THE ECOLOGY OF LOWER-CAMBRIAN BUILDUPS FROM ZUUNE ARTS, MONGOLIA - IMPLICATIONS FOR EARLY METAZOAN REEF EVOLUTION

Reefal buildups in western Mongolia of mid-Early Cambrian (late Atdaba nian) age flourished during a period when shelf seas were globally wid espread. The succession at Zuune Arts records the transition from shal low marine siliciclastic sediments (Bayan Gol Formation) to shallow ma rine, but still clastic-influenced, carbonates (Salaany Gor Formation) . The Salaany Gol Formation is interpreted as having been deposited as a series of shallowing upwards cycles on a shallow, gently inclined s helf in a rapidly subsiding epicontinental sea. Cycles commenced with the growth of subtidal metazoan-calcimicrobe aggregative communities o n an open shelf. The resultant buildups were commonly engulfed by exte nsive, massive microbial stromatolites when they grew in agitated inte rtidal conditions. Latterly, they were smothered by ooid shoals in res ponse to rapid sea level rise. Four types of reefal buildup are distin guished: (1) green-coloured calcimicrobe (Tarthinia, Epiphyton, Gordon ophyton and Renalcis) boundstones;(2) red- or green-coloured Cambrocya thellus-Tarthinia-Epiphyton bafflestones; (3) red-coloured Okulitchicy athus bindstones; and (4) red-coloured radiocyath-archaeocyath-cribric yath bioherms. Each is interpreted as having grown at increasing depth s and possibly sedimentation rates. The buildups are commonly enclosed within graded and planar bedded bioclastic grainstones and packstones , and are best developed towards the top of the formation, when sea le vel was high. Thickets of large, solitary archaeocyaths are also infer red in the deeper interbiohennal areas. These buildups had synoptic re lief and constructed porous structures with growth-framework cavities housing diverse coelobiontic communities. Extensive synsedimentary cem ents are present, including pseudomorphed aragonitic fans and possible pseudomorphed aragontic botryoids. These early reefs thus have geolog ical fabrics similar to later Phanerozoic representatives. It is propo sed however, that this ecosystem was largely composed of generalist an d opportunistic filter and suspension feeders which were dependent upo n a far higher input of nutrients than modern day reefal developments. Bacteria were probably the main primary producers, from both planktic and benthic cyanobacterial communities. The diversity of each buildup assemblage appears to be controlled by primary cavity size, the riche st fauna belonging to the highly tiered radiocyath-dominated community inferred to have lived in the deepest waters. The communities at Zuun e Arts can be compared with other buildups from the early Cambrian, an d with Lower Ordovician receptaculid-calcimicrobe-solitary sponge bioh erms known from the USA and Siberia.