بیوهرم های‌ آرکئوسیاتیدی‌ در بین توالی سیلیسی آواری: یافته‌‌ نو در‌ سازند لالون، البرز خاوری

Introduction

The Early Cambrian time is well known for the development of first metazoan reefs composed of calcimicrobes and archaeocyathan association (see Zhuravlev and Wood, 1995; Copper, 2001; Rowland and Shapiro, 2002). Archaeocyath-calcimicrobe reefs 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 often with considerable siliciclastics present (Rowland and Gangloff, 1988; Zhuravlev, 2001; Rowland and Shapiro, 2002).

To date, there is no report of any metazoan reef in the Toyonian or older Lower Cambrian succession of Iran and adjacent countries. Recent studies in the vicinity of Tuyeh-Darvar of 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 Formaton. These 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 main constituent of the bioherms (Lasemi and Amin-Rasouli, 2005b).

This paper documents the occurrence, sedimentology, and depositional environment of theses Toyonian bioherms, the only known Lower Cambrian metazoan bioherms of northern Gondwana. The Toyonian buildups of the Alborz are unusual in two ways. First, the reefs are composed of a framework entirely constructed by constructional metazoans and are overgrown by laminated microbialite (stromatolite). Second, the reefs (composed of individual and compound bioherms) occur as two horizons of limited lateral extent within an entirely siliciclastic succession. This study is based on detailed facies analysis of a stratigraphic section containing the bioherms. Several samples for thin and polished sections were collected at close intervals. The carbonates are classified on the basis of Dunham (1962) textural schemes and Embry and Klovan system (1971).

Stratigraphy and Regional Correlation

The Lalun Formation is a part of the lower Sauk sequence and constitutes the upper unit of over 2 kilometers platform succession deposited on the northern margin of Gondwana during the Lower Cambrian (Lasemi, 2001). Assereto (1963) divided the Toyonian Lalun Formation, in the type locality, into three lithostratigraphic units: (1) Sandstone unit (498 meters of red sandstone with red shale intercalations), (2) Shale unit (35 meters of red shale with thin sandstone interbeds) and (3) Top Quartzite unit (50 meters of white quartzarenite and arkosic sandstone).

The Shale and Top Quartzite units of the Lalun Formation in the study area are about 70 meters thick. About 12 meters of well-bedded colored shale constitutes the lower part of the Shale unit, which contains a few carbonate beds of limited lateral extent and some laminae or very thin beds (1-2 centimeter thick) of very fine-grained sandstone/siltstone throughout. Two dolomitized reef horizons with total thickness of up to 2.5 meters occur in the upper part of the colored shale (Amin-Rasouli, 1999; Lasemi and Amin-Rasouli, 2003, 2005a). The colored shale gradationally changes to red, interbedded sandstone and shale layers of the upper part of the Shale unit.

The contact between the Shale and Top Quartzite units is gradational but the Shale unit overlies the lower sandstone unit with a 50-centimeters thick transgressive pebbly chertarenite (Lasemi, 2001; Lasemi and Amin-Rasouli, in press). Lasemi (1995, 2001) recognized a heavily weathered sandstone horizon/red paleosol at the top of the Top Quartzite unit of the Lalun Formation corresponding to the Lower-Middle Cambrian boundary. Deposition of the upper part of the regressive Top Quartzite during a forced regression (Lasemi and Amin-Rasouli, 2003) also supports the development of a regional unconformity at the end of the Top Quartzite deposition. This distinct interregional unconformity separates the Top Quartzite (uppermost Lower Cambrian) from the overlying clean shallow marine carbonates (Member 1) of the Middle Cambrian Mila Formation (Lasemi, 2001; Lasemi and Amin-Rasouli, 2003; Lasemi and Amin-Rasouli, in press).

Reef facies

Reefs of the Shale unit of the Lalun Formation are rigid framework with well-defined syndepositional relief constructed by a consortium of calcified metazoans and stromatolite (stromatolite only overgrows the upper part the buildups). They occur as two reef zones of limited lateral extent containing reef complexes that are surrounded by well-bedded colored shale/volumetrically less important pebbly rudstone and consist of cabbage- or sack- shaped individual and compound buildups.

The lower reef zone consists of reef complexes composed of individual bioherms with colored shale as interbiohermal matrix. They rarely reach 50 centimeters in height and their diameter ranges from 5 to 70 centimeters. The upper reef zone is composed of reef complexes containing individual and compound metazoan bioherms that are overgrown by laminated stromatolite. Each lenticular reef complex begins, along the reef horizon, with individual bioherms and changes laterally to larger compound buildups. The compound buildups consist of individual bioherms stacked one upon the other (Kalyptrate buildups of Rowland and Gangloff, 1988). They are oval or circular in plan with irregular periphery, less than 2 meters thick and 0.75 to 1.2 meters in diameter. The upper surface of some compound buildups is flat with raised edges, show concentric layering with a core of an individual buildup. The skeletons in the buildups increase in size upwards and outwards.

Metazoans with fairly well-preserved skeletons appear to be the only constructors in the buildups. The skeletons are distinguishable in the field and in hand-specimens but because of pervasive dolomitization, the details of their structures are not completely preserved. They are solitary and colonial types and their skeletal morphology indicates bioconstruction by archaeocyath/sponges, coralomorphs and other undefrentiated metazoans. Skeletons resembling archaeocyath/sponges (especially in the upper reef zone) appear to be diverse, with vase, bowl and cup shaped and cylindrical (single or double wall individual and thin to thick branching with finger- sized digits) forms (e.g., Copper, 2001; Zhuravlev, 2001) present. The next and equally important constructional organisms identified in the reef zones appear to be the coral like skeletons or coralomorphs. Another less important reef forming organism, in the upper reef zone, is laminated stromatolite which bind the skeletons at the surface and overgrow the upper part of single and compound buildups.

Discussion

The individual and compound (kalyptrate) metazoan bioherms of the Lalun Formation are the only known Toyonian reefs in Iran and adjacent countries, the northern 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 Sauk I supersequence (time slice 1 of Golonka and Kiessling, 2002).

The Toyonian reefs of the Alborz are unique in that they lack the skeletal calcimicrobes, which dominated the Early Cambrian archaeocyaths-calcimicrobes reefs (e. g., Rowland and Gangloff, 1988; Kruse et al., 1995; Zhuravlev, 2001; Rowland and Shapiro, 2002). In the Toyonian reefs of Iran, the only associated microbialite is laminated stromatolite, which appears to be the only calcimicrobial structure enveloping the upper part of the bioherms. Although coralomorphs constitute a minor part of the reef building communities in the Lower Cambrian reefs (e. g., Savarese et al., 1993; Copper, 2001), they appear to be one of the dominant reef builders in the Toyonian buildups of northern Iran.

The Toyonian meter-scale reefs of Iran flourished in turbid waters associated with high siliciclastic input in the high energy abandoned tidal channels of the middle estuarine depositional setting. This is in accord with some of the Lower Cambrian archaeocyaths-calcimicrobes reefs, especially those with branching archaeocyaths (see Zhuravlev, 2001), which developed in siliciclastic dominated settings (e. g., Savarese, 1993; Rowland and Gangloff, 1988; Zhuravlev, 2001). 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 when other conditions are favorable, siliciclastic input and water turbidity is not a limiting factor for reef development. The absence of calcimicrobes in the Toyonian metazoan reefs of the Alborz suggests strong turbidity conditions during deposition that prevented calcimicrobial growth. Only during early late highstand, when sea-level fall put the emerged upper portion of the reefs in intertidal/supratidal positions (climax stage of the reef succession), laminated stromatolite overgrew the upper part of the buildups. The advance of the fluvial siliciclastics of the upper part of the Shale unit, during late highstand sea-level fall, resulted in the demise of the reef building organisms.

Reef growth appears to have occurred in shallow, near shore, high-energy brackish to normal marine setting. Thick branching archaeocyathids/sponges and reef rudstone with intraclasts, ooids, quartz grains and rip up clasts are evidences of agitation. Partial infilling of the primary inter-biohermal cavities by large reef clasts/cross-laminated siltstone to very fine sandstone, suggests occasional disturbances by storms and periodic influx of siliciclastics because of rainstorms and floods. The absence of sparry calcite cement and the presence of reddish colored infiltrated mud between the skeletons indicate that the brackish under-saturated water/fine siliciclastics prevented cementation.

The intrregional unconformity between the Top Quartzite and Member 1 of the Middle Cambrian Mila Formation is traceable in many parts of the world (e.g., Palmer, 1981; Rowland and Gangloff, 1988; Millson et al., 1996; Meng et al., 1997; Fedo and Cooper, 2001). The red beds of the Zagoon Formation and the lower Sandstone unit of the Lalun Formation of Iran signify a pause in the Cambrian transgression and are most likely the result of the early Toyonian regression (e.g., Zhuravlev, 1986; Rowland and Gangloff, 1988). The regressive Top Quartzite (Lasemi and Amin-Rasouli, 2003) of the Lalun Formation indicate a second major Toyonian regression, the Hawke Bay extinction event (Palmer and James, 1980) that resulted in total destruction of the Lower Cambrian metazoan reef builders. Only thrombolite and stromatolite reefs developed in the lower member of the Middle Cambrian Mila Formation (Lasemi, 2001), as a result of the post Early Cambrian reef crisis.

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