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GUIDELINES ON THE
DESCRIPTION AND
CLASSIFICATION OF ROCKS OF
THE TUEN MUN FORMATION
Hong Kong Geological Survey
GEO/CEDD
Rod Sewell
ObjectivesObjectives
• to review all known lithologies of the Tuen Mun Formationbased 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 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 OutlineTalk Outline
• History of classification and nomenclature of the Tuen Mun
Formation
• Geoguide 3 and international standard practice on the
description and classification of rocks
• Key lithologies of the Tuen Mun Formation
• Terms used for structural and metamorphic features
Work in Progress
An example of the Classification Scheme
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 MunFormation
Tuen MunFormation
Tuen MunFormation
Tsing Shan Formation
Tin Shui WaiMember
Decades of DebateDecades of Debate
Tuen MunTuen Mun
Formation
TsingTsing Shan FormationShan FormationLangford et al. (1989)Langford et al. (1989)
Definition: Fine-grained, quartziticsandstone, metasiltstone and phyllite, with subordinate tuff, tuffite and conglomerate
Location: Confined to the western foothills of the Tuen Mun Valley. 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.
TuenTuen MunMun FormationFormationLangford et al. (1989)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 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.
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 MunFormation
Tuen MunFormation
Tuen MunFormation
Tsing Shan Formation
Tin Shui WaiMember
Classification and NomenclatureClassification and Nomenclature
Tuen MunTuen Mun
Formation
Tin Tin ShuiShui WaiWai MemberMemberFrost (1992)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. 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 tuff-breccia, fine to coarse andesitic tuff and tuffite, tuffaceous siltstone, lapilli tuff, and andesitic conglomerate”.
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 MunFormation
Tuen MunFormation
Tuen MunFormation
Tsing Shan Formation
Tin Shui WaiMember
TuenTuen MunMun FormationFormationSewell et al. (2000)Sewell et al. (2000)
Definition: The Tuen Mun Formation is defined as a fault-boundedDefinition: The Tuen Mun Formation is defined as a fault-boundedvolcaniclastic and epiclastic sequence of fine-grained, cross-bedded,well-graded quartzitic metasandstone, metasiltstone and phyllite, withsubordinate 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 exposurespredominate in the south and west, whereas the east and north are mostlycovered by superficial deposits
Age: Early to Middle Jurassic based on poorly constrained Ar-Ar age (180 Ma)
• 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
Map UpdatingMap Updating
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
• 10,700 boreholes reviewed
• 400 rock specimens considered
• 120 thin sections examined
Methodology for TMV ReviewMethodology for TMV Review
In addition
• Careful examination of major boundaries
• Field visits to carry out spot checks of critical sites
• Review of structure and metamorphism
• Extensive use of GeoInfo
• GI Reports with photos considered most reliable
• Site visits to examine core
Borehole ReviewBorehole Review
• 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)
GeoguideGeoguide 3 and International Practice3 and International Practice
• 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 Proportion of PyroclasticPyroclastic MaterialMaterial
Percentage Volume
of Pyroclastic
Material
Rock Type Rock Name
0%Non-Pyroclastic
Volcanic RocksAndesite, Dacite, Rhyolite lava
75% - 100%Pyroclastic
Volcanic Rocks
Agglomerate, Pyroclastic Breccia,
Tuff Breccia, Lapilli-stone,
Lapilli Tuff, Ash Tuff
25% - 75% TuffitesTuffaceous Breccia/Conglomerate/
Sandstone/Siltstone/Mudstone
0% - 25% Epiclastic RocksBreccia, Conglomerate, Sandstone,
Siltstone, Mudstone
(after Le Maitre et al., 2002 and Schmidt, 1981)
Proportion of Marble Proportion of Marble ClastsClasts
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 MineralsProportion 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 MetamorphismType of Metamorphism
Type of
MetamorphismTerms Description
All Types 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.identified.
Dynamic
MetamorphismMylonitic
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
Altered
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).
(after GCO, 1988; Brodie et al., 2007 and Schmid et al., 2007)
Key Key LithologiesLithologies
• 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 OneStep One
Tuen Mun Formation
Andesitic rocks(Andesite lava flow/dyke/sill)
PyroclasticComponent?
Andesite Lava?
Y N
Y N
Tuffites(Tuffaceous Breccia/
Conglomerate/Sandstone/Siltstone/Mudstone)
PyroclasticVolcanic Rocks(Agglomerate, Pyroclastic
Breccia, Tuff Breccia, Lapilli-stone, Lapilli Tuff,
Ash Tuff)
Epiclastic Rocks(Sedimentary Breccia,
Conglomerate, Sandstone, Siltstone, Mudstone)
Proportion of pyroclasticmaterial?
Sedimentary Rock Name
Y N
(75-100%) (0-25%)(25-75%)
Massive
Carbonate
Rocks Go to
Step 3
Step TwoStep Two
Rocks identified in “Step One”
refer
Percentage volume of
marble clasts?
Marble clast-bearing?
Y N
(5-20%) (20-50%) (>50%)
Breccia/Conglomerate/Sandstone/Siltstone/
Mudstone withoccasional marble clasts
(<5%)
Breccia/Conglomerate/Sandstone/Siltstone/
Mudstone withsome marble clasts
Breccia/Conglomerate/Sandstone/Siltstone/
Mudstone withmany marble clasts
Marble Breccia/Conglomerate
Step ThreeStep Three
Rocks identified in “Step One” and “Step Two” refer
% Carbonate Matrix?
CarbonateMatrix?
Y N
Impure Marble (Rock Name)
Marble(Rock Name)
Calcareous (Qualifier)
(>95% CaCO3) (50-95% CaCO3) (10-50% CaCO3)
Includes
marble clast-
bearing rocks
with <50% by
volume marble
clasts
There is no known massive marble in
the Tuen Mun Formation
Step FourStep Four
Rocks identified in “Step One” and “Step Two” refer
Type of metamorphism?
Metamorphosed?
Y N
MyloniticMeta- Altered
(Unknown) (Dynamic)(Metasomatism or
hydrothermally altered)
Tuff Tuff BrecciaBreccia
Strong, dark grey mottled light grey, slightly decomposed TUFF BRECCIA
Step 1: Pyroclastic component? (Yes, >75%)
Step 2: Marble clast component? (No)
Rock Name: Tuff Breccia (angular lapilli and blocks of coarse ash TUFF dominant)
Step 3: Carbonate matrix? (No)
Step 4: Metamorphism? (No)
TuffaceousTuffaceous Marble Marble BrecciaBreccia
Strong to very strong, dark grey to dark greenish grey, dappled white, slightly
decomposed, altered, mylonitic, tuffaceous MARBLE BRECCIA
Step 1: Pyroclastic component? (Yes, 25-75%)
Step 2: Marble clast component? (Yes, >50%)
Rock Name: Tuffaceous Breccia (angular cobbles dominant)
Marble Breccia
Step 3: Carbonate matrix? (No)
Step 4: Metamorphism? (Yes, both dynamic and contact (mylonitic and altered)
decomposed, altered, mylonitic, tuffaceous MARBLE BRECCIA
TuffaceousTuffaceous BrecciaBreccia
Very strong, dark greenish grey, slightly decomposed, altered, calcareous, tuffaceous
Step 1: Pyroclastic component? (Yes, 25-75%)
Step 2: Marble clast component? (Yes, 20-50%)
Rock Name: Tuffaceous Breccia (angular cobbles dominant)
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 Marble BrecciaBreccia
Strong, light grey, streaked grey and dappled white, slightly decomposed, mylonitic,
MARBLE BRECCIA (with skeletal residuum, i.e. dissolution of deformed marble clasts)
Step 1: Pyroclastic component? (No)
Step 2: Marble clast component? (Yes, >50%) (ie. Marble breccia)
Rock Name: Breccia (angular cobbles dominant)
Step 3: Carbonate matrix? (Yes, but <10% CaCO3 because marble clast supported)
Step 4: Metamorphism? (Yes, some sign of dynamic) (ie mylonitic)
MARBLE BRECCIA (with skeletal residuum, i.e. dissolution of deformed marble clasts)
Calcareous Calcareous BrecciaBreccia
Strong to very strong, dark greenish grey, streaked dark grey, mottled white, altered,
mylonitic, calcareous BRECCIA with many marble and quartzite clasts
Step 1: Pyroclastic component? (No)
Step 2: Marble clast component? (Yes, 20-50%)
Rock Name: Breccia (angular cobbles dominant)
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)
mylonitic, calcareous BRECCIA with many marble and quartzite clasts
Calcareous Calcareous MetasandstoneMetasandstone
Strong to very strong, dark greenish grey, locally streaked white, calcareous
Step 1: Pyroclastic component? (No)
Step 2: Marble clast component? (Yes, but <5%)
Rock Name: SANDSTONE
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
BrecciaBreccia grading to Sandstonegrading to Sandstone
(with some marble clasts)
(with occasional marble clasts)Tuffaceous sandstone
Tuffaceous Sandstone
Tuffaceous Breccia (with many Marble clasts)