TRIASSIC REEF PATTERNS

Abstract

Abstract The evolution of Triassic reefs is characterized by a three-step development accentuated by three brief reef blooms, a long-lasting evolutionary renewal of reef biota, a strong diversification of "reef windows" including anactualistic siliciclastic settings, and a global latitudinal reef expansion from the northern to the southern hemisphere starting during the Carnian. Triassic reefs were formed in the Tethys and in western and eastern parts of the Paleo-Pacific Ocean of Panthalassa during a time range of > 50 Myr Tethyan reefs are concentrated in the Western Tethys, in a southern reef zone off the northern coasts of Gondwana, and on various plates drifting from Gondwana to the north. Most Panthalassan reefs are known from terranes originally situated in the ancient Pacific and now attached to Asia and western North America. Compositional reef types include microbial reefs, sponge reefs, coral reefs, dasycladacean algal reefs, pelecypod reefs, and serpulid reefs. The end-Permian extinction event resulted in the sudden disappearance of high-diversity Late Permian reefs in southern China and a global gap of inetazoan reefs in the Early Triassic and Early Anisian, lasting > 8 Myr. A few Permian-type Lazarus taxa reappeared in the Norian, indicating a prolonged existence in unknown refugia, possibly on the shelves of Panthalassan or Tethyan terranes. The Early Triassic is characterized by the exclusive dominance of microbial reefs. The recovery of metazoan-domina ted reefsduringthe Middleand Late Anisian lasted about2 Myrand took place in the Western Tethys and on the South China Plate, The Ladinian to the Norian-Rhaetian interval exhibits phases of differentiation, faunal turnovers, extinctions, and the continuous latitudinal expansion of reef domains from the Ladinian to the Norian-Rhaetian within a time range of > 40 Myr. Four evolutionary reef units are evident both for scleractinian corals and sphinctozoid sponges as well as for dasycladacean green algae with respect to the taxonomic composition and diversity of the reef faunas: Middle Anisian to Early Ladinian, Late Ladinian to Early Carnian (Cordevolian or Julian J, first reef optimum), Late Carnian to Early Norian (faunal turnover), and Norian-Rhaetian (second reef optimum). The Triassic reef ecosystem underwent a distinct three-step development starting with Scythian microbial reefs, followed by Anisian to Carnian and Norian-Rhaetian metazoan-dominated reefs. The second and the third intervals are distinguished by the biotic composition, dominating reef builders, size and abundance of reef structures, widths of reef windows, frequency of reef domains, depositional setting, and latitudinal distribution patterns. The change and turnover between the steps 2 and 3 took place during thelale Carnian-early Norianinterval within a time span of about 13 Myr. During this tune intervalmajorextinctions, climatic perturbations, and significant paleooceanographic changes occurred. High late Carnian-early Norian extinction rates of reef biota were compensated by synchronous high origination rates. The significant transition from middle Triassic and Carnian sponge-dominated reefs to late Triassic coral-dominated reefs indicates a change in principal environmental controls, possibly connected with rearrangements of oceanic surface circulation patterns and related nutrient input. Norian-Rhaetian microbial-serpulid reefs in the northwestern Tethys represent a peculiar reef type different from coeval sponge-coral reefs facing open-marine basins. All Triassic reefs were severely affected by a global crisis near the Triassic-]urassic boundary and disappeared totally in the Western Tethys and Southern Tethys as well as on the Pacific mieroplates. Lowermost Jurassic (Hettangian) reefs are extremely rare. Some late Triassic reef organisms (corals, encrusters) continued into the Middle Liassic The recovery of the metazoan reef system in the Liassic lasted at least 10 Myr. Quantitative survey: Reef abundance increased from the Middle Anisian to the Late Anisian and Early Ladinian, and then abruptly in the Ladinian, followed by a first reef optimum in the earliest Carnian (Cordevolian). A decrease from very abundant to common reef occurrence took place during the Late Carnian. The frequency and regional distribution of reefs were set back in the Early Norian, followed by a new gradual increase, resulting in a second reef optimum in the Late Norian. During this time platform margin reefs in the Tethys and reefal buildups in oceanic settings and in back-arc positions of the Pa nthalassan Paleo-Pacific were established The frequency of Rhaetian reefs was similar to that after the first reef optimum. The two reef optima, defined by great abundance and wide regional distribution of reefs, embrace remarkably short time intervals (Cordevolian about 1-2 Myr, Sevatianabout2 Myr). Similarly, the Illyrian reef bloom spanned only a relatively short time range. There are conspicuous