Kg. Taylor et Jhs. Macquaker, Early diagenetic pyrite morphology in a mudstone-dominated succession: theLower Jurassic Cleveland Ironstone Formation, eastern England, SEDIMENT GE, 131(1-2), 2000, pp. 77-86
Diagenetic pyrite in the mudstones and ironstones of the Lower Jurassic Cle
veland Ironstone Formation of eastern England exhibits two distinct morphol
ogies: framboidal pyrite, commonly associated with organic matter, and euhe
dral pyrite, associated with detrital clay pellets. These two morphologies
are mutually exclusive in occurrence. Framboidal pyrite is present in clay-
rich mudstones, ooidal ironstones, apatite-rich units and some silt-rich mu
dstones. Euhedral pyrite is present in silt-rich and sand-rich mudstones. d
elta(34)S isotopic analysis of six samples of pyrite suggests that both typ
es of pyrite morphology precipitated during early diagenesis from porewater
s with open access to overlying sea-water, although both probably acted as
sites for continued pyrite precipitation during burial. It is proposed that
framboidal pyrite precipitated from iron-dominated porewaters at sites of
sulfide supply (i.e. in the region of organic matter as a result of bacteri
al sulfate reduction) where, locally, sulfide production rates were high en
ough for porewaters to reach supersaturation with respect to FeS. Euhedral
pyrite also precipitated from iron-dominated porewaters, but sulfide produc
tion rates from organic matter was such that FeS saturation was not reached
at the sites of sulfide production. Instead, euhedral pyrite was precipita
ted directly from porewater when FeS? saturation was reached. The control o
ver pyrite morphology was probably the amount and reactivity of the organic
matter within the deposited sediments. The sand-rich mudstones contained l
ess reactive organic matter due to elastic dilution and deposition in shall
ower environments with O-2-rich bottom waters. The ironstones and apatite-r
ich units were deposited under very low sedimentation rates, and as a resul
t organic matter contents were very low and iron reduction dominated early
diagenesis, which inhibited sulfate-reduction. The presence of minor frambo
idal pyrite within these units, however, suggests that sulfide reduction to
ok place in micro-environments during early diagenesis. (C) 2000 Elsevier S
cience B.V. All rights reserved.