Indurated calcarenite and calcilutite dikes cut vertically through lower Oligocene volcanic sediments in the Oamaru region, east coast, North Otago, South Island, New Zealand. Individual dikes extend more than 10 m vertically and can be followed for much greater distances along regular or irregular strike. Field relations and paleontologic data suggest the limestone dikes were filled from above; at least 2 younger Oligocene formations contributed debris. Most dikes comprise several sharply defined layers of differing texture, composition, or structure parallel with dike walls. Subhorizontal layering, with either grading or cross-stratification, is also present but less common. Most composite dikes are less than 10 cm thick, but they range up to 30 cm; many of the dikes pinch and swell along both strike and dip. Flow structures are well developed between 2 vertical layers exposed in strike section along one dike. Limestone filling the interstices of a pillow lava was apparently supplied through channels now preserved as dikes.
Some dike layers are well-sorted bioclastic sand, but most layers consist of calcareous mud, with varying proportions of fossils (planktonic and benthonic forams, bryozoans, echinoderm debris, mollusk debris, brachiopod fragments, calcareous algae, sponge spicules), glauconite, tuff fragments, quartz, feldspar, rare pyroboles, clay minerals, intraclasts (probably mostly from older dike layers), and rare pellets.
The polyphase dikes are interpreted as infillings of fissures that repeatedly opened on the flanks of volcanic mounds. Infillings appear to be more varied than the possible source rocks and probably record transient sedimentation.