Archaeocyathan buildups within an entirely siliciclastic succession: New discovery in the Toyonian Lalun Formation of northern Iran, the Proto-Paleotethys passive margin of northern Gondwana

Sedimentary Geology 201 (2007) 302 – 320 www.elsevier.com/locate/sedgeo

Archaeocyathan buildups within an entirely siliciclastic succession: New discovery in the Toyonian Lalun Formation of northern Iran, the Proto-Paleotethys passive margin of northern Gondwana Yaghoob Lasemi ⁎, Hadi Amin-Rasouli Department of Geology, Tarbiat Moallem University, Tehran, Iran Received 8 June 2006; received in revised form 19 April 2007; accepted 17 May 2007

Abstract Meter-scale buildups constructed exclusively by archaeocyaths have been recognized within the uppermost Lower Cambrian siliciclastic succession of Eastern Alborz in northern Iran. They are the only known Toyonian reefs in Iran and adjacent countries and occur in the lower part of the Shale unit of the Lalun Formation. The reefs are of limited lateral extent, reach a maximum thickness of 2.5 m and consist of several reef complexes containing cabbage- or sack-shaped buildups surrounded by well-bedded, colored shale. Each reef complex in the main reef zone consists of meter-scale individual and kalyptrate (compound) buildups that are overgrown by laminated stromatolite. The individual buildups are 12 to 75 cm thick and 5 to 50 cm in diameter, but the compound buildups are up to 2 m thick and their diameter ranges from 75 to 120 cm. In contrast to most Lower Cambrian reefs, these compound buildups demonstrate a complete ecological succession including pioneer and climax phases. Archaeocyaths in the buildups are solitary and colonial types showing great diversity of growth forms, with vase-, bowl- and cup-shaped and cylindrical forms present. The cups range from less than 1 mm to 1.5 cm in diameter and are up to 4 cm tall in the majority of the skeletons, but they may reach up to 4 cm in diameter in the massive colonial forms or in the branching forms of the upper outer part of the compound bioherms. The limited lateral extent of the reef horizons, and lateral facies changes in the colored shale toward various reefs supports deposition in drowned tidal channels in an estuarine depositional environment. A change of the individual bioherms to larger compound buildups suggests lateral depth variation of the tidal channels. Partial infilling of the primary inter-biohermal cavities by large reef clasts and cross-laminated siltstone to very fine sandstone suggests occasional disturbances by storms and periodic influx of coarse siliciclastics. In striking contrast to other Lower Cambrian reefs, the archaeocyathan buildups of the Alborz flourished in a completely siliciclastic setting and lack the skeletal calcimicrobes which dominated the Lower Cambrian reefs. Abrupt lateral and vertical facies changes to colored shale and fluvial red beds, the presence of infiltrated shale between the skeletons, and in central cavities and the intervallum of the archaeocyaths suggest highly turbulent and turbid incoming water with abnormally high concentrations of fine siliciclastic material during reef development. High concentrations of fine, suspended siliciclastics could well have prevented the light penetration which was necessary for calcimicrobial growth. In addition, the relatively small size of the archaeocyaths and absence of reef dwellers is very likely a consequence of the high terrigenous mud content and, perhaps, belownormal salinity of the ambient sea water. In the absence of calcimicrobes and suspension feeder metazoans in the dark and muddy water of the mid-estuarine setting, archaeocyathans became the only bioconstructors of the Iranian buildups. Abnormally high

⁎ Corresponding author. Fax: +98 21 8830 9293. E-mail address: [email protected] (Y. Lasemi). 0037-0738/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2007.05.015

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concentration of nutritious fine siliciclastics suggests that photosymbionts and oligotrophic conditions were not needed by archaeocyaths; particular hydrodynamic conditions along with high nutrient flux, rather than light, were essential for archaeocyathan communities. © 2007 Elsevier B.V. All rights reserved. Keywords: Alborz Mountains (Iran); Archaeocyaths; Kalyptrate buildups; Toyonian; Fine siliciclastic input; Estuarine; Toyonian regression

1. Introduction The Early Cambrian is well known for the development of reefs composed of calcimicrobes and archaeocyathan association (Zhuravlev, 1986; Rowland and Gangloff, 1988; James and Gravestock, 1990; Kruse et al., 1995; Zhuravlev and Wood, 1995; Wood, 1999; Copper, 2001; Rowland and Shapiro, 2002). These meter-scale reefs are common in the Lower Cambrian rocks worldwide (e.g., Kruse et al., 1995; Zhuravlev, 2001; Rowland and Shapiro, 2002) with the calcimicrobes in most cases being the dominant reef building organisms. Reefs with archaeocyath-calcimicrobe consortia first appeared at the base of Tommotian Stage in the Siberian Platform; they spread worldwide throughout the rest of the Early Cambrian, in a variety of low latitude depositional settings — many with considerable siliciclastic input (Rowland and Gangloff, 1988; Zhuravlev, 2001; Rowland and Shapiro, 2002). Late Early Cambrian (Toyonian) archaeocyath-calcimicrobe reefs have been described from Siberia, eastern Canada and Nevada (James and Kobluk, 1978; James and Debrenne, 1980; Debrenne and James, 1981; James et al., 1988; Rowland and Gangloff, 1988; Rowland and Shapiro, 2002). To date, there have been no reports of any metazoan reefs in the Toyonian or older Lower Cambrian succession of Iran and adjacent countries. The Toyonian succession of Iran comprises up to 1000 m of fluvial red beds and marginal marine siliciclastics assigned to the Zaigoon and Lalun Formations. Recent studies in the vicinity of Tuyeh (Fig. 1) in the Alborz Mountains of northern Iran (Amin-Rasouli, 1999; Lasemi, 2001; Lasemi and Amin-Rasouli, 2003, 2005a) revealed the presence of a few carbonate beds, including two resistant reef zones, in the lower part of the Shale unit of the Lalun Formation. The reef zones were tentatively described as thrombolite and stromatolite bioherms by Lasemi and Amin-Rasouli (2003, 2005a). Close inspection of the internal structure of these reefs showed metazoan skeletons as the only constituents of the bioherms with laminated stromatolite as capping facies (Lasemi and Amin-Rasouli, 2005b).

This paper documents the occurrence, structure, sedimentology, and depositional environment of the Toyonian archaeocyathan bioherms of the Alborz Mountains of northern Iran. These buildups, the only known Lower Cambrian (Toyonian) metazoan bioherms of northern Gondwana, are unusual in two ways. First, the reefs are composed of a framework entirely built by archaeocyaths, which are overgrown, in the main reef zone, by laminated stromatolite. Second, the reefs (composed of individual and compound bioherms) show ecological zonation and occur as two horizons of limited

Fig. 1. Location map of the study area. Locations of the Tuyeh and Shahmirzad sections are shown by empty circles.

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Fig. 2. Early Cambrian (520 Ma) plate tectonic reconstruction (modified from Kiessling, 2001; Golonka, 2002). The solid circle at the eastern edge of the Proto-Paleotethys Ocean is the approximate location of the archaeocyathan buildups of the Alborz, the Proto-Paleotethys passive margin of northern Iran.

lateral extent within an entirely siliciclastic succession. This study is based on detailed facies analysis of the lower part of the Shale unit of the Toyonian Lalun Formation in eastern Central Alborz (Fig. 1), where the bioherms were discovered. Several samples for thin and polished sections were collected at close intervals. The carbonates are classified on the basis of the Dunham (1962) textural scheme and the Embry and Klovan (1971) system. 2. Geological setting and stratigraphy The study area is located on the east side of Tuyeh (about 48 km southwest of Damghan) in northern Iran (Fig. 1) in the Alborz Mountains, a range that extends for 2000 km along the south margin of the Caspian Sea from eastern Turkey to Afghanistan. According to global plate tectonic and paleogeographic maps (Kiessling, 2001; Golonka, 2002), the study area was positioned in an equatorial setting of northwest Gondwana during the Early Cambrian (Fig. 2) . During the Cambrian Period, the Alborz Mountains of northern Iran was part of an extensive platform, extending from the Arabian Shield to the northern Iranian margin of Gondwana (present-day coordinates) in the Proto-Paleotethys (Lasemi, 2001) (Paleoasian Ocean of Zonenshain et al., 1990) passive margin (Fig. 2). The Lower Cambrian strata of the Alborz Mountains of northern Iran comprise a succession of alternating carbonate and siliciclastic units (e.g., Alavi-Naini, 1993;

Hamdi, 1995; Lasemi, 2001) reflecting eustatic sea level changes. These units, from bottom to top, include the alternating shale and carbonates of the upper part of the Soltanieh Formation (lowermost Lower Cambrian), alternating carbonates and siliciclastics of the Barut Formation (middle Lower Cambrian), and the upper Lower Cambrian siliciclastics (mainly red beds) of the Zaigoon and Lalun Formations (Fig. 3). The uppermost Precambrian to Upper Cambrian deposits of northern, central and southwest Iran are part of a platform succession related to a passive margin that was already rifted in Late Precambrian times (Lasemi, 2001). In the Toyonian Stage, possibly due to the Toyonian regression (e.g., Rowland and Gangloff, 1988; Fedo and Cooper, 2001; Flugel and Kiessling, 2002), the extensive carbonate platforms of the northern passive margin of Gondwana were exposed and become the site of deposition of over 1000 m of fluvial red beds and marginal marine siliciclastics assigned to the Zaigoon and Lalun Formations (Lasemi, 2001). The uppermost Lower Cambrian Lalun Formation conformably overlies the Zaigoon Formation and unconformably underlies, throughout Iran, the mainly shallow marine carbonates of the Middle-Upper Cambrian Mila Formation (Lasemi, 2001; Lasemi and Amin-Rasouli, 2006). It is a part of the lower Sauk sequence (Sloss, 1963; Palmer, 1981) and constitutes the upper unit of a passive margin succession up to 3 km thick that was deposited on the northern margin of Gondwana (present day coordinate) during the Early Cambrian (Lasemi, 2001).

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The Shale and Top Quartzite units of the Lalun Formation in the study area are about 70 m thick. About 12 m of well-bedded, colored shale near the base of the Shale unit (Figs. 3 and 4B–C) contains a few carbonate beds of limited lateral extent (including two dolomitized reef horizons with a total thickness of up to 2.5 m) and some laminae or very thin beds (1–2 cm thick) of very fine-grained sandstone/siltstone throughout (AminRasouli, 1999; Lasemi and Amin-Rasouli, 2003, 2005a). The colored shale gradationally changes to the red, interbedded sandstone and shale layers of the upper part of the Shale unit (Figs. 3, 4B–C and 6C). 3. Reef facies Reefs in the Shale unit of the Lalun Formation have a rigid framework with well-defined syndepositional relief (true reef of Kiessling and Flugel, 2002) and were constructed by archaeocyaths (see below). They occur in two reef zones (Fig. 4B–D) of limited lateral extent that are composed of lenticular reef complexes (Figs. 4E, 5A). The reef complexes consist of cabbage- or sackshaped individual and compound buildups (Figs. 5A-C, 6A–F and 7A–C), surrounded by well-bedded colored shale (Figs. 4C–F, 6A–D) and volumetrically less important pebbly rudstone. 3.1. Lower reef zone Fig. 3. Stratigraphic nomenclature of the Lower and lower Middle Cambrian succession in the Alborz Mountains of northern Iran. Diagonal lines indicate documented unconformities. Right columns show more detailed stratigraphy of the Shale and Top Quartzite units, as well as the lower part of the Shale unit (colored shale) of the Lalun Formation in the Tuyeh section.

Assereto (1963) divided the Lalun Formation, in its type locality in the Central Alborz Mountains, into three lithostratigraphic units: (1) Sandstone unit (498 m of red sandstone with red shale intercalations), (2) Shale unit (35 m of red shale with thin sandstone interbeds) and (3) Top Quartzite unit (50 m of white quartzarenite and arkosic sandstone) (Figs. 3, 4A). The contact between the Shale and Top Quartzite units is gradational but the Shale unit overlies the lower sandstone unit with a transgressive pebbly chertarenite 50 cm thick (Lasemi, 2001). The contact of the Top Quartzite unit of the Lalun Formation with that of Member 1 carbonates of the Mila Formation (Middle Cambrian) is unconformable (see below) and corresponds to the Lower-Middle Cambrian boundary (Lasemi, 1995, 2001; Lasemi and Amin-Rasouli, 2003, 2006).

The lower reef zone occurs near the base of the colored shale (Figs. 3, 4B–C) and consists of reef complexes (Figs. 4F, 5A) that are composed of individual bioherms (Fig. 5B–C) with colored shale as the interbiohermal matrix. It starts with a basal microconglomerate (transgressive lag deposit) overlain by interlaminated skeletal grainstone and stromatolite boundstone (Fig. 5C), changing upwards to low diversity, millimeter-sized archaeocyaths (Fig. 5C–F) indicating the pioneer phase (stabilization and colonization stages) of Walker and Alberstadt (1975). The buildups in the lower reef zone rarely reach 50 cm in height and their diameter ranges from 5 to 40 cm. At the edge of one of the reef lenses, skeletal grainstone/rudstone fills the voids between the skeletons of a few small bioherms. 3.2. Upper reef zone The upper reef zone constitutes the upper part of the colored shale (Figs. 3, 4B–C) and is composed of reef complexes (Figs. 4E, 6C–D and 7A–B) containing individual and compound archaeocyathan bioherms. The largest reef complex found in the study area is 2 m high,

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Fig. 4. (A) Panorama (view to the northeast) of the uppermost Lower Cambrian Lalun Formation and Middle Cambrian carbonates of the Mila Formation in the Tuyeh section (stratigraphic top to the right). SS: Lower Sandstone unit, Sh: Shale unit, TQ: Top Quartzite unit, M1 and M2: Members 1 and 2 (Middle Cambrian carbonates) of the lower part of the Mila Formation. (B) Panorama (view to the southwest) of the upper part of the Sandstone and Shale units of the Lalun Formation (stratigraphic top to the left). Two reef zones (R) occur in the colored shale of the lower part of the Shale unit (the interval between the base of the Shale unit and the top of the upper reef zone is 12 m thick). (C–D) Close up views of B showing reef complexes surrounded by colored shale. (E) A lenticular reef complex of the upper reef zone (hammer in circle for scale; circle diameter is 40 cm). (F) A lenticular reef complex of the lower reef zone (length of the scale is 10 cm).

extends laterally for 20 m, and consists of several individual and compound buildups. Each lenticular reef complex begins, along the reef horizon, with individual

bioherms (Fig. 6A-B) and changes laterally to larger compound buildups (Figs. 6E–F and 7A–B). The individual buildups are 12 to 75 cm thick, 15 to 75 cm in

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Fig. 5. (A) Field photograph of a reef complex (center of picture) of the lower reef zone built by cabbage-shaped individual bioherms overlain by colored shale (scale in circle is 10 cm long; circle diameter is 20 cm). Top of the photograph shows the uppermost layer of the red-colored Sandstone unit of the Lalun Formation. (B) Enlarged portion of the middle part of photograph A (scale is 10 cm long). (C) Vertical section through the lower reef zone showing the basal skeletal micro-conglomerate laminae, which are overlain by interlaminated skeletal grainstone and stromatolite boundstone followed by low diversity archaeocyathid skeletons. (D) Photograph of a small bioherm of the lower reef zone showing low diversity solitary archaeocyathid forms. (E) Broken surface of a sample from the lower part of a compound buildup showing solitary individual and massive colonial forms. The ill-defined central cavities could be the result of expansion from the inner wall of irregular archaeocyath cups. Note the inter-skeletal and intra-skeletal voids are filled with colored shale. (F) Photograph of a hand sample from the upper part of the lower reef zone showing single-walled (upper left) and double-walled (upper center) archaeocyath growth forms. Note the well developed central cavities, septa, intervallum and porous walls. The inter-and intra-skeletal voids are filled with colored shale.

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Fig. 6. (A–B) Solitary buildups of the upper reef zone that are completely overgrown by laminated stromatolite and surrounded by colored shale (diameter of lens cap is 5.5 cm). (C) A lenticular reef complex (center of photograph) consisting of compound buildups surrounded by colored shale (circle diameter is 20 cm). It is overlain by interbedded shale and sandstone of the Shale unit (stratigraphic top towards lower left). The upper right of the photograph shows the uppermost layer of the Sandstone unit of the Lalun Formation. (D) A different view of C showing the transgressive skeletal micro-conglomerate of the base of the buildups (the height of bioherms is about 1 m). (E) One of the kalyptrate buildups (compound buildup) of the upper reef zone in which individual bioherms are stacked one upon the other to make larger buildups (the height of the reef is 80 cm). Note that the sack-shaped individual buildups and their skeletons increase in size upwards, but their numbers increase and then decrease upward. The upper part of the buildup is overgrown by laminated stromatolite (darker appearance in the upper part of the compound buildup). (F) A view from the top of 5E (pen is 15 cm long). Note that this compound buildup consists of only two larger buildups at its top. Note also the vase- or cup-shaped skeletons in the outer body of the buildup.

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Fig. 7. (A and C) Compound buildups of a reef complex with concentric layering and irregular/raised margins. They are capped by laminated stromatolite and overlain by colored shale. Notice vase- or cup-shaped skeletons on the outer body of the compound buildup (left side of the hammer head in the center of photograph) that become larger upward. (B) Several compound buildups with circular upper surfaces. Note the cylindrical branching skeletons (by the hammer head) that change to larger vase- or cup-shaped skeletons upward (upper right) at the outer body of the bioherms. The size of the skeletons increases away from the center. (D) Photograph of a hand sample showing transverse and oblique sections of archaeocyath cups showing septa, central cavity and pore canals/pore tubes. All of the intra-skeletal voids are filled with red-colored mud. (E) Transverse section of a hand sample showing single-walled and double-walled individual and double-walled massive colonies. Note cylinder, vase- or cup-shaped archaeocyath growths forms. (F) Enlarged view of upper left corner of E. Note the well developed intervallum, septa, central cavity and porous inner wall in the double-walled form in the left center of the photograph. Red-colored mud fills the intra- and inter-skeletal voids.

diameter and are overgrown by laminated stromatolite (Fig. 6A–B). The compound buildups are up to 2 m thick, 75 to 120 cm in diameter and capped by laminated

stromatolite (Figs. 6A–B, 7A–C and 8A–D). They are composed of stacked sack-shaped individual bioherms (kalyptrate buildups) that become larger upward (Fig. 6E).

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Fig. 8. (A) One of the compound buildups of Fig. 7A with raised edge and concentric layering capped by stromatolite. Length of hammer is 36 cm. (B) Close up view of A showing a variety of archaeocyath forms covered by laminated stromatolite in the left side of the photograph (lens cap diameter is 5.5 cm). (C) A sample from the upper surface of A, showing details of stromatolite that binds and overgrows the outer surface of the skeletons. (D) Thin section photomicrograph of C showing silt-size grains that are trapped in the stromatolite laminae.

Rowland and Gangloff (1988) defined the term kalyptrate as a compound buildup constructed by concavo-convex or biconvex individual bodies of boundstone (kalyptra) up to 2 m in diameter and 1 m tall, lying in close association with similar bodies to form a compound buildup. Smith (1981) and James (1983) used the term saccolith for such buildups. As shown in Fig. 6E, the kalyptrate buildups in the upper reef zone consist of stacked, closely associated, sack-shaped biconvex individual buildups that lie in the upright position within the buildup. In these buildups, the size of the individual bioherms tends to increase upward, but their numbers increase toward the center and then decrease upward (at the expense of smaller bioherms) to form one or two larger individual bioherms (Fig. 6E-F). The kalyptrate buildups occur as individuals or as clusters within a reef complex, and are oval or circular in plan view with an irregular periphery (Figs. 6C–F, 7A–B and 8A). The upper surface of some compound buildups is flat with raised edges and shows concentric layering

with a core of an individual buildup (Figs. 7A and 8A). Skeletons in the buildups increase in size upwards and outwards (Figs. 6E–F, 7A–C). The upper reef zone shows a complete ecological succession from the pioneer (stabilization and colonization) to climax (diversification) stages (Walker and Alberstadt, 1975; Copper, 1988). Each kalyptrate bioherm consists of a basal microconglomerate-pebbly sandstone transgressive lag/storm deposit (Figs. 6D–E, 7C) containing undifferentiated skeletal grains (stabilization stage) overlain by small bioherms containing low diversity millimeter-sized solitary forms (Fig. 5D-F) of pioneer or colonization stage before changing to a larger threedimensional buildup composed of larger sack-shaped bioherms consisting of millimeter-centimeter-sized archaeocyathids (diversification or climax stage) showing higher diversity and a greater variety of morphologies (Figs. 7C–F, 9A–F). The succession is overgrown by laminated stromatolite (Figs. 6E–F, 7A–C, and 8A–D) without the development of the domination stage.

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Fig. 9. Side (A) and top (B) views of a colony of finger-sized cylindrical archaeocyaths with ill-defined central cavities that are filled by colored shale. (C) Enlarged view of part of B showing concentric growth forms that were probably developed due to inner wall expansion. (D) Hand specimen showing a vertical section of vase-shaped archaeocyaths. Note two single-walled forms on the right and an oblique double-walled form with septa and pore tubes in the left side of the photograph. (E–F) Vertical (E) and horizontal (F) sections of cup- or bowl-shaped single-walled growth forms. The individual skeletons are stacked one upon the other, giving rise to various diameters in horizontal section (the voids are filled with colored shale).

312 Y. Lasemi, H. Amin-Rasouli / Sedimentary Geology 201 (2007) 302–320 Fig. 10. (A–J) Photomicrographs of microscopic archaeocyaths that are found in the rudstone layers associated with the buildups. These forms are not present in the bioherms and are interpreted to have been transported from the offshore area of an estuarine setting to the reef site (all scale bars are 0.1 mm). (A–B) Parts of transverse to tangential sections of archaeocyaths showing septas and voids related to the intervallum areas between the outer and inner walls of a cup. (C and G) Transverse sections of concentric forms with off-center canals that are filled due to outgrowths from the inner wall. (D) Transverse section of a cup similar to K that is filled by mud. (E, F and J) Transverse sections of dolomite-cemented growth forms showing central cavities filled with colored mud and silt-sized sediments. Note well-developed radial septas in E and J. (H) Concentric form with off-center central cavity showing inner wall expansion. (I) Transverse and oblique section of a concentric doublewalled inverted cone of a small archaeocyath showing inner wall expansion and central cavity filled with red-colored mud. (K) Longitudinal section of a vase-shaped archaeocyath with longitudinal septa and central cavity filled with mud-sized sediment. Other grains are smaller archaeocyaths, quartz and shale clasts.

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3.3. Biotic composition Archaeocyaths with fairly well-preserved skeletons appear to be the only constructors in the buildups. The skeletons are easily distinguishable in the field and in hand-specimens but, except for well preserved microscopic forms (Fig. 10), the details of their structure are not completely preserved because of pervasive dolomitization. The archaeocyaths are solitary and colonial types (Figs. 5D–F, 7D–F, 9A–F and 10K) and show a great diversity of growth forms, including vase, bowl and cup shapes and cylindrical forms (e.g., Hill, 1972; Wood, 1999; Copper, 2001). The cups range from less than 1 mm to 1.5 cm in diameter and are up to 4 cm tall in the majority of the skeletons. Cups may reach up to 4 cm in diameter in the massive colonial forms or in the branching forms of the upper outer part of the compound bioherms (Fig. 7D). Cylindrical forms consist of millimeter-sized individuals (Fig. 7E) and thin to thick branching types with finger-sized digits (Fig. 9A-B). In plan view, single walled forms (Figs. 5F, 7E, 9D–F and 10D), double walled forms without septa (Fig. 10C, F) or with radial/ longitudinal septa (Figs. 5F, 7D–F, 9D, 10A–B, E, H–K and 11B), concentric (Fig. 10C, F or double walled irregular forms with curved and wavy plates (pseudosepta) indicating lateral expansion from the inner wall (e. g., Hill, 1972) with ill-defined central cavities (Figs. 5E, 9B–C and 10C) are recognized. Porous walls or horizontal pore canals/pore tubes are preserved in some skeletons (Figs. 5F, 7D–F and 11B). Some microscopic forms that are found in the reef rudstone (Fig. 10C-I) are a fraction of a millimeter in size and show oval/concentric growth forms. Some of these small growth forms show radial/longitudinal septa (Fig. 10E, G, I-K) and others show nicely developed central (Fig. 10E-F, I-K) or off-centered (Fig. 10C, G-H) cavities that are filled by infiltrated red shale. The thicknesses of the laminates or the intervallum vary along the periphery of some concentric forms, leading to off-centered central canals (Fig. 10G-H). These forms could be transverse sections of microscopic archaeocyaths, a part of their skeletons (perhaps skeletal tissue laminate), or larval and young archaeocyath growth forms (e. g., Hill, 1972). The small forms with concentric laminae and ill-defined central canals are likely the consequence of outgrowth from the inner wall of small larval/juvenile irregular archaeocyaths or they may be transverse sections of the lower part (holdfast) of the cups. The size, growth forms and abundance of the skeletons differ in different buildups within each reef complex. Solitary buildups consist of smaller skeletons with

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low growth form diversity, but larger compound buildups contain larger skeletons and a greater variety of skeletal morphologies. Reef dwellers such as trilobites, hyolithids, brachiopods and echinoderms, which are commonly present in other archaeocyathan buildups, are absent. Another less important reef forming organism is non-calcified microbialite (laminated stromatolite) that binds the skeletons at the surface and caps the single and compound buildups in the upper reef zone (Figs. 6A–B, E–F, 7A–C and 8A–D). Some of the stromatolite laminae contain silt size quartz grains (Fig. 8F). 3.4. Associated sediments Reef debris is rare near the buildups. Only a few buildups change laterally (along the same horizon) to a pebbly rudstone layer (Fig. 11A-B) at the edge of a few reef complexes before changing to colored shale. The rudstone facies consists of archaeocyath skeletons, skeletal reef debris, red shale/siltstone pebbles, and sand-to silt-sized chert and quartz grains (Figs. 10, 11C–D). Well-bedded colored shale directly overlies the buildups with a sharp contact, covering the upper surfaces of skeletons or capping stromatolites (Figs. 4C–F, 6C–D and 7A). Sparry calcite cement is rare and the reddish colored, infiltrated mud of the enclosing colored shale typically fills the voids between skeletons, the central cavities and the intervallum areas (Figs. 5C–F, 6E–F, 7D–F, 9A–F and 10). Except for a few compound buildups in which reef debris (up to several centimeters in diameter) and bioclast-bearing, cross-laminated red siltstone to very fine sandstone partially fills the primary inter-biohermal cavities (Fig. 11E-F), the matrix between the buildups consists of the red shale and siltstone of the enveloping colored shale. 4. Discussion The individual and compound (kalyptrate) archaeocyathan bioherms of the Lalun Formation are the only known Toyonian reefs in Iran and adjacent countries, along the northern (present coordinates) passive margin of Gondwana. They form transgressive and early highstand systems tracts of the Shale unit depositional sequence (Lasemi and Amin-Rasouli, 2003) in the upper part of the Sauk I super sequence (time slice 1 of Golonka and Kiessling, 2002). 4.1. Regional correlation and age The reefs encountered in this study occur within the Shale unit of the Lalun Formation (upper Lower

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Fig. 11. (A) Reef rudstone facies containing sand and pebble-sized siliciclastic grains and microscopic archaeocyaths, which occur at the edge of some buildups in the upper reef zone (lens cap is 5.5 cm in diameter) (B) A massive archaeocyath colony showing concentric growth form/septa and pore canals. Colony is associated with siliciclastics-bearing rudstone. (C–D) Thin section photomicrographs of the reef rudstone showing shale, siltstone and quartz clasts and reef debris. Spherical grains (in the upper left of C) resemble archaeocyaths. (E) Large reef debris and (F) pebbly finegrained sandstone with a reef clast as interbiohermal matrix.

Cambrian) of northern Iran (Fig. 3). The following evidence supports a Toyonian (latest Early Cambrian) age for the archaeocyathan buildups of northern Iran. Due to lack of guide fossils in the Lower-Middle Cambrian boundary interval, there have been conflicting

views on the position of the Lower-Middle Cambrian boundary among the geologists working on the Cambrian deposits of Iran. Stocklin et al. (1964), Stocklin and Setudehnia (1971) and Kushan (1978), on the basis of stratigraphic position, suggested the boundary to be the

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upper surface of the Top Quartzite unit of the Lalun Formation. Alavi-Naini (1991, 1993), and Aghanabati (2004) proposed that the boundary lies in the gradational contact between the Shale and Top Quartzite units of the Lalun Formation. Lasemi (1995, 2001) recognized a distinctive, red-weathering sandstone/red paleosol on the upper surface of the Top Quartzite that is recognized in the Alborz, Central Iran and Zagros Mountains of southwest Iran. This regional unconformity corresponds to the Lower-Middle Cambrian boundary, the Sauk I-II unconformity of Palmer (1981). The Sauk I–II unconformity records the culmination of the global Toyonian regression (Palmer and James, 1980; Rowland and Gangloff, 1988; Rowland and Shapiro, 2002) and corresponds to the Hawke Bay extinction event, which resulted in the demise of the Lower Cambrian metazoan reef builders (e.g., Zhuravlev, 1996; Brasier, 1996; Flugel and Kiessling, 2002; Rowland and Shapiro, 2002). Following the late Toyonian event, only thrombolite and stromatolite reefs occur in the Middle Cambrian successions of the world, including Members 1 and 2 of the Mila Formation of Iran. In the Shahmirzad section of the eastern Central Alborz Mountains, about 60 km to the west of the archaeocyath reef-bearing Tuyeh section (Fig. 1), the base of the lower member of the Mila Formation is a transgressive tract containing, near its unconformable lower boundary, a thrombolite reef zone up to 3 m thick (Lasemi, 2001, Lasemi and Amin-Rasouli, 2006). It consists of upward-thickening (1 cm to 1 m thick), stacked individual, or compound domical thrombolite bioherms. Thrombolite buildups normally appear during times of major environmental change, including transgressive events, metazoan reef crises and the recovery interval following mass extinction events (e. g., Brett, 1995; Glumac and Walker, 1997; Lasemi et al., 1998; Whalen et al., 2002; Mancini et al., 2004; Adams et al., 2005). Above the thrombolite zone, only stromatolite reefs dominate the Middle Cambrian succession of the Alborz Mountains (Members 1 and 2 of the Mila Formation) and other parts of Iran (e. g., Kohansal Ghadimvand, 1993; Lasemi, 1995; AminRasouli, 1999; Rastgar, 2000; Lasemi, 2001; Lasemi and Amin-Rasouli, 2003, 2006). The meter-scale sponge-microbial patch reefs in the Mila Formation (Hamdi et al., 1995) of the Shahmirzad section (Fig. 1) occur in the middle part of Member 3 of the Mila Formation, over 250 m above the LowerMiddle Cambrian unconformity. Hamdi et al. (1995) had envisioned a conformable contact between the Lalun and Mila Formations and erroneously reported the reefs to occur at the base of Member 3. Due to the low preservation potential of trilobites in the partially re-

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crystallized bioclast (trilobite, brachiopod and crinoid fragments) grainstone facies of the host rocks, the suggested latest Middle Cambrian age (Hamdi et al., 1995) for these reefs is questionable. Based on the documented late Middle Cambrian age of Member 2 (e. g., Kushan, 1978; Wittke, 1984) and sequence stratigraphic analysis of the Cambrian deposits of the Shahmirzad section (Lasemi, 1995, 2001) the sponge-microbial reefs of the Alborz belong to the Upper Cambrian deposits. There is an excellent correlation between the Sandstone, Shale and regressive Top Quartzite units of the Lalun Formation and the upper Lower Cambrian succession of the Appalachian Mountains of Eastern North America (e. g., Debrenne and James, 1981). The unconformity-bounded upper Lower Cambrian succession of Labrador and Newfoundland (Toyonian), like the Iranian example, starts with the fluvial Bradore Formation or its equivalent units (Zaigoon and lower Sandstone unit equivalent) overlain by a unit of limestone (including archaeocyathan bioherms), siltstone and shale called the Forteau Formation (Shale unit equivalent), which is capped by a regressive sandstone named the Hawke Bay Quartzite (Top quartzite equivalent). 4.2. Depositional setting and siliciclastic input The silty and sandy colored shale (and the archaeocyathan reefs) of the Shale unit were deposited in an estuarine depositional setting (Fig. 12). Amin-Rasouli (1999) and Lasemi and Amin-Rasouli (2003, 2005a), based on facies analysis, suggested a wave dominated, inner to middle estuary depositional setting for the Shale unit and an outer estuary/shoreface depositional environment for the overlying regressive Top Quartzite unit of the Lalun Formation. The estuarine depositional setting developed in the study area during the transgression that followed the early Toyonian regression (see below) and the southward migration of the meandering river system that deposited the upper Lower Cambrian fluvial red beds of the Zaigoon Formation and the lower Sandstone unit of the Lalun Formations (Lasemi, 2001). This fluvial succession that unconformably underlies the reef-bearing Shale unit of the Lalun Formation is very thick (close to 1000 m) and covers a vast area in southern, central and northern Iran and the adjacent countries of northern/ northwestern Gondwana (e.g., Stocklin and Setudehnia, 1971; Alavi-Naini, 1993; Hamdi, 1995; Lasemi, 2001). The succession is dominated by flood plain mud rock, which was the likely source of the terrigenous mud that fed the river system of the associated estuarine environment. The archaeocyathan reefs developed within the high energy drowned channels of an estuarine depositional

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Fig. 12. Depositional model of the Shale unit in the Tuyeh area of northern Iran. Reef complexes and archaeocyathan buildups are interpreted to have developed in an estuarine depositional setting.

setting that existed probably landward of an offshore carbonate/siliciclastic barrier (Fig. 12). The limited lateral extent of the reef horizons and lateral facies change of various reefs to colored shale support deposition in drowned tidal channels of an estuarine depositional environment. Individual small bioherms change laterally to larger compound buildups (containing larger skeletons of greater diversity and growth morphology), which suggest lateral variation in the depth of the tidal channels in the estuarine depositional setting. The rarity of marine cement, the shale infiltrated into the inter-skeletal and intra-skeletal cavities, and the enveloping colored shale (Figs. 4C–F, 5C–F, 6A–F, 7A–F, 9A–F, and 10) indicate that the abundant fine siliciclastic particles and, perhaps, brackish water, undersaturated with respect to calcite prevented cementation. The rare presence of siliciclastic-bearing reef rudstone and partial infilling of the primary inter-biohermal cavities by large reef clasts and cross-laminated siltstone to very fine sandstone (Fig. 11E-F) suggest episodic disturbances by storms and periodic influxes of coarse siliciclastics because of rainstorms and floods. Some of the bioclasts and intraclasts/rip up clasts in the reef rudstone/basal transgressive lag deposits are interpreted to have been transported from the offshore area to the reef site. 4.3. Reef biota and paleoecology The Toyonian archaeocyathan reefs of the Alborz Mountains are unique in that they lack the skeletal calcimicrobes, which dominated other Lower Cambrian

archaeocyath-calcimicrobe reefs (e. g., Rowland and Gangloff, 1988; Kruse et al., 1995; Wood, 1999; Stanley, 2001; Zhuravlev, 2001; Rowland and Shapiro, 2002; Alvaro et al., 2006). In the Alborz reefs, the only associated microbialite is laminated stromatolite, which appears to be the only microbial structure that caps the bioherms in the upper reef zone (Figs. 6A–B, 6E–F, 7A–C and 8A–D). A review of the existing literature on the Lower Cambrian reefs (e. g., Rowland and Gangloff, 1988; Savarese et al., 1993; Zhuravlev, 2001; Rowland and Shapiro, 2002) indicates that archaeocyaths and their associated calcimicrobes developed in carbonate dominated settings commonly with considerable siliciclastic material present. According to Hill (1972) and Rigby and Gangloff (1987) archaeocyaths flourished best in sediments that contained up to one-third insoluble residues. The archaeocyathan bioherms of the Alborz, on the other hand, flourished in a completely siliciclastic setting. Abrupt lateral and vertical facies changes to colored shale and fluvial red beds, the presence of infiltrated shale between the skeletons, in central cavities and the intervallums of the archaeocyaths, suggest highly turbulent incoming water with abnormally high concentrations of fine siliciclastics during reef development. High concentrations of fine suspended siliciclastic particles in the water could very well prevent or sharply reduce the light penetration necessary for calcimicrobial growth. Except for a few stromatolite laminae that developed at the start of the lower reef zone (before their submergence) and those that capped the buildups of the upper reef zone during the reef emergence, only

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archaeocyaths could flourish in the highly muddy ambient water of the estuary. In addition, the relatively small size of the archaeocyaths, and the absence of reef dwellers such as trilobites, hyolithids, brachiopods and echinoderms, which are commonly present in other archaeocyathan buildups (e. g., Debrenne and James, 1981; Fagerstrom, 1987; Wood, 1999; Zhuravlev, 2001), was very likely a consequence of the high terrigenous mud content and perhaps below-normal salinity of the ambient sea water. Results from this study and other Lower Cambrian examples, as well as development of coral reefs in restricted or coastal, clastic-dominated marginal marine environments (Kiessling, 2002; Wilson, 2005) indicate that siliciclastic input was not a limiting factor for archaeocyathan growth. Turbulent water and abnormally high concentrations of nutritious fine siliciclastics suggest, as was pointed out by Wood et al. (1993) and Wood (1999), that photosymbionts and oligotrophic conditions were not needed for archaeocyaths to build their mineralized skeletons. It appears that hydrodynamic conditions and high nutrient flux rather than light were essential for archaeocyathan communities (e. g., Wood et al., 1992, 1993; Zhuravlev, 2001). In the absence of calcimicrobes and suspension feeder metazoans in the dark and muddy water of the estuarine setting, archaeocyaths became the sole bioconstructors of the Iranian buildups. 4.4. Ecological succession and sea level changes: Walker and Alberstadt (1975) proposed a four-stage sequence for ancient reef ecosystems. These include: the basal stabilization stage composed of lime sand, the colonization stage consisting of low diversity frame builders, the diversification stage consisting of diverse reef builders, and the domination stage composed of low diversity reef fauna adapted to high energy conditions. The concept of ecological succession has been applied to many Phanerozoic reefs (e. g., James, 1983; Rowland and Gangloff, 1988; Zhuravlev, 2001). Some authors (e. g., Wood, 1999; Pratt et al., 2000) believe that the Lower Cambrian reefs did not show any vertical zonation. Rowland and Gangloff (1988) and Rowland and Shapiro (2002), on the other hand, documented a complete Walker–Alberstadt succession in the non-kalyptrate reefs of the Botomian Poleta Formation of western Nevada, USA. Rowland and Gangloff (1988) interpreted the vertical zonation of the Poleta reef to be the result of cratonward facies migration that accompanied the Early Cambrian Transgression and interpreted the absence of vertical zonation in the kalyptrate reefs of the Forteau Formation to be a result of the Toyonian regression. The

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ecologically zoned Toyonian reefs of the Alborz (see below), however do not support their interpretation. Similar to the Poleta Formation, the Toyonian kalyptrate reefs of northern Iran also display a well-developed ecological succession. In these compound buildups, after the development of their pioneer phase, probably during a fourth-order sea level rise, the reef constructors kept pace with slowly rising see level during early highstand, expanded in lateral and vertical directions (the size and number of sack-shaped individual buildups increased) at first, then continuously aggraded (keep-up reef of James and Macintyre, 1985) (Fig. 6E–F). As a consequence of continuous decrease in the rate of sea level rise or during sea level stillstand, in the later part of early highstand, the reefs grew to sea level and formed fewer but larger individual buildups (catch-up reef of James and Macintyre, 1985). During early highstand of sea level, the range and diversity of archaeocyath growth forms increased, leading to larger three-dimensional compound buildups, the diversification or climax stage. Gradual sea level fall at the end of early highstand positioned the reefs in the intertidal/supratidal setting that resulted in the demise of archaeocyaths and the growth of the capping stromatolite. Eventually, the advance of the fluvial siliciclastics of the upper part of the Shale unit, during late highstand sea level fall, resulted in the total demise of the Lower Cambrian reef ecosystem. It appears that in the Toyonian reefs of northern Iran, the low diversity domination stage is absent and the final stage of reef development is marked by maximum archaeocyathan diversity (climax stage of Copper, 1988). The lower reef zone occurred in the transgressive tract, when sea level rise was faster (Lasemi and Amin-Rasouli, 2003), therefore, it did not develop fully and an arrested succession (Copper, 1988) was developed. Although the Toyonian regression interval encompasses the entire Toyonian Stage, the results of this study and sequence stratigraphic analysis of the upper Lower Cambrian deposits (Lasemi, 2001; Lasemi and AminRasouli, 2003) indicate that there are two transgressive– regressive cycles (Shale and Top Quartzite depositional sequences) bounded by two regional unconformities. The upper Lower Cambrian fluvial succession (the red beds of the Zaigoon Formation and the lower Sandstone unit of the Lalun Formation), which is bounded by two major unconformities (Fig. 3) reflects the first pause in the Cambrian transgression, which is here referred to as the early Toyonian regression. The regressive Top Quartzite (Hawke Bay sandstone equivalent) that underlies the well-documented Lower-Middle Cambrian boundary (see above) represents another major regression, here referred to as the late Toyonian regression, which resulted in the demise of the archaeocyaths and the destruction of the

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first metazoan-calcimicrobe reef building ecosystems (e.g., Zhuravlev, 1996; Brasier, 1996; Flugel and Kiessling, 2002; Rowland and Shapiro, 2002). 5. Conclusions The Toyonian archaeocyathan reefs of the Alborz Mountains developed in the Proto-Paleotethys passive margin of northern Gondwana during latest Early Cambrian (Toyonian) time. The reefs consist of reef complexes of limited lateral extent within an entirely siliciclastic succession. The main reef zone of the Alborz Mountains include individual and compound bioherms (Kalyptrate buildups) composed solely of archaeocyaths. The compound bioherms, in contrast to most Lower Cambrian reefs, display a well developed ecological succession. The reefs developed in turbid waters associated with high energy drowned tidal channels of an estuary depositional setting. Strong turbidity with abnormally high and constant input of fine siliciclastics prevented the growth of skeletal calcimicrobes, which dominated the Lower Cambrian archaeocyath-calcimicrobe reefs. The very muddy and perhaps brackish, under-saturated water of the estuary also prevented marine cementation and only reddishcolored mud infiltrated the intra and inter-skeletal voids. The Toyonian archaeocyathan reefs of the Alborz Mountains represent the final stage of metazoan reef building before the late Toyonian regression (the Hawke Bay extinction event) that resulted in the demise of the Lower Cambrian metazoan reef builders. As a consequence of the late Toyonian reef crisis, only thrombolite and stromatolite reefs developed in the Middle Cambrian carbonates of Iran. Acknowledgements We are indebted to B. W. Sellwood, the editor of Sedimentary Geology and two anonymous reviewers for critical reviews of the first draft of the manuscript. Their thorough reviews, suggestions and constructive criticisms, which led to significant improvement of the paper, are gratefully acknowledged. Critical review of the final version of the manuscript by Rod Norby and Jonathan Goodwin of the Illinois State Geological Survey is greatly appreciated. References Adams, E.W., Grotzinger, J.P., Watters, W.A., Schroder, S., McCormic, D.S., Al-Siyabi, H.A., 2005. Digital characterization of thrombolite reef distribution in a carbonate ramp system (terminal Proterozoic, Nama Group, Namibia). AAPG Bull. 89, 1293–1318.

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