has also been incorporated into the group study submission.
Figure 2.
Hydrothermal alteration zoning displayed through the use of vellum paper to clearly see the
zones of alteration in association with the geologic map. This figure has been incorporated into
the group study submission.
Figure 3.
Geologic cross section from A-A’ spanning the length of the study area with key incorporation for
interpretation of color and symbolism. The present day erosional surface constitutes the
topographic profile and no vertical or horizontal exaggeration has been used. This figure has
been incorporated into the group study submission.
Figure 4.
All of the following stereonet figures have been derived from the correlation of structural
measurements taken in the field. These stereonents all display the North trending azimuth
(000,360) at the top portion of the net. Through quantitative analysis these measurements are
used to support the geologic distribution observed over the course of this study. The number of
data points used for each stereonet analysis is displayed:

9. (Stereonet data appears on the following pages)
A.
Figure 4 A. Poles plotted to bedding measurements taken over the course of this filed study from several location spanning the
map region.
B.
Figure 4 B. Fault bedding planes plotted from various outcrop locations from throughout the study area.

10. C.
Figure 4 C. Poles plotted to fault bedding planes from Figure 4 B. The distribution of poles is seen to form loose clusters. There
are three distinct clusters.
D.
Figure 4 D. Fault azimuth orientations plotted through the use of stereonet techniques. There is a clear trend of major faults
within the N-E quadrant. There is an encompassing trend seen with the direction of faulting relative to the mapping area.

11. Figure 4 E. Bedding planes from joint measurements measured throughout the course of this field study.
Figure 4 F. Poles to joints plotted through the use of stereonet analysis.

12. Figure 4 G. Slickenline poles plotted through the use of stereonet analysis.