F,O, S I' ' 35- 21T '10-'314Abb'I1Tab'IER "GE"'0 01

Silurian Reefal Buildups: Spiti-Kinnaur, Himachal Himalaya, India

Silurische Rifle aus dem Himachal Himalaya, Indien

O m N. B h a r g a v a und Udai K. Bass i , Chand iga rh

KEYWORDS: PALEOECOLOGY - FACIES - REEF ENVIRONMENT - CORALS - ALGAE - HIMACHAL HIMALAYA - INDIA -. SILURIAN

SUMMARY

The Silurian sequence in Spiti and Kinnaur was studied at Takche, Gechang, Muth-Shian, Leo and Man- chap. At all these localities the sequence commences with an intertidal - near-shore argillo-arenaceous succession. The overlying part at Takche comprises arenaceous dolomite, calcareous sandstone and dolomite. It sporadically contains rugose and tabulate corals deposited in a shallow undathem near the shore. The succession at Gechang is more calcareous and in some parts rich in corals; stromatoporoids occur locally. Here bioclastic wacke/packstones and framestones formed a buildup which was possibly formed in a lagoon. The Muth-Shian area is characterized by bioclastic mudstones, bioclastic wacke/packstones and framestones. Reef builders are corals, stromatoporoids and solenoporoids. These sed- iments seem to be a lateral equivalent of the coastal arenaceous sequence exposed at Baldar and may represent a fringe reef.

The diversity of the fauna and microfacies increases in the Leo area, where mud/wackestones, bioclastic pack/grainstones, bioclasfic floatstones, bindstones and framestones occur. The main reef-builders are tabulate corals (Halysites predominates), laminar to domal stromatoporoids, algae and bryozoa which grew on a protected reef fiat near the back of a reef.

The Manchap reef is built by tabulate corals, bryozoa, stromatoporoids and algae. Vermiporella inhabited open spaces of Halysites chains, forming a typical community. Tabulate corals grew on stromatoporoids, rugose corals-

are encrusted by stromatoporoids. Wacke/pack/grainstones, float/rudstones and frame/-

baffle/bindstones occur. The facies and the biocommuni- ties indicate deposition on a reef flat and flank. Upward growth of the reef into a turbulent zone, its later destruction and regrowth possibly account for intimate interlayering and intertonguing of these diverse facies types. All the buildups were formed in protected areas with moderate wave energy.

The cements are siliceous, ferruginous micritic and spafitic and rarely show ferruginous subequant calcite. The reefal sequences are overlain by arenaceous rocks en- closing a few tabulates and Psilophyton. The reef growth was depth-controlled, caused by transgressive and regres- sive cycles. Reef growth started slowly in muddy water and reached its maximum in clear water.

1 INTRODUCTION

Several rugose and tabulate corals and stromatoporoids (HAYDEN 1904, REED 1912) are known from the Silurian limestone of the Spiti area. This paper describes Silurian reefal buildups in the Spiti and Kinnaur areas of the Himachal Himalaya (Fig. 1).

Silurian reefs are known from various parts of the world. Those in North America (LOWENSTAM 1950) and Gotland (MANTEN 1971) have been extensively investigated. RIDING (1981) and BURCHETTE (1981) provide excellent reviews of European Silurian and Devonian reefs. In Pakistan (STAUFFER 1968) the

Addresses: Dr. O. N. Bhargava, 529, Sector 18 B, Chandigarh 160018; Dr. U. K. Bassi, 98-100, Sector 17 C,

Chandigarh 160017, India

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Fig. 1. Geological map of the Spiti-Kinnaur area.

Nowshera Reef, though somewhat different in faunal and lithological components, occupies a stratigraphic position similar to the one described here.

2 GEOLOGY

The Palaeozoic to Mesozoic sequences in the Spiti and Kinnaur area occur in two isolated synclinofia having comparable lithostratigraphic and faunal contents. We, therefore, consider them to represent part of the same basin, now separated due to erosion along a structural high.

The rocks roughly assignable to the Silurian were formally designated as the Takche Formation (SRIKANTIA 1981) in the Spiti Valley and as the Manchap Formation in the Kinnaur (BASSI et al. 1983). The stratigraphic position of the Takche/Manchap Forma- tion is illustrated in Fig. 2. The Ordovician-Silurian boundary in the Spiti Valley was fixed by REED (1912) with the help of Propora himalaica and P. (Lyella) amer/cana (EDWARD & HAIME). The Silurian-Devonian boundary is not well defined in this area.

Throughout the Spiti Valley, the Takche Formation is extensively developed. Towards the northwest (in the Lahaul Valley) it becomes arenaceous and is almost

Fig. 2. Relative stratigraphic position of the Takche/Manchap Formation. (1) Batal Formation (Eocambrian), carbonaceous phyllite, slate, quartzite. (2-3) Kunzam La Formation . (2) Dcbsa Khad Member (Eocambrian to lowermost Cambrian), grey shale-slam, sandstone, locally carbonaceous. (3) Parahio Member (Lower to ?Middle Cambrian), grey-green shale, slate, sandstone, dolomite. (4) Thango Formation (Ordovician to earliest Silurian), red siltstone, fine-grained sandstone, conglomerate at ,the base and shale intercalations in the upper part. (5) Takehe/Manchap Formation with reefal buildups. (6) Muth Formation (Early Devonian to Early Upper Devonian), white mottled sandstone.

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Fig. 3. Variations in the lithology, microfacics and biofacies of the Takchc/Manchap Formation.

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indistinguishable from the underlying and overlying arenaceous Thango and Muth Formations. In the Spiti Valley, the Takche Formation is mainly exposed along the snow-clad ridges soaring to more than 5500m above sea level. This sequence was studied only in the valleys at Takche, Gechang, Muth-Shian, in the Pin Valley and Leo Dogri in the Lipak Valley. In Kinnaur, the Manchap Formation was examined at Manchap meadow. All these areas are located between 3600 and 4500m altitude.

3 DESCRIPTION OF THE LOCALITIES

The Thango Formation, which is mainly arenaceous, displays intercalations of shales in its upper part in all the areas. With an increasing percentage of calcareous cement it passes at the top into the Takche and Manchap Formations. In the Manchap area Pentamerus appears in the upper part of the Thango Formation.

3.1 TAKCHE In this area the Takche Formation (Fig. 3A) comprises

a thick sequence of grey shales (50%) calcareous sand- stone/siltstone (36%) changing to arenaceous dolomites (13%) which are reddish brown on weathered surfaces. Lenses of dolomite (1%) occur locally within the sequence. The calcareous sandstones and arenaceous dolomites consist of well-sorted subangular to subrounded quartz grains (40-50%) cemented by ferruginous micrite (50-60%) which occasionally replaces quartz grains along fissures. The shales are brownish and well-developed in the middle part of the formation. The m i c r o f a c i e s consistsof A. bioclastic quartz wackestones and calcareous quartz wackestones and B. bioclastic wackestones with crinoids, brachiopods and corals in decreasing order of abundance. F o s s i l c o n t e n t atTakche: The shales intercalated in the upper middle part of the sequence are rich in trace fossils (Arthrophycus, P alaeophyc us, P lanolites).

Silicified rugose corals (Slreptelasmatids, Chonophyll- urn) are sporadically present in the upper part of the sequence. Rare and small colonies (3cm x 4cm) of silicified Favosites and Halysites are preserved primarily in the upper part of the sequence. North of Takche a few nodular colonies of Plasmopora are developed in silica-free dolomite found in the middle to upper part of the sequence.

Fragments of crinoids are mainly present in the basal part of the sequence. Brachiopods (Orthis, Atrypa) are preserved in the middle and upper parts of the sequence. Trilobites are usually broken. Flask-shaped problematica occur as 1.5 to 2cm long casts in the sandstone. They were described by REED (1912) as Apidium indicum REED and compared with ostracods.

3.2 GECHANG The percentage of shales (37%) and sandstones (3%)

is smaller in this section than in the Takche section. The

sequence starts with a grey crinoidal limestone (Fig. 3B). It is overlain by alternating shales, calcareous sandstones and arenaceous dolomites and dolomites. In the upper part there is a conspicuous 7m thick nodular light-grey dolomite.The topmost part of the section consists of thin- bedded limestone/dolomite, 50m thick. Cross-bedding, wa- vy bedding and cuspate ripple marks are present. The m i c r o f a c i e s consists of A. Bioclastic wacke/packstones which are rather common. The sediment is bioturbated; fossils have been recrytallized. Crinoids, brachiopods, trilobites, corals and rare echinids occur as bioclasts. B. Thamnopora-framestones with a micritic matrix. F o s s i l c o n t e n t atGechang: Trace fossils (Arenicolites, Planolites) are preserved in the sandstone-shale sequence. Tryplasma and Streptelasmatids were found in the basal part and Chonophyllum in the upper part of the section. Tabulate corals (Thamnopora, Favosites spitiensis REED, Halysites wallichi REED) are present in dolomites and arenaceous dolomites. ?Protaraea (PI.10/1) colonies are confined to the dolomites. The chambers of Halysites are filled with spherulitic chert and open spaces with equant ferruginous calcitic cement.

Fragments of crinoids are distributed throughout the sequence but most prominently in the basal part. Brachiopods (Orthis, Atrypa, rynchonellids), trilobites (on- ly fragments) and echinoids were found.

3.3 MUTH-SHIAN In this locality, the Takche Formation is pre-

dominantly calcareous (70%). The shale (23%) and sand- stone (7%) intercalations are mainly restricted to the basal and top parts. Nodular bedding in the middle and syndepositional slumping in the upper part are con- spicuous in this section (Fig. 3C). Three m i c r o f a c i e s types occur: A. bioturbated mudstones, B. bioclastic wacke/packstones with bioclasts of crinoids, corals, brachiopods and trilobites. The rock is extensively recrystalliTed and dolomitized. C. framestones with stromatoporoids. F o s s i l c o n t e n t atMuth-Shian: Trace fossils (Planolites) are found in the shale-sandstone intercalations. From Muth-Shian the following fossils are known: Stromatopora concentrica GOLDFUSS, Cyathophyllum (recorded by REED, 1912, but not observed by us), Halysites catenularia kanurensis I/El=D, H. wallachi REED, Favosites spitiensis REED, Propora, and Parachaetetes, brachiopods (Orthis, Chonetes, Atrypa), Tentaculites (rare), trilobites (Calymene, Cheirurus), gastropods (Pleurotomaria), cephalopods, Orthoceras, rare) and crinoidal debris.

3.4 LEO The sequence here can be broadly subdivided into four

units (Fig. 3D): U n i t I comprises calcareous sandstones to arenaceous

limestones with rare fossils. U n i t II consists of argillaceous to marly dolomites with

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Fig. 4. Sketch of polished handspecimen showing reef composition in the upper part of the Leo Reef. B: bryozoa, C: rugose corals, Ch: Chonophyllum, Cr: crinoids, F.D: fossil debris, H: Halysites, R: rynchonellids, S: stromatoporoids, St: Streptelasmatid.

spherical colonies of tabulate corals occasionally growing on top of stromatoporoids. U n i t III consists of thick-bedded dolomite with abundant corals and common stromatoporoids (Fig. 4) and displays micro-unconformities. U n i t IV is made up of calcareous sandstone rich in brachiopods and Psilophyton in the upper part. Five m i c r o f a c i e s types can be distinguished: A. Mud/wackestones (5%): True mudstone is rare, found only between colonies of tabulate corals. The bioclasts in wackestones occur mostly as debris, only a few as well- preserved fossils. The fossil content is the same as in the

pack/grainstones. B. Pack/grainstones (60%) with three subtypes: bioclastic packstones, bioclastic lithoclastic packstones and layered bioclastic packstones. Homogeneous micrite to ferruginuous and argillaceous micrite forms the matrix (30 to 80% of the rock). Along with debris (15%) the bioclasts include crinoids (5%) mostly in the basal part of the section, brachiopod shells (20%), echinoid spines (1%), bryozoans (15%), ostracods (1%), corals (38%) and recrystallized stromatoporoids. Bryozoa occur as frag- ments and as encrustations on corals. Lithoclasts are micritic, rarely sparitic. C. Bioclastic floatstones (10%) contain stromatoporoids, brachiopod shells, and corals as clasts. D. Bindstones (12%): Main binders are clathrodictyonids, Parachaetetes, and bryozoa. (PI. 10/4,5). E. Framestones (13%) consist of stromatoporoids (PI. 10/2) and thriving Halysites (P1. 10/2) which add a considerable volume to the framework. The matrix is

micritic. Burrows in stromatoporoids are filled in places with ferruginuous micrite. Clear sparry calcite, rarely micrite, (sometimes with many generations) fill fossils and cracks in the rocks. F o s s i l c o n t e n t atLeo: Corals and bryozoans are common throughout the sequence. Stromatoporoids and brachiopods appear in the middle part of the sequence, becoming common in the upper part. Algae occur. Most of the fossils have been recrystallized.

From the I2o locality the following fossils are known: Rugose corals: Chonophyllum (PI. 10/3, most common) and streptelasmatids (strongly recrystallized and ferrugin- ized; common in the upper part of the section). Tabulate corals: Halysites catenularia kanaurensis REED (PI. 10/2) and H. wallichi REED are most common. In certain layers up to ten colonies (each with a section-area of 250cm 2) are developed within one square meter. They are associated with stromatoporoids. Favosites spitiensis REED occurs as spherical colonies (P1. 13/4). Its chambers are filled with micrite and clear sparite. Colonies of Favosites are much smaller than those of Halysites. Protaraea kanaurensis (REED) (P1. 12/5) occurs in the basal part of this section. Stromatoporoids are highly recrystallized and have therfore been classified after KERSHAW & RIDING (1978) as laminar, low domal (P1. 10/2), extended domal and rarely dendroid (P1. 10/4,5). Bryozoa: HaUopora (P1. 10/4, highly branched), Fenestella, ?Fistulipora and Coenites (both fragmented), and some unidenfifiable bifoliate forms (PI. 10/4). Algae:

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Parachaetetes (PI. 10/6; nodular colonies and as a binder). Brachiopods (Orthis, Chonetes, rynchonellids). Crinoids: rare but rather big. Trilobites usually broken, rare. Cephalopods: Orthoceratids (rare). Gastropods: rare. Echinids: rare spines and fragments. Ostracods: extremely r a r e .

3.5 MANCHAP The Manchap Formation at its type locality can be

divided into four litho-units (Fig. 3E): U n i t I commences with grey ferruginous cross-bedded sandstone (with casts of Orthis and Pentamerus) and is followed by U n i t II with grey argillaceous marly limestones and sub- ordinate calcareous shales characterized by crinoids and basket-shaped colonies of Favosites (diameter 20cm). U n i t ~I consists of bluish to dark grey dolomites with large corals and brachiopods, while U n i t IV is a calcareous quartzite with Halysites, brachiopod shells and Psilophyton. Seven m i c r o f a c i e s -types characterize the type locality of the Manchap Formation: A. Mudstones (2%) are laminated, syndepositionaUy deformed, evenly laminated and bioturbated. B. Three types of bioclastic wackestones (15%): 1. Bioclastic quartz wackestone restricted to the basal part of the section yields clasts of echinid spines, tabulate corals, gastropods and problematic Umbellinaceans (PI. 10/9). 2. Sponge spicule wackestone is rare and confined to Unit III. 3.Vermiporella-echinid wackestone. C. Bioclastic pack/grainstones (55%): 1. Vermiporella- packstone (PI. 10/7). 2. Bryozoan-trilobite packstone (PI. 11/8). 3. Coral-bryozoan packstone (PI.11/4). 4. Bio- turbated algal pack/wackestone. 5. Layered algal-bryozoan packstone. 6. Vermiporella grainstone (PI. 11/5). 7. Bra- chiopod-lamellibranch pack/grainstone with geopetal fabrics. D. Float/rudstones (10%): 1. Coral-algal float/rudstone contains clasts derived from a reef enclosing in situ colonies of Vermiporella and Favosites (PI. 12/1). 2. LameUibranch-brachiopod float/packstone (PI. 10/8, 11/6). 3. Rugose coral- stromatoporoid floatstone. 4. Codiacean- coral floatstone (P1. 11/7). 5. Bryozoan-trilobite floatstone. 6. Coral-brachiopod floatstone (Pls. 12/2, 13/3,6). 7. Brachiopod-ostracod-bryozoan float/packstone (PI. 13/1). E. Framestones: 1. Halysites-Vermiporella framestone. 2. Plasmoporella-Vermiporella framestone (P1. 11/10). 3. Stromatoporoid-rugose coral framestone. F. Bindstones: 1. Bryozoan bindstone (P1. 11/I). 2. Girvanella bindstone (Pls. 10/12; 11/3). 3. Vermiporella bindstone (P1. 12/4). G. Bafflestones with Hallopore (P1. 13/2). F o s s i 1 s from Manchap: Trace fossils (Arthrophycus, Chondrites, Planolites, and Rusophycus (BHARGAVA et al. 1984) occur in the basal part of the Manchap Formation. Rugose corals: Radiastraea (PI. 12/6) nodular colonies in the uppermost part of the Manchap Formation. Streptelasmatids (PI. 13/3,5,6), Tryplasma, and Ptychophyllum (PI. 13/6) are found in the

middle and upper parts of the sequence. Tabulate corals: Favosites spitiensis RI~.~D is most dominant in the basal part and forms irregular, inverted basket-shaped colonies. The chambers are f'dled with clear silica which has been partially replaced by micrite (P1. 11/2,9). It is also found overgrowing stromatoporoids and even encrusted by them (P. 11/2). Halysites catenularia kanaurensis (PI. 13/10) is common in the middle and upper parts of the sequence. The open space in some of the colonies is inhabited by Vermiporella. Plasmoporella occurs as large nodular and lenficular colonies often surrounded and interwoven with Vermiporella (P1. 11/10). Plasmoporella colonies are enclosed within spherical stromatoporoids. Plasmopora is interlayered with a recrystallized ?stromatoporoid. ?Heliolites occurs as nodular colonies. Stromatoporoids are laminar and low domal, their internal structure has been largely obliterated by recrystallization. Stromatoporoids are sparsely distributed throughout the sequence, though a bit more pronounced in Unit III. Low domal and spheroidal stromatoporoids enclose Plasmoporella. Stromatoporoids occur overgrowing Favosites spitiensis REED (PI. 11/2), as a substrate for Favosites or as encrustation on rugose corals. The diameter of the stromatoporoids are not larger than 20crm Algae: Girvanella occurs as layered filaments (PI. 10/12) in bindstones, in a spherical form (PI. 11/3) and as an encrustation.Vermiporella (PIs. 10/11; 13/7, 9) grew (PI. 12/4) in the open spaces of Halysites and also surrounding or enclosed within Plasmoporella (PI. 11/lO).Vermiporella is common around coral colonies. ?Solenoporoids (PI. 13/8) occur as nodules in bindstones. Probable ?Codiacean algae (PI. 11/7) form nodular colonies with rugose corals. Bryozoans (Hallopora most common) form ramose colonies or encrust ostracods (FI. 13/1). Coenites (PI. 12/3) occurs in the bindstones and floatstones.Fenestella and fistuliporids occur in floatstones.

Rare nodosariids (Pl. 10/7) and unidentifiable foraminifera occur in the basal part of the sequence. Ostracods are rare: some tests are encrusted by bryozoans (PI. 13/1). Crinoids occur mostly in the basal and upper parts of the section (PI. 13/1). Brachiopods: Orthis, Atrypa, Pentamerus. Gastropods: Euomphalus (PI. 10/10), Pleurotomaria and a few unidentified forms (PI. 10/7). LameUibranchs are rather common as fragments. Ce- phalopods: Straight orthoceratids (0.5-5cm in size) are found sporadically (P1. 11/7). Fragments of spines and plates of echinids. Fragments of trilobites of different sizes are found in packstone and floatstone facies (P1. 11/8). Sporadic sponge spicules are present in the wackestone facies. Tentaculites are rare. Problematica: rare flask- shaped (PI. 10/9) Umbellinaceans, uniserial chambered forms possibly Kamaena, star-shaped possible bryozoa (Pls. 10/13; 12/7) and unidentifiable ?dasyclads are found in thin-sections.

4 GENERAL COMMENTS ON THE SEDIMENT

Bioturbation is known from all the localities described

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Area Bedding

Takche Cross- bedded

Gechang Cross, thick to massive bedding

Muth- Cross, Shian nodular ,

syndepo- sltional slumping

Leo Thick to massive

Manchap Massive

Main microfacies

Arenaceous dolomite to d o l o m i t i c sand-. stones

Bioclastic arena- ceous waeke/pack- stones. Thamnopora framestones; I n t e r - fingered and inter- layered

Bioclastic pack/ wackestones, frame- stones

Mud/wackestones, bio- e l a s t i c pack/ Rrain- s tones ~ b i o c l a s t i c floatstones; i n t e r - f ingered

Bioclastic wacke- s tones , pack /gra in - s tones , f l o a t / r u d - s tones , frame/bind/ bafflestones

Main builders

Sparse ly d i s t r i b u t e d F a v o s i t e s , H a l y s i t e s , and rugose c o r a l s in sandy dolomi tes .

Rugose c o r a l s , Thamnopora, F a v o s i t e s , Haly- s i t e s

Rugose c o r a l s , Syringopora, F a v o s i t e s , Haly- s i t e s , Parachaete tes , rare s tromatoporoids

Rugose c o r a l s (common), H a l y s i t e s (abundant) , F a v o s i t e s , Protaraea , Hal lopora, e n c r u s t i n g laminar to low domal s tromatoporoids coupon in the upper part

Rugose c o r a l s - inc lud ing ?Radiastraea, F a v o s i t e s (dominant in the basal and upper p a r t s ) , Plamoporel la , H a l y s i t e s , rare s tromatoporoids a l l through, s p h e r i c a l and e n t r u s t i n g ( : i rvanel la , Pararhaete tes , ?Snlenopora, Vermiporel ln , Hal lopors , sponge sp icu les

Palaeoenvlronment

Shallow undathem, near the shore.

Lagoon, occasionally ravaged by storms to form paekatones. Quartz i n d i c a t e s near- shore environment

Low energy protected env i ron - ment, poss ib ly a lagoon.

Reef f l a t p a r t i a l l y protected in proximity of a back-ree f e s p e c i a l l y in the bas~l and middle part s of the s e c t i o n s . Local ly ? fores lope c o n d i t i o n s

P a r t i a l l y to l a r g e l y s h e l - tered organic ree f and f o r e - s lope areas , l o c a l l y pro- t ec ted n iches for the growth of Vermiporel la provided by H a l y s i t e s and Plasmopore l la . Phottc i n t e r t i d a l zone near wave base in a t r o p i c a l sea. F l u c t u a t i o n s in sea l e v e l .

Table 1. Paleoenvironment of the reefal buildups in the Takehe/Manchap Formation in the Spiti-Kinaur area, Himachal Himalaya, India

here, but is most common in the Manchap area. Two types can be recognized: Type A with a circular arrangement of the matrix particles, also forming the filling material.These "circles" are syndepositional and simultaneously f'dled by the unconsolidated material of the host rocks. Type B filled by ferruginous or sparitic material. These burrows occur in slIomatoporoids too. Fenestral fabrics are almost absent except in Gechang and Leo areas where they are rare.Cements: Clear siliceous and chalcedonic cement form the first generation of cements in many Favosites and Halisites-framestones (PI. 2/2,9). Replacement by micritic and sparitic cements is partially common. Clear sparite is found in open spaces, veins, fractures and chambers of fossils. Ferruginous micritic and sparitic cements fill the chambers of corals. The sparitic cements seems to be the first generation of the cement. Ferruginous subequant calcite is a rare type, noticed in the open spaces of Halysites colonies at Gechang and may represent a fresh water phreatic environment with active water circulation (LONGMAN 1980). Fibrous and micritic cement in alternating layers was only found in one floatstone occurring in the uppermost part of Unit III of Leo. Sparry calcite is a primary precipitate and also due to recrystallization of micrite with which it has a hazy and irregular boundary. Stylolites are mostly of low amplitu- des, irregular (stylomottled) and at places anastomising (stylobrecciated). They are thickest at crests and troughs, in rare cases uneven thickening patterns are noticed. Inso- luble ferruginous matter is observed along the stylolitic seams. Rarely black bituminous material is present e.g., in the Plasmoporella-Vermiporella framestones. Stylolites are present along contacts of different grains and fossils

(P1. 11/10), but rare in uniform textured rocks.

5 PALEOENVIRONMENTS (Tab. 1)

At all the localities, the basal and uppermost parts of the Takche and Manchap Formations comprise arenaceous sequences. Only the middle part of these formations displays a limestone/dolomite succession with local sandstone and shale intercalations.

The arenaceous sequences composed of cross-bedded calcareous, well-sorted siltstone/sandstone with interbed- dad shales are characterized by Arenicolites, Arthro- phycus, Chondrites, Palaeophycus, Pianolites, Rusophy- cus, Skolithos, indicating neritic to littoral environments (BHARGAVA et al. 1984). The arenaceons dolomitic limestone roughly corresponds to the energy indices II/3 and IV 4 (PLUMI.EY et al. 1962). The predominance of quartz suggests a deposition in a tidal flat near the shore line.

The paleoenvironmental interpretation of the reef of the Manchap/Takche Formation is based on the following considerations: 1. Microfacies: Framestones represent an organic reef/lagoon, wackestones quiet protected water, pack- stones the foreslope and also possibly turbulent lagoonal flanks, rud/floatstones the fore.slope or reef flank environments (WILSON 1975; FLOGEL 1982). 2. Halysites indicates high energy but protected environ- ments and Favosites, especially basket-shaped growth forms, indicates a basin with feeble currents (WILSON 1975).

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3. Bryozoa thrived best in a low energy/protected environment. 4. Spherical and encrusting forms of Girvanella indicate reef facies (WRAY 1977; TSIEN & DRICOT 1977). 5. Parachaetetes and Solenopora are typical of back-reef areas though they are also recorded in reefs (WRAY 1977). 6. Algae thrive best in the partly protected intertidal photic zone of tropical seas (WRAY 1977). 7. Dasyclads can build wave-resistant structures (RIDING & WATI'S 1981). Vermiporella, which is locally abundant, may be compared here to the dasycladaceans. 8. Laminar and low domal stromatopomids point to a high energy environment (KERSHAW & RIDING 1978). 9. The conspicuous presence of quartz in carbonates indicates its derivation from coastal sand-dunes near a shoreline (WILSON 1975).

The reefoid carbonate sequence is terminated by ferruginous cross-bedded and ripple-marked sandstones with brachiopods, a few Favosites and Halysites together with the plant fossil Psilophyton. This sequence shows a shallowing up throughout the basin. The overlying Muth Formation represents a beach sand deposit.

6 AGE OF THE REEFAL BUILDUPS

The lower boundary of the reef is well within the Silurian as indicated by the occurrence of Pentamerus in the upper part of the underlying Thango Formation. The reefal buildup is, therefore, placed in the Early to Middle Silurian.

Halysites is considered to range up to but not later than the Silurian (I4llJ. & STUMM 1963). Halysites occurs up to the top of the 12o and Manchap reefs which are overlain by Psilophyton-bearing sandstones. There- fore, the upper boundary of the reefs must still lie within the Silurian.

7 CONTROL OF THE REEF

The top of the Middle-?Upper Cambrian Parahio Member, in the Spiti Valley is marked by a sedimentolo- gical break. The deposition of the Thango Formation over the Parahio Member and older rocks was caused by Ordovician transgression: basal units of the Thango Formation slowly transgraded over different parts of the Parahio Member and even older rocks. Due to the extended transgression, the basin gradually deepened and at a particular depth reef growth started. As the reefs in Leo and Manchap were growing upward they were partially destroyed by wave action, a process which was repeatedly active in forming the pack/float/rudstones. In addition, with possible fluctuation of the sea-level an intimate association of in situ colonies, boundstones and pack/float/rudstones originated, characterizing a protected

reef flat or reef flank facies. A regressive cycle terminated the reef growth and covered the reefs with sediments of the intertidal zone. The shallowing extended during the Muth Formation, when clean beach sands were deposited. The growth of the reefs in Spiti and Kinnaur was thus depth-controlled, promoted by transgressive and obliter- ated by regressive cycles. Except for a pegmatite, intrusive in the Thango Formation at Phalong Danda (Lahaul), there is no evidence of more large-scale events during the Takche/Manchap Formation.

8 REEF GROWTH

The reef growth was generally Ix)or where siliciclastic sediments were deposited. Its growth in all the areas studied started on crinoid-bearing sediments in still muddy environments, as indicated by a sequence of argillaceous limestone/dolomites and bedded calcareous shales. Reef-- builders were usually Favosites and bryozoa, but as the basin became less turbid, rugose and tabulate corals, bryozoa and algae contributed to the formation of the reef. In large parts of the reef the tabulate corals and subordinate stromatoporoids were the main reef-builders. Bryozoa acted as trappers and binders of the sediments. In the basal part of the reef they were joined by Girvanella, which was replaced by Vermiporella and ?Codiacean algae in the middle and upper parts. The layers with Vermiporella helped in binding the sediments. The occurrence of Vermiporella in the rudstone facies suggests that it thrived in high energy areas ravaged by storms and possibly that it formed wave resistant frameworks similar to those formed by other dasyclads in Gotland (RIDING & WATFS 1981). Solitary and massive rugose corals in the reefs did not just add skeletal material, but where densely populated, acted as retarders of the current, thus forcing it to unload its sedimentary burden.

An intergrowth of chlorophytaVermiporella and tabu- late corals can be observed in the Manchap area. Vermipo- rella grew within the chains of Halysites and all around Plasmoporella.

9 REEF TYPES

Between Gechang and Leo, the reef growth in the Takche Formation is more or less continuous, though the thickness of the reefal section varies. At Takche in the north (towards Lahaul) the thickness of the corM-bearing beds is considerably reduced and they pass laterally into sandstones of shoreline facies. A similar relationship between Muth-Shian and its equivalent at Baldar is evident. The reef at Muth-Shian appears to be growing near the coast and is thus forming some type of fringe reef (WILSON 1975), though in a protected environment. The Leo Reef extends laterally over 1.5kin and the Manchap Reef over 5km. They both were buik on a protected reef- flat or on reef-flanks or may even be variants of barrier reefs. Possibly all these reefs belong to one major reefal

43

complex. Further east in Kumaun, KHANNA et al. (1985) reported massive stromatoporoids from the Young Limestone, denoting perhaps a high energy area of a reef core. More work in adjacent areas is still required for a definite interpretation of the reef types.

ACKNOVvq.~r~GMENTS

We are grateful to Prof. Dr. E. Fliigel for reading the manuscript. Prof. Dr. S. B. Bhatia helped in the identification of bryozoa. Drs. R. Riding and T.P. Burchette generously provided the literature. Dr. A. D. Ahluwalia, K. V. Rama Naidu and A. Banerjee helped with the photomicrography. Panchi R a m and Jagjit Kumar prepared hundreds of large thin-seelions in record time.

1331-1350, Colorado TSIEN, H.H. & DRICOT, E. (1977): Devonian Calcareous Algae from the Dinant and Namur Basins, Belgium. - In: FL~OGEL, E. (Ed.): Fossil Algae, 344-350, Berlin ISpringer) WILSON, J.L. (1975): Carbonate facies in geologzcal time. - 1- 147, Heidelberg (Springer) WRAY, T.L (1977): Calcareous Algae. - Developments in Paleontology, Slratigr. 4, 1-185, New York (Elsevier)

REFERENCES

BASSI, U.K., CHOPRA, S. & DATYA, B.M. (1983): A new Phanerozoic Basin in Kinnaur/Himachal Himalaya. - J. Geol Soe. India 24/6, 281-290, Bangalore BHARGAVA, O.N., BASSI, U.K. & CHOPRA, S. (1984): Trace Fossils from the Ordo-Silurian rocks of Kinnaur, Hima- chal Himalaya. - J. Geol. Soc. India 25/3, 175-186, Bangalore BURCHEI'TE, T.P. (1981): European Devonian Reefs: A review of current concepts and models. -Soc . Econ. Petrol. Min., Sl~c. Publ. 30, 85-142, Tulsa FLOGI~L, E. (1982): Microfacies Analysis of ! jmestones. - 1-163, Heidelberg-Berlin-New York (Springer) HAYDEN, H.H. (1904): The geology of Spiti, with parts of Bashahar and Rupshu. - Mere. Geol. Surv. India 36/1, 1-121, Calcutta HILL, D. (1963): Rugosa. - In: MOORE, R.C. (Ed.): Treatise on Invertebrate Palaeontology. Part F. Reprint, 1-498, Kansas (Univ. g~n~as Press) HILL, D & STUMM, E.C. (1963): Tabulata. - In: MOORE, R.C. (Ed.): Treatise on Invertebrate Palaeontology. Part F, ]~PRrint. 1-498, Kansas (Univ. Kansas Press)

SHAW, S. & RIDING, R. (1978): Parametefization of Stromatopomids' Shape. - Lethaia 2, 233-242, Oslo KHANNA, A.K., SINHA, A.K. & SAH, S.C.D. (1965): Yong Limestone of Tethys Himalaya - Stratigraphic status and palynological fossils. - J. Geol. Soc. India 26/3, 191-198, Bangalore LONGMAN, M.W. (1980): Carbonate diagenetic textures from nearshore diagenetic environments. - Amer. Ass. Petrol. Geol. Bull. 64/4, 461-187, Tulsa LDWENSTAM, H.A. (1950): Niagaran Reefs of the Great Lakes Area. - J. Geol. 58/4, 430-487, Chicago MANTEN, AA. (1971): Silurian Reefs of Got.land. - Develoments in Sedimentology 13, 1-539, Amsterdam (Elsevier) PLUMLEY, WJ., RISLEY, G.A., GRAVES, R.W. & CALEY, M.E. (1962): Energy index for limestone interpretation and classification. -Mere Amer. Assoc. Petrol. Geol. 1, 85-107, Tulsa RIDING, R. (1980): Composition, Structure and Environmental Setting of Silurian Bioherms and Biostromes in Northern Europe. -Soc. Econ. Petrol. Min., Spec. Publ. 30, 41-83, Tulsa RIDING, R. & WATTS, N. (1981): Silurian Algal Reef Crest in Gotland. - Naturwiss. 68, 91-92, Heidelberg REED, F.R.C. (1912): Ordovician and Silurian Fossils from Cenlral Himalaya. - Pal. Indica, Series 15, 7/2, 1-168, Calcutta SRIKANTIA, S.V. (1981): The lithostratgraphy, sedimentation and structure of Protemzoic-Phanerozoic Formations of Spiti basin in the Higher Himalaya of Himachal Pradesh, India. - In: SINHA, A.K. (Ed.): Contemporary Geoscientific Researches in Himalaya, Vol. 1, 31-48, Dehradun (Blshen Singh and Mabendra Pal Singh) STAUFFER, K.W. (1968): Silurian-Devonian Reef Complex near Nowshera, West Pakistan. - Bull. GeoL Soc. Amer. 79,

44

P l a t e

Fig. 1.

Fig. 2.

Fig. 3.

Fig. 4.

Fig. 5.

Fig. 6.

Fig. 7.

Fig. 8.

Fig. 9.

Fig. 10.

Fig. 11.

Fig. 12.

Fig. 13.

10 Reefal Buildups in the Himachal Himalaya, India

?Protaraea colony. Loc. Gechang. Scale in mm.

ttalysites catenularia kanaurensis REED encrusted by a low domal stromatoporoid. Framestone facies with mi- critic matrix. Loc. leo. (Negative print), x 2.5

Chonophyllum filled with ferruginous micrite (white) and sparite (dark)./_x~c. Leo. (Negative print), x 1.2

Partial encrustations of Streptelasma by a dendroid Clathrodictyonid. Hallopora (top, left comer) and bifoliate bryozoa also occur in micritic matrix. I.x)c. Leo. (Negative print), x 2.2

Enlargment of Fig. 4: Dendroid Clathrodictyonid. (Negative prin0, x 5

Transverse and tangential section of Parachaetetes. Loc. Leo. (Negative print), x 4

Moderately sorted, normally packed gastropod-Vermiporella packstone with nodosariids. Gastropod Idled with algae. Loc. Manchap. (Negative print), x 7.5

Poorly to moderately sorted lameUibranch-brachiopod float/packstone. Micrite (white) due to an umbreUa effect under the shells, l_x)c. Manchap. (Negative print), x 3.2

Flask-shaped umbellinacean in a fine sparitic matrix. Filled with coarser sparite. Loc. Manchap. x 30

Euomphalus.Loc. Manchap. x 1

Transverse section of Vermiporella f'dled with sparite, l_x~c. Manchap. x 25

Filamentous Girvanella in a bindstone. Loc. Manchap. x 30

Unidentified bryozoa encrusted by Girvanella. Loc. Manchap. x 40

TAFEL 10

46

P l a t e

Fig. 1.

Fig. 2.

Fig. 3.

Fig. 4.

Fig. 5.

Fig. 6.

Fig. 7.

Fig. 8.

Fig. 9.

Fig. 10.

11 Silurian Reefal Buildups in the Himachal Himalaya, India

Branched indeterminable ?bryozoa in a bindstone. Loc. Manchap. (Negative print), x 3.2

Longitudinal section of Favosites possibly encmsted by stromatoporoids. Filled with silica (black) and micrite (white). Loc. Manchap. (Negative print), x 3.5

Girvanella sp. as a spherical form and also as single filaments in the sediment. Loc. Manchap. x 30

Layered bryozoan-coral floatstone. The layering is uneven and discontinuous. ?Coenites (A), streptelasmatid co- ral (B),and Halysites (C). A rugose coral occurs as a "dropstone" (D). Loc. Mancbap. (Negative print), x 4.5

Vermiporella grainstone. I_xx:. Manchap. x 20

Brachiopod-lamellibranch pack/grainstone showing geopetal fabric. Fine sparitic matrix. Micritic material (white) is preserved below the shells due to an umbrella effect. Loc. Manchap. (Negative print), x 3.5

Nodular colonies of ?codiacean alga. The alga displays sparitic material (dark) in its center (A). I.zx:. Manchap. (Negative print), x 4

Bryozoa-trilobite packstone. Other clasts are micritic. I.xx;. Manchap. (Negative print), x 6

Cross-section of Favosites filled with silica (primary cemen0 largely replaced by micrite (white). Note irregul- ar boundary between the two, Loc. Manchap. (Negative print), x 6

Vermiporella-Plasmoporella-~ame/bindstone. Tangential section of Plasmoporella showing arched tabulae in ver- tic'al cut. Loc. Manchap./Negative print), x 3.5

TAFEL 11

48

P l a t e

Fig. 1.

Fig. 2.

Fig. 3.

Fig. 4.

Fig. 5.

Fig. 6

Fig. 7.

12 Silurian Reefal Buildups in the Himachal Himalaya, India

Vermiporella-Favosites reef rud/floatstone. Angular fragments of in situ colonies occur in ferruginous sparitic matxix. Cracks are/'tiles by clear sparite. Loc. Manchap. (Negative print), x 7

Coral-bryozoa floatstone. The coral with a wide globular dissepimentarium has been recrystallized. The bryo- zoa is Hallopora. Loc. Manchap. (Negative prin0, x 4

Bifoliate bryozoa and Coenites in a bindstone. The clasts in the rock include Halysites, brachiopod and organic debris. I-~c. Manchap. (Negative print), x 4

Vermiporella bindstone. A gastropod partially filled with sparite occurs in the rock. Some of the algae are fil- led with sparite (black). Loc. Manchap. (Negative print), x 7

Protaraea kanaurensis (REED) colony. Loc. Leo. (Negative print), x 4

Nodular colony of ?Radiastraea. LOC. Manchap. x 2

Spherical Girvanella encrusting a bryozoa in the bindstone facies. Loc. Manchap. x 30

TAFEL 12

50

P l a t e

Fig. 1.

Fig. 2.

Fig. 3.

Fig. 4.

Fig. 5.

Fig. 6.

Fig. 7.

Fig. 8.

Fig, 9,

Fig. 10.

13 Silurian Reefal Buildups in the Himachal Himalaya, India

Btachiopod shell (A), ostracod (B), bryozoa (C), crinoid (D) in a floats(one. Bryozoa encrusting ostracod tests. The matrix is microsparitic. Loc. Manchap. (Negative print), x 7

tlallopora bafflestone with a micrific matrix. Loc. Manchap. (Negative print), x 4

Coral-bryozona floats(one, Longitudinal section of a streptelasmatid coral (A), The matrix is micritic with fine organic debris. I_z~. Manchap, (Negative print), x 3

Favosites corallites partly filled with sparite (dark) and micfite (white). Loc. Leo. (Negative print), x 3

Streptelasma in growth position in a ?baffiestone together with debris of crinoids and bryozoans. The matrix is micrite. LOc. Manchap. (Negative print), x 2.5

Streptelasmatid coral (A) floak~tone. Other clasts are tabulate corals and fine organic debris. Loc. Manchap. (Negative print), x 2.2

Thalli of Vermiporella.Transverse section filled with rnicrite, Loc. Manchap. x 25

Solenoporid. Thin-section of a noduIar colony, Loc. Manchap, (Negative print), x 3

Thalli of Vermiporella. Tangential secUon fiIled with sparitic cement. Loc. Manchap. x 20

ttalysites catenularia kanaurensis REED. Loc. Manchap. Scale in ram.

TAFEL 13