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Isotopic and trace element evidence for submarine lithification of hardgrounds in the Jurassic of eastern England
Author(s) -
MARSHALL JAMES D.,
ASHTON MICHAEL
Publication year - 1980
Publication title -
sedimentology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.494
H-Index - 108
eISSN - 1365-3091
pISSN - 0037-0746
DOI - 10.1111/j.1365-3091.1980.tb01178.x
Subject(s) - lithification , geology , subaerial , cementation (geology) , aragonite , carbonate , geochemistry , mineralogy , calcite , cement , sedimentary rock , composite material , materials science , metallurgy
Geochemical and petrographic data suggest early submarine cementation of hardgrounds from the Lincolnshire Limestone Formation, Middle Jurassic, England. The three hardgrounds, from Cowthick, Castle Bytham and Leadenham quarries, developed in tidal‐inlet, on‐barrier and lagoonal sub‐environments of a carbonate barrier‐island complex. At Cowthick early composite (acicular‐bladed) radial‐fibrous cements, which pre‐date aragonite dissolution, completely fill intergranular pore‐space at the hardground surface; away from it isopachous fringing cements decrease in thickness. Microprobe analyses demonstrate zoning within the fringes with magnesium concentrations (> 2 wt % MgCO 3 ) higher than those in allochems or later, ferroan cement (≅0.5 wt % MgCO 3 , 1.7 wt % FeCO 3 ). At Castle Bytham early granular isopachous cements, which post‐date aragonite dissolution, occur within 5 cm of the surface. At Leadenham early lithification is superficial and represented by ferruginous crusts and micritic internal sediment. Late blocky cement fills residual pore‐space in all three examples. Carbon and oxygen isotopic composition of whole‐rock samples taken at intervals away from each hardground surface demonstrate the increasing proportion of late 18 O depleted cements (δ 18 O – 8 to – 10). Early cements must have a marine isotopic composition; different δ 18 O values from each hardground reflect the intensity of early lithification and exclusion of late cements at the hardened surface. There is no isotopic evidence for subaerial cement precipitation during possible emergence at Castle Bytham. Oyster samples (with δ 18 O, – 2.9 and δ 13 C, 2.4) give estimated palaeotemperatures of 22–25°C. Early cements from Cowthick are enriched in 18 O and 13 C (δ 18 O = 0 δ 13 C ≅ 3‰) compared to the oyster values. In conjunction with trace element data this is interpreted as evidence for high‐magnesium calcite precursor cements which underwent replacement in a system with a low water: rock ratio. The intensity of early lithification is related to depositional environment: maximum circulation of sea‐water producing the most lithified hardground (Cowthick). This is directly analogous to the formation of Recent hardgrounds.