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 μm. 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).