GUIDELINES ON THE DESCRIPTION AND CLASSIFICATION OF ROCKS OF THE TUEN MUN FORMATION

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1 GUIDELINES ON THE DESCRIPTION AND CLASSIFICATION OF ROCKS OF THE TUEN MUN FORMATION Rod Sewell Hong Kong Geological Survey GEO/CEDD

2 Objectives to review all known lithologies of the Tuen Mun 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 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

3 Talk Outline History of classification and nomenclature of the Tuen Mun Geoguide3 and international standard practice on the description and classification of rocks Terms used for structural and metamorphic features Key lithologies of the Tuen Mun Work in Progress An example of the Classification Scheme

4 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 Repulse Bay Tuen Mun Tuen Mun Tuen Mun Tsing Shan Tin Shui Wai Member Tuen Mun Tuen Mun

5 Tsing Shan 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.

6 Tuen Mun 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.

7 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 Repulse Bay Tuen Mun Tuen Mun Tuen Mun Tsing Shan Tin Shui Wai Member Tuen Mun Tuen Mun

Faulted against Carboniferous rocks in the north, and rest unconformably on older rocks in the south.

8 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.

9 Tin Shui Wai Member From (Darigo, 1989)

10 Tuen Mun 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 Repulse Bay Tuen Mun Tuen Mun Tuen Mun Tsing Shan Tin Shui Wai Member Definition: The Tuen Mun 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)

11 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

12 Map Updating Published Published Published

13 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

14 Borehole Review Extensive use of GeoInfo GI Reports with photos considered most reliable Site visits to examine core

15 Rock Samples and Thin Sections 400 Rock Samples 120 Thin Sections

16 Geoguide 3 and International Practice Due to the complexity of lithologiesin the TuenMun, some rock names used in Geoguide3 need to be further qualified with reference to international classification schemes Classification of pyroclasticrocks 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

17 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 0% - 25% Epiclastic Rocks Tuffaceous Breccia/Conglomerate/ Sandstone/Siltstone/Mudstone Breccia, Conglomerate, Sandstone, Siltstone, Mudstone (after Le Maitre et al., 2002 and Schmidt, 1981)

18 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)

19 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)

20 Type of Metamorphism Type of Metamorphism Terms Description All Types Meta- Dynamic Metamorphism Mylonitic Metasomatism or Hydrothermal Alteration Altered 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. 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. 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; Brodieet al., 2007 and Schmidet al., 2007)

21 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)

22 Step One Tuen Mun Andesite Lava? Y N Andesitic rocks (Andesite lava flow/dyke/sill) Y Pyroclastic Component? N Proportion of pyroclastic material? (75-100%) (25-75%) (0-25%) Sedimentary Rock Name Massive Carbonate Rocks Go to Step 3 PyroclasticVolcanic Rocks (Agglomerate, Pyroclastic Breccia, Tuff Breccia, Lapilli-stone, LapilliTuff, Ash Tuff) Tuffites (Tuffaceous Breccia/ Conglomerate/Sandstone/ Siltstone/Mudstone) Epiclastic Rocks (Sedimentary Breccia, Conglomerate, Sandstone, Siltstone, Mudstone)

23 Step Two Marble clastbearing? Y N Percentage volume of marble clasts? Rocks identified in Step One refer (<5%) (5-20%) (20-50%) (>50%) Breccia/Conglomerate/ Sandstone/Siltstone/ Mudstone with occasional marble clasts Breccia/Conglomerate/ Sandstone/Siltstone/ Mudstone with some marble clasts Breccia/Conglomerate/ Sandstone/Siltstone/ Mudstone with many marble clasts Marble Breccia/ Conglomerate

24 Step Three Y Carbonate Matrix? N % Carbonate Matrix? Rocks identified in Step One and Step Two refer (>95% CaCO 3 ) (50-95% CaCO 3 ) (10-50% CaCO 3 ) Marble (Rock Name) Impure Marble (Rock Name) Calcareous (Qualifier) Includes marble clastbearing rocks with <50% by volume marble clasts There is no known massive marble in the Tuen Mun

25 Step Four Metamorphosed? Y N Type of metamorphism? Rocks identified in Step One and Step Two refer (Unknown) (Dynamic) (Metasomatismor hydrothermally altered) Meta- Mylonitic Altered

ash TUFF dominant) Step 2: Marble clast component?

26 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?

27 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

(Yes, 20-50%) With many marble clasts, some quartzite, and occasional andesite clasts Step 3: Carbonate matrix?

28 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% CaCO 3 ) 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?

29 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% CaCO 3 because marble clastsupported) 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)

30 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% CaCO 3 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

(Yes, but <5%) With occasional marble clasts and some quartzite clasts Step 3: Carbonate matrix?

31 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% CaCO 3 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

32 Breccia grading to Siltstone Siltstone Sandstone Breccia

33 Breccia grading to Sandstone Tuffaceous sandstone (with occasional marble clasts) (with some marble clasts) Tuffaceous Breccia(with many Marble clasts) Tuffaceous Sandstone

34 THANK YOU

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