Fritz Neuweiler

HALYSIS HØEG, 1932 IN ORDOVICIAN CARBONATE MOUNDS, TARIM BASIN, NW CHINA

Abstract The calcareous microproblematicum Halysis Høeg 1932 occurs in abundance in Ordovician fine-grained, reddish carbonate mounds rich in spar-cemented cavities (Katian, Tarim Basin, NW China). Morphological analysis of Halysis suggests a U-shape skeleton punctually attached to a soft substrate of carbonate sand and silt. The skeletons have a maximum width of 12 mm and consist of laterally branching tubes. The tubes display circular to laterally flattened outlines with a mean diameter ~ 125 &mgr;m. The tube walls consist of an inner and an outer layer of microcrystalline calcite, and a central layer of imbricated, radially arranged calcite tablets. An affinity of Halysis with extant siphonous calcareous green algae (Bryopsidales, Udoteaceae) is likely. If correct, Halysis represents a case of a green alga that acquired its skeleton de novo in accordance with sea-water chemistry (low genetic control, low-magnesium calcite, Ordovician calcite sea). Halysis carbonate mounds are low-relief, internally bedded, shallow-water packstone to grainstone banks. Spar-cemented cavities are Halysis-supported shelter cavities (~ 90%) and depositional cavities (~ 10%) produced from sediment-laden flows. The mounds formed as part of a shallow-subtidal carbonate ramp dominated by algal-pellet sand sheets. Autocyclic drivers (sand propagation via tides, storms) produced increments of sediment burial followed by episodes of omission and algal growth. These mounds should not be mistaken for “shallow-water carbonate mudmounds” nor for algal framework reefs. In terms of facies, texture, biostratinomy and primary porosity, these mounds are considered a miniature version of bryozoan-rich carbonate banks. Twisted and imbricated aggregates of fronds of Halysis produced shelter cavities making up ~ 5% of the total mound rock volume, thereby reducing accommodation space in sufficient quantity to explain mound formation. A review shows that Halysis presented herein displays the largest and most robust intrageneric growth form compared to occurrences of more basinal settings (Ordovician) as well as much younger carbonate deposits (Silurian to Devonian).

TAPHONOMIC FILTERING IN ORDOVICIAN BRYOZOAN CARBONATE MOUNDS, TRENTON GROUP, MONTMORENCY FALLS, QUEBEC, CANADA

ABSTRACT Late Ordovician bryozoan carbonate mounds are present in the upper part of the Deschambault Formation (Trenton Group) at the Montmorency Falls locality, northeast of Quebec City. These mounds are local features within a stratigraphic interval otherwise characterized by a bedded sequence of bryozoan-rich deposits. From the core of the mound to its margins and into the well-bedded off-mound sediments, there is a taphonomic gradient in terms of preservation, automicrite formation, fragmentation, transport, growth versus shelter porosity, and marine cementation. By ruling out both local seepage of nonmarine fluids (nonspecialist fauna, normal-marine carbon and oxygen stable isotopes, near PAAS rare earth element distribution patterns) and differential growth rate (bryozoan zooecium size), mound formation is explained by a positive taphonomic feedback mechanism. Centimeter-size patches of automicrite in mound cores are considered crucial in explaining subtle variations of microtexture, microfacies, and subsequent mound formation. Automicrite (M1) is dark gray, contains microbioclasts, has sharply defined, scalloped outer margins, is commonly gravity defying, and occasionally joins up to bridge multiple bryozoan skeletons. Clusters of microtubules are present within M1 and a first generation of infiltrated microcrystalline carbonate sediment (M2). The size, geometry, and arrangement of these microtubules suggest an origin either from the attachment of nonspicular, keratosan sponges, or from the assimilatory action of marine fungi (so-called Wedl tunnels). In conclusion, a subtle and small-scale secondary reinforcement of bryozoan skeletons by cryptobiontic ephemeral substrates is critical for the development of mounds out of an essentially level-bottom bryozoan community.