The document summarizes the stratigraphy and characteristics of the Stillwater Complex, a layered mafic intrusion in Montana. It describes the different rock series that make up the Complex from bottom to top, including a Basal Series containing ores, an Ultramafic Series with peridotite and orthopyroxenite zones exhibiting rhythmic layering, and a Banded Series containing norites and gabbros. It notes the Complex was uplifted and eroded, exposing its steeply dipping layers. The document also provides general characteristics of layered intrusions, such as their perpendicular layering that can be cryptic or rhythmic and extend over large areas.

1. Stratigraphy of Stillwater complex &
Characteristics of Layered Mafic
Intrusions
Sharik Shamsudhien
183104008

2. The Stillwater complex
• The Precambrian Stillwater Complex in southwestern Montana
outcrops in a belt about 45 km long and up to 6 km wide .
• It was uplifted, tilted, and eroded prior to the Middle Cambrian,
and sediments of this age lie unconformably on the eroded top
surface, so an unknown thickness of the upper sequences is
lost.
• Later tilting and erosion now exposes an excellent cross
section of the steeply northward-dipping remaining portions of
the intrusion.
• The exposed stratigraphy of the Stillwater is similar to that of
the Bushveld and has been subdivided into a Lower Zone, an
Ultramafic Series, and a Banded Series.

4. Stratigraphy of Stillwater Complex
•Basal Series: a 50-150 m thick complex unit consisting of a
sill/dike complex, norites, gabbros, and o-pyroxenite. The
series also contains xenoliths of country rock (pyroxene
cordierite hornfels) and is the site of massive sulfides (Cu-Ni
ores)
•Banded series: Complex group of cumulates all of which
contain cumulus plagioclase (norites, gabbros,
gabbronorites, anorthosites, troctolites). The main Pt-Pd reef
occurs ~ 400 m above the contact between the Ultramafic
series and the Banded series
•Ultramafic Series: The base of this series is marked by
first appearance of cumulus olivine (phase layering). The UM
series consists of two zones:
o Lower Peridotite Zone
o Upper Orthopyroxenite Zone

5. o Lower Peridotite Zone: >20 cycles (each ranging from 20-150 m
thick) of macrorhythmic layering with a distinctive sequence of
lithologies. The lowermost unit is an olivine plus chromite cumulate
with intercumulus opx, plag and minor cpx, overlain by harzburgite
(olivine + opx cumulate with intercumulus cpx and plag) in turn
overlain by orthopyroxenite (opx cumulate with intercumulus cpx
and plag)
o Upper Orthopyroxenite Zone: single, thick (up to 1070 m), fairly
uniform sequence of orthopyroxenite (cumulus opx with
intercumulus cpx and plag) and rare thin layers of olivine-rich
harzburgite.
•Banded series: Complex group of cumulates all
of which contain cumulus plagioclase (norites,
gabbros, gabbronorites, anorthosites, troctolites).
The main Pt-Pd reef occurs ~ 400 m above the
contact between the Ultramafic series and the
Banded series

6. Figure 12.20. Schematic illustration of a model for the development of a cyclic
unit in the Ultramafic Zone of the Stillwater Complex by influx of hot primitive
magma into cooler, more evolved magma. From Raedeke and McCallum (1984)
J. Petrol., 25, 395-420.

7. Characteristics of Layered Intrusions
• The defining characteristic of layered intrusions is the layering
of often ultramafic or mafic units.
• The typical sequence is something like
• Chromitite, dunite, pyroxenite, norite, leuconorite, anorthosite
• Mixed chromitite – anorthosite layers reflect mixing as a new
magma batch comes in
• Layers are nearly always perpendicular to the sides of the
magma chamber and may be continuous over very large
distances (kms)
• Layering can be either cryptic or rhythmic
• Cryptic layering represents layers whose composition
changes progressively in response to fractional crystallisation
occuring in the parent magma
• Rhythmic layering represents layers of alternating
composition
• All the rock types are normally fairly coarse grained, all the
grain sizes are visible to the naked eye.

8. THANK YOU