The Orera section (Calatayud Basin, NE Spain) - Universiteit Utrecht

Chapter 3
Paleoenvironmental reCOllstruction of a
middle Miocene alluvial fan to cyclic
shallow lacustrine depositional system
ill the Calatayud Basin (NE Spain)
Abstract
The middle Miocene sedimentary fill ofthe Calatayud Basin in northeastern Spain consists of
proximal to distal alluvial fan-floodplain and shallow lacustrine deposits. The sediments in the
study area, which accumulated along the northeastern margin ofthe basin, are divided into four
main facies groups which are characteristic for different sedimentary environments: i) proximal
and medial alluvial fan facies that comprise clast-supported gravels and subordinate sandstones
and mudstones, the latter exhibiting incipient pedogenic features; ii) distal alluvial fan facies,
formed mainly ofmassive mudstone, carbonate-rich paleosols and local carbonate pond deposits;
iii) lake margin facies, which show two distinct lithofacies associations depending on their
distribution, i.e. front or lateral, with respect to the alluvial fan system; and iv) cyclic shallow
lacustrine (to floodplain?) facies showing a remarkable cyclical alternation of green-grey and/or
red mudstone units and white dolomitic carbonate beds.
A variety of carbonate deposits is present in distal alluvial fan, lake margin and shallow
lacustrine facies. The dominant mineralogy of these carbonate deposits is dolomite. Varied
percentages of calcite are observed in carbonate-rich paleosols of the distal alluvial-fan facies
association and in lake margin carbonates. b 18 0_ and b'3C-values range from -7.9 to 3.0%0 PDB
and -9.2 to -1.7%0 PDB for carbonate-rich paleosols from the distal alluvial fan facies. More
negative b 18 0 and b 13 C values come from carbonate-rich paleosols and lake margin carbonates,
which extended in front ofthe alluvial fan systems where the carbonate beds show a wide scatter
of isotopic values. The mineralogy of these carbonates consists of calcite and dolomite. More
positive b'80 and b 13 C values correspond to dolomite beds deposited in a shallow lacustrine
environment. The isotopic composition of the dolomitic carbonates shows similarities with
carbonates from recent and ancient evaporitic lake systems.
The cyclic shallow lacustrine succession, as exposed in the Orera Composite Section (OCS), is
dominantly cOITlposed ofsm.all-scale m.udstone-carbonate cycles. Deposition ofthese cycles took
place in a low-gradient, shallow lake basin developed in a "shadow zone" between two major
alluvial fans. The accommodation space for the deposits was provided by gradual alluvial fan
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