Harvey R. Young

Burial-dominated cementation in non-tropical carbonates of the Oligocene Te Kuiti Group, New Zealand

Abstract Cementation of bryozoan-echinoid-benthic foraminiferal temperate shelf carbonates of the Oligocene Te Kuiti Group, North Island, New Zealand, occurred mainly during subsurface burial. The calcite cements in the limestones are dominated by equant and syntaxial rim spar which typically becomes ferroan (given an iron supply) and, compared to the skeletal material with normal marine δ18O values from +2 to −1‰ , more depleted in 18O with depth of burial, the δ18O composition of bulk cement samples ranging from −1 to −7‰ . These trends reflect the establishment in pore waters during sediment burial of reducing conditions and gradually increasing temperatures (20–50°C), respectively. The δ13C values ( 0 to +3‰ ) of the cements remain the same as the host marine shells, suggesting the source of carbon in the cements was simply redistributed marine carbonate derived from shell dissolution. Two gradational burial diagenetic environments influenced by marine-derived porewaters are arbitrarily distinguished: shallow burial phase and moderate burial phase. During the shallow burial phase, down to 500–600 m sub-bottom depth, the carbonates lost at least 25% of their original porosity by mechanical compaction and were selectively cemented by non-ferroan or usually ferroan, variably luminescent, slightly 18O-depleted sparry calcite cement (δ18O −2 to −4‰ ), mainly as syntaxial rims about echinoid grains. These shallow-burial cements form less than about 10% of total cement in the majority of the limestones and their source was probably mainly mild intergranular dissolution of calcitic skeletal fragments accompanying the onset of chemical compaction. During the moderate burial phase, between about 600 and 1100 m sub-bottom depth, porosity loss continued (typically to about 70% of its original value) as a result of pressure-solution of calcitic bioclasts associated with more advanced stages of chemical compaction. This involved development of a wide variety of non-sutured and microstylolitic solution seams, including both single and composite, wispy or continuous, bedding-parallel types and non-parallel reticulate forms. The released carbonate was precipitated as ferroan (or non-ferroan where iron supply was negligible), dull luminescent, strongly 18O-depleted (δ18O −4 to −7‰ ), mainly equant calcite spar cement, occluding available pore space in the limestones.

Endolithic biodegradation of cool-water skeletal carbonates on Scott shelf, northwestern Vancouver Island, Canada

Abstract Biodegradation of shell material is widespread in the cool-water skeletal carbonate deposits on Scott shelf, northwestern Vancouver Island, and is especially evident in the large aragonitic bivalves, Glycymeris and Humilaria , major primary sediment contributors. Ten types of endolithic microborings have been identified in the shells, including representatives of green algae (e.g., Ostreobium quekettii ), blue-green algae (e.g., Plectonema terebrans, ?Scytonema sp.), fungi, bacteria and clionid sponges, as well as macroborings of phoronids, polychaetes and naticid gatropods. Microcrystalline carbonate is not precipitated in vacated bores. Boring physically weakens the shells, rendering them more prone to mechanical abrasion during sediment transport and bioturbation, and to biological abrasion by grazing benthos. Tumbling experiments demonstrate that the rate of carbonate mud production is much greater for bored as compared to fresh bivalve shells, and that mud production rates decrease with tumbling time because most endolithic microborings are confined to the periphery of grains. Boring also increases significantly the porosity and surface area of skeletal grains, and destroys their organic matrix, making them susceptible to maceration and dissolution on cool-water shelves. Fostered by the generally low rates of carbonate production and accumulation, many aragonitic bivalve shells on Scott shelf have become thoroughly degraded through a combination of endolithic microboring, maceration and dissolution within about 1000 years in ambient sea water. In geologic terms, such selective taphonomic loss of skeletal material may be considerable in ancient temperate-shelf limestones and should be evaluated when interpreting their paleoecology and paleoenvironments.

Sedimentology and petrography of mass‐emplaced limestone (Orahiri Limestone) on a late Oligocene shelf, western North Island, and tectonic implications for eastern margin development of Taranaki Basin

Abstract The Te Kuiti Group in North Wanganui Basin, North Island, New Zealand, of Oligocene ‐ earliest Miocene (Whaingaroan‐Waitakian) age, is dominated by calcareous siltstone, calcareous sandstone, and skeletal limestone. Exposures in the southwestern corner of the basin at Awakino Tunnel are distinctive because, compared with elsewhere, the group is generally thicker (>300 m), has strong dips (25–45°E), exhibits an up‐section decrease in the amount of dip, and the capping Orahiri Limestone includes several thick (up to 3 m) mass‐emplaced units containing a variety of 1–10 cm sized calcareous lithoclasts of older Te Kuiti Group rocks. Petrographic and δI8O and δ13C data suggest that the source deposits of these lithoclasts were cemented at relatively shallow subsurface burial depths (100–500 m) before their uplift and erosion. The lithoclasts so produced were rounded by abrasion in shoal water, often bored profusely by pholad bivalves, and sometimes encrusted by coralline algae, before being periodical...

Evidence of former evaporites in the Cambro-Ordovician Durness Group, northwest Scotland

Abstract Quartz nodules, exhibiting the morphology of nodular anhydrite, occur in two zones near the top of the Sangomore Formation (Cambro-Ordovician Durness Group) in a section on Balnakiel Bay, northwest Scotland. The nodules, up to 4 cm in diameter, occur in thinly laminated dolomite and are composed of megaquartz and length-slow chalcedony (lutecite and quartzine). Lath-like textural relicts, outlined by opaque inclusions, and minute crystals of anhydrite occur in the megaquartz and lutecite. The quartz nodules are interpreted as silica pseudomorphs after early diagenetic anhydrite nodules which developed in a peritidal setting. Quartz nodules in stratigraphically higher parts of the Durness Group may record the existence of other peritidal sediments in the same geographic area.

Silicification in Mississippian Lodgepole Formation, northeastern flank of Williston basin, Manitoba, Canada

Five types of replacement silica are recognized in the Lower Mississippian Virden Member carbonates on the northeastern flank of Williston basin: microcrystalline quartz, chalcedonic quartz, anhedral megaquartz, euhedral megaquartz, and stringy megaquartz. Silica tends to replace various bioclasts, and all except the stringy megaquartz also occur as non-replacive void-filling cement or as silica forming chert nodules and silicified limestone. Although crinoids, brachiopods, corals, bryozoans, molluscs, trilobites, forams, and ostracodes are present in the sediments studied, only the first three show evidence of silicification. Crinoids are commonly replaced by microcrystalline quartz whereas brachiopods typically by spherules of length slow chalcedony. Coalesced spherules, often in concentric rings (beekite rings), may form sheet-like masses on the surface of corals and brachiopods. Although bryozoans are common in the Virden Member, none showed any evidence of silicification. The difference in the susceptibility to silicification may be related to the shell microstructure, biological group, size of organism, skeletal mineralogy, and organic content of the bioclasts. Biogenic silica derived from the dissolution of siliceous sponge spicules is considered to be the most likely silica source for silicification. Most silica is believed to be released during early diagenesis before the sediments were deeply buried. The Virden Member carbonate may have experienced two episodes of replacement, the first affecting the bioclasts, the second producing silicified limestone and chert nodules.