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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 THANK YOU