High-frequency fluctuations in heavy mineral assemblages from Upper Jurassic sandstones of the Piper Formation, UK North Sea: Relationships with sea-level change and floodplain residence

Upper Jurassic sandstones deposited in a shallow-marine deltaic setting in the Piper Field of the Outer Moray Firth area, North Sea, show high-frequency fluctuations in apatite:tourmaline ratios that appear to be related to sea-level change. Because apatite and tourmaline are both stable during burial diagenesis and have similar hydraulic behavior, variations in the apatite:tourmaline ratio indicate either differences in sediment provenance or in the extent of floodplain weathering, apatite being unstable during weathering. Other provenance-sensitive heavy mineral ratios (rutile:zircon, monazite:zircon, chrome spinel:zircon) and mineral-chemical data from detrital garnet assemblages show that sandstones with high apatite:tourmaline have the same provenance as sandstones with low apatite:tourmaline. Fluctuations in apatite:tourmaline ratios are therefore attributed to the extent of weathering during floodplain residence prior to the sediment entering the marine system. Sedimentological data indicate that sandstones with high apatite:tourmaline were deposited during sea-level highstands, whereas sandstones with low apatite:tourmaline were deposited during lowstands. The implication of this observation is that during sea-level lowstands, sediment undergoes more prolonged floodplain residence than during highstands, apparently the direct result of the increase in areal extent of the floodplain. The fluctuations in apatite:tourmaline offer an opportunity for high-resolution correlation in the Piper Field. If similar patterns become apparent in other areas, variations in apatite:tourmaline ratios could also provide a basis for identifying highstand and lowstand events, and help establish whether deep-water submarine fan sandstones were deposited during highstands or lowstands.