Download guidelines on the description and classification of rocks

GUIDELINES ON THE
DESCRIPTION AND
CLASSIFICATION OF ROCKS OF
THE TUEN MUN FORMATION
Rod Sewell
Hong Kong Geological Survey
GEO/CEDD
Objectives
• to review all known lithologies of the Tuen Mun Formation
based on literature review, field mapping, drill core logging,
and examination of rocks samples and thin sections
• to propose a classification scheme that enables all rocks
of the Tuen Mun Formation to be described accurately and
consistently according to published guidelines on rock
description in Geoguide 3 and in alignment with
international practice
• to introduce a lexicon which facilitates drill core logging
and surface mapping in the Tuen Mun Valley to aid
practitioners in the description and characterisation of
rocks of the Tuen Mun Formation
Talk Outline
• History of classification and nomenclature of the Tuen Mun
Formation
• Geoguide 3 and international standard practice on the
description and classification of rocks
• Terms used for structural and metamorphic features
• Key lithologies of the Tuen Mun Formation
Work in Progress
An example of the Classification Scheme
Decades of Debate
Williams et al.
(1945), Davis
(1952)
Ruxton (1960)
Allen &
Stephens
(1971)
Langford et
al. (1989)
Frost (1992)
Sewell et al.
(2000)
Repulse Bay
Volcanics
Plover Cove
Formation
Repulse Bay
Formation
Tuen Mun
Formation
Tuen Mun
Formation
Tuen Mun
Formation
Tsing Shan
Formation
Tin Shui Wai
Member
Tuen Mun
Formation
Tsing Shan Formation
Langford et al. (1989)
Definition: Fine-grained, quartzitic
sandstone, metasiltstone and
phyllite, with subordinate tuff,
tuffite and conglomerate
Location: Confined to the western
foothills of the Tuen Mun Valley.
Contact with granite is an
intrusive shear zone, inclined
steeply to the west. Eastern
boundary is faulted along at least
part of its length.
Description: Cross-bedded and cross-laminated sandstone is interbedded with
foliated siltstone, tuff, tuffite and conglomerate. Tuff and tuffite include abundant
quartz , feldspar and lithic lapilli, whereas “sandstone and siltstone carry a scatter of
quartz lapilli”. Conglomerate comprises subangular to subrounded boulders, cobbles
and gravel, within a cemented sand matrix. Clasts include sandstone, vein quartz
and andesite. Up to 2m thick beds, fine upwards through coarse to fine sandstone.
Tuen Mun Formation
Langford et al. (1989)
Definition: Metavolcanic rocks
underlying much of the lower
Tuen Mun Valley between Castle
Peak Bay in the south, and Ha
Tsuen in the north.
Location: Type area is located on
the low hills which overlook
northwestern Tuen Mun where
the best exposures of fresh rock
are found. Eastern outcrops are
concealed by superficial deposits.
Description: Consists mostly of dark grey or greenish grey massive andesite lavas
which may include subangular to subrounded andesite lava clasts, or rarely
quartzite. Locally interbedded with lapilli-bearing ash crystal tuff and tuffite. Much of
the formation has been dynamically metamorphosed, with foliation most
conspicuous in the vicinity of the western granite contact. Thought to be in fault
contact with the underlying Tsing Shan Formation.
Classification and Nomenclature
Williams et al.
(1945), Davis
(1952)
Ruxton (1960)
Allen &
Stephens
(1971)
Langford et
al. (1989)
Frost (1992)
Sewell et al.
(2000)
Repulse Bay
Volcanics
Plover Cove
Formation
Repulse Bay
Formation
Tuen Mun
Formation
Tuen Mun
Formation
Tuen Mun
Formation
Tsing Shan
Formation
Tin Shui Wai
Member
Tuen Mun
Formation
Tin Shui Wai Member
Frost (1992)
Definition: Marble clast-bearing
tuff breccia and conglomerate
interbedded with fine-ash tuff and
siltstone (Darigo, 1989)
Location: Confined to the eastern
margin of Tuen Mun Valley.
Faulted against Carboniferous
rocks in the north, and rest
unconformably on older rocks in
the south. Gradational contact
with andesite and tuff in south.
Description: Comprises “interbedded sequence of volcaniclastic rocks, including tuffbreccia, fine to coarse andesitic tuff and tuffite, tuffaceous siltstone, lapilli tuff, and
andesitic conglomerate”.
Tin Shui Wai Member
From (Darigo, 1989)
Tuen Mun Formation
Sewell et al. (2000)
Williams et al.
(1945), Davis
(1952)
Ruxton (1960)
Allen &
Stephens
(1971)
Langford et
al. (1989)
Frost (1992)
Sewell et al.
(2000)
Repulse Bay
Volcanics
Plover Cove
Formation
Repulse Bay
Formation
Tuen Mun
Formation
Tuen Mun
Formation
Tuen Mun
Formation
Tsing Shan
Formation
Tin Shui Wai
Member
Definition: The Tuen Mun Formation is defined as a fault-bounded
volcaniclastic and epiclastic sequence of fine-grained, cross-bedded,
well-graded quartzitic metasandstone, metasiltstone and phyllite, with
subordinate tuff, tuffaceous metasiltstone, metasandstone,
breccia-conglomerate and andesite lava (Sewell et al. 2000)
Location: Tuen Mun Valley, forming a 2 km-wide north-trending outcrop
between Tuen Mun in the south and Tin Shui Wai in the north. Rock exposures
predominate in the south and west, whereas the east and north are mostly
covered by superficial deposits
Age: Early to Middle Jurassic based on poorly constrained Ar-Ar age (180 Ma)
Map Updating
• Since 2004, HKGS has been carrying out updating
of 1:20,000-scal geological maps on a digital
platform
• For the past five years, the HKGS has been
reviewing data for Map Sheets 2, 5, and 6
This is not a full field resurvey
• Desk study involving review of all rock samples,
thin sections and boreholes
• Field visits to carry out spot checks of critical
sites
Map Updating
Published
Published
Published
Methodology for TMV Review
• 10,700 boreholes reviewed
• 400 rock specimens considered
• 120 thin sections examined
In addition
• Careful examination of major boundaries
• Field visits to carry out spot checks of critical sites
• Review of structure and metamorphism
Borehole Review
• Extensive use of GeoInfo
• GI Reports with photos
considered most reliable
• Site visits to examine core
Rock Samples and Thin Sections
400 Rock Samples
120 Thin Sections
Geoguide 3 and International Practice
• Due to the complexity of lithologies in the Tuen Mun Formation,
some rock names used in Geoguide 3 need to be further qualified
with reference to international classification schemes
• Classification of pyroclastic rocks has followed the IUGS
(Le Maitre et al., 2002; Schmidt, 1981)
• We have followed recommendations of the BGS and IUGS for
refining the classification and description of marble clast-bearing
rocks
• Terms used for structural and metamorphic features have followed
IUGS
Proportion of Pyroclastic Material
Percentage Volume
of Pyroclastic
Material
Rock Type
Rock Name
0%
Non-Pyroclastic
Volcanic Rocks
Andesite, Dacite, Rhyolite lava
75% - 100%
Pyroclastic
Volcanic Rocks
Agglomerate, Pyroclastic Breccia,
Tuff Breccia, Lapilli-stone,
Lapilli Tuff, Ash Tuff
25% - 75%
Tuffites
Tuffaceous Breccia/Conglomerate/
Sandstone/Siltstone/Mudstone
0% - 25%
Epiclastic Rocks
Breccia, Conglomerate, Sandstone,
Siltstone, Mudstone
(after Le Maitre et al., 2002 and Schmidt, 1981)
Proportion of Marble Clasts
Percentage Volume of Marble
Clasts (in Gravel or larger size)
Rock Name
>50%
Marble Breccia/Conglomerate
20% - 50%
Breccia/Conglomerate/Sandstone/
Siltstone/Mudstone with many marble clasts
5% - 20%
Breccia/Conglomerate/Sandstone/
Siltstone/Mudstone with some marble clasts
<5%
Breccia/Conglomerate/Sandstone/
Siltstone/Mudstone with occasional marble clasts
(after GCO, 1988 and Hallsworth & Knox, 1999)
Proportion of Carbonate Minerals
Percentage Volume of Carbonate
Minerals (in Sand or finer size)
Rock Name
95% - 100%
Pure Marble
50% - 95%
Impure Marble
10% - 50%
Calcareous
Sandstone/Siltstone/Mudstone
(after Hallsworth & Knox, 1999 and Rosen et al., 2007)
Type of Metamorphism
Type of
Metamorphism
All Types
Terms
Description
Meta-
Prefix used with an intrusive, volcanic or sedimentary rock name
(e.g. metagranite, metatuff and metasiltstone) to indicate that the
rock has been partially metamorphosed, without specifying the
type of metamorphism.
The prefix should only be applied when the original rock can be
identified.
Dynamic
Mylonitic
Metamorphism
Qualifier used with an intrusive, volcanic or sedimentary rock name
(e.g. mylonitic granite/tuff/siltstone) to indicate a very fine-grained,
crystalline, cohesive, foliated metamorphic rock with streaked or
banded texture produced by shearing, fracturing of original grains
(i.e. tectonic reduction of grain size) during faulting.
Metasomatism
or
Hydrothermal
Alteration
Qualifier used with an intrusive, volcanic or sedimentary rock name
(e.g. altered granite/tuff/siltstone) to indicate the rock has
undergone
metasomatism
or
hydrothermal
alteration
(e.g. silicification, chloritisation, epidotisation, propylitisation and
sericitisation).
Altered
(after GCO, 1988; Brodie et al., 2007 and Schmid et al., 2007)
Key Lithologies
• Andesite (lava flows, dykes, sills)
• Pyroclastic rocks (coarse ash tuff, tuff breccia)
• Tuffaceous sedimentary rocks
• Sedimentary rocks
(Many of these lithologies have been subjected
to metamorphism and hydrothermal alteration)
Step One
Tuen Mun Formation
Andesite Lava?
Y
N
Andesitic rocks
(Andesite lava flow/dyke/sill)
Y
Proportion of
pyroclastic
material?
(75-100%)
PyroclasticVolcanic Rocks
(Agglomerate, Pyroclastic
Breccia, Tuff Breccia,
Lapilli-stone, Lapilli Tuff,
Ash Tuff)
(25-75%)
Tuffites
(Tuffaceous Breccia/
Conglomerate/Sandstone/
Siltstone/Mudstone)
Pyroclastic
Component?
N
Sedimentary
Rock Name
(0-25%)
Massive
Carbonate
Rocks Go to
Step 3
Epiclastic Rocks
(Sedimentary Breccia,
Conglomerate, Sandstone,
Siltstone, Mudstone)
Step Two
Marble clastbearing?
Y
N
Rocks identified
in “Step One”
refer
Percentage
volume of
marble clasts?
(<5%)
Breccia/Conglomerate/
Sandstone/Siltstone/
Mudstone with
occasional marble clasts
(5-20%)
Breccia/Conglomerate/
Sandstone/Siltstone/
Mudstone with
some marble clasts
(20-50%)
Breccia/Conglomerate/
Sandstone/Siltstone/
Mudstone with
many marble clasts
(>50%)
Marble Breccia/
Conglomerate
Step Three
Carbonate
Matrix?
Y
% Carbonate
Matrix?
(>95% CaCO3)
Marble
(Rock Name)
(50-95% CaCO3)
Impure Marble
(Rock Name)
There is no known massive marble in
the Tuen Mun Formation
N
Rocks identified in
“Step One” and
“Step Two” refer
(10-50% CaCO3)
Calcareous
(Qualifier)
Includes
marble clastbearing rocks
with <50% by
volume marble
clasts
Step Four
Metamorphosed?
Y
Type of
metamorphism?
(Unknown)
Meta-
(Dynamic)
Mylonitic
N
Rocks identified in
“Step One” and
“Step Two” refer
(Metasomatism or
hydrothermally altered)
Altered
Tuff Breccia
Step 1: Pyroclastic component? (Yes, >75%)
Rock Name: Tuff Breccia (angular lapilli and blocks of coarse ash TUFF dominant)
Step 2: Marble clast component? (No)
Step 3: Carbonate matrix? (No)
Step 4: Metamorphism? (No)
Strong, dark grey mottled light grey, slightly decomposed TUFF BRECCIA
Tuffaceous Marble Breccia
Step 1: Pyroclastic component? (Yes, 25-75%)
Rock Name: Tuffaceous Breccia (angular cobbles dominant)
Step 2: Marble clast component? (Yes, >50%)
Marble Breccia
Step 3: Carbonate matrix? (No)
Step 4: Metamorphism? (Yes, both dynamic and contact (mylonitic and altered)
Strong to very strong, dark grey to dark greenish grey, dappled white, slightly
decomposed, altered, mylonitic, tuffaceous MARBLE BRECCIA
Tuffaceous Breccia
Step 1: Pyroclastic component? (Yes, 25-75%)
Rock Name: Tuffaceous Breccia (angular cobbles dominant)
Step 2: Marble clast component? (Yes, 20-50%)
With many marble clasts, some quartzite, and occasional andesite clasts
Step 3: Carbonate matrix? (Yes, 10-50% CaCO3)
Step 4: Metamorphism? (Yes, some sign of silicification) (ie altered)
Very strong, dark greenish grey, slightly decomposed, altered, calcareous, tuffaceous
BRECCIA with many marble clasts
Marble Breccia
Step 1: Pyroclastic component? (No)
Rock Name: Breccia (angular cobbles dominant)
Step 2: Marble clast component? (Yes, >50%) (ie. Marble breccia)
Step 3: Carbonate matrix? (Yes, but <10% CaCO3 because marble clast supported)
Step 4: Metamorphism? (Yes, some sign of dynamic) (ie mylonitic)
Strong, light grey, streaked grey and dappled white, slightly decomposed, mylonitic,
MARBLE BRECCIA (with skeletal residuum, i.e. dissolution of deformed marble clasts)
Calcareous Breccia
Step 1: Pyroclastic component? (No)
Rock Name: Breccia (angular cobbles dominant)
Step 2: Marble clast component? (Yes, 20-50%)
With many marble clasts and quartzite clasts
Step 3: Carbonate matrix? (Yes, but 10-50% CaCO3 minerals) (ie. calcareous)
Step 4: Metamorphism? (Yes, some sign of dynamic and chloritization) (ie mylonitic, altered)
Strong to very strong, dark greenish grey, streaked dark grey, mottled white, altered,
mylonitic, calcareous BRECCIA with many marble and quartzite clasts
Calcareous Metasandstone
Step 1: Pyroclastic component? (No)
Rock Name: SANDSTONE
Step 2: Marble clast component? (Yes, but <5%)
With occasional marble clasts and some quartzite clasts
Step 3: Carbonate matrix? (Yes, but 10-50% CaCO3 minerals) (i.e. calcareous)
Step 4: Metamorphism? (Yes, but uncertain) (META)
Strong to very strong, dark greenish grey, locally streaked white, calcareous
METASANDSTONE with occasional marble and quartzite clasts
Breccia grading to Siltstone
Siltstone
Sandstone
Breccia
Breccia grading to Sandstone
Tuffaceous sandstone
(with occasional marble clasts)
(with some marble clasts)
Tuffaceous Breccia (with many Marble clasts)
Tuffaceous Sandstone
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