guidelines on the description and classification of rocks...

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.








Tuen MunFormation

Tuen MunFormation

Tuen MunFormation


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”.

Tin Tin ShuiShui WaiWai MemberMember

From (Darigo, 1989)








Tuen MunFormation

Tuen MunFormation

Tuen MunFormation


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

Published

Map UpdatingMap Updating

Published

Published

• 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

Rock Samples and Thin SectionsRock Samples and Thin Sections120 Thin Sections400 Rock Samples

• 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%)


Mudstone withsome marble clasts


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 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 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 Siltstonegrading to Siltstone

Siltstone

SandstoneSandstone

Breccia

BrecciaBreccia grading to Sandstonegrading to Sandstone

(with some marble clasts)

(with occasional marble clasts)Tuffaceous sandstone

Tuffaceous Sandstone

Tuffaceous Breccia (with many Marble clasts)

THANKYOU

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