Concha Arenas Abad

This work provides new insights to assess the factors controlling carbonate
deposition in the siliciclastic fluvial systems of rift basins. Sedimentological
and stable-isotope data of microbialites and associated carbonate facies,
along with regional geological information, are shown to reveal the influence
of climate and tectonics on the occurrence and attributes of carbonate
deposits in these settings. The Vega Formation – a 150 m thick Lower Kimmeridgian
siliciclastic fluvial sequence in Asturias Province (northern
Spain) – constitutes a candidate for this approach. This unit includes varied
facies (stromatolites; rudstones, packstones and wackestones containing oncoids,
intraclasts, charophytes and shell bioclasts; marlstones and polygenic
calcareous conglomerates) that formed in a low-gradient fluvial–lacustrine
system consisting of shallow, low-sinuosity oncoid-bearing channels and
pools within marshy areas, with sporadic coarse alluvial deposition. The
sedimentological attributes indicate common erosion by channel overflow
and rapid lateral changes of subenvironments caused by water-discharge
variations. The carbonate fluvial–lacustrine system developed near uplifted
marine Jurassic rocks. The occurrence of the system was conditioned by normal
faults (active during the deposition of the unit) that favoured: (i) springs
of HCO3–Ca-rich water from a Rhaetian–Sinemurian carbonate rock aquifer;
and (ii) carbonate deposition in areas partially isolated from the adjacent
siliciclastic fluvial system. The microbialite d13C and d18O values support
deposition in a hydrologically open system, fed by ambient-temperature
meteoric water, with riparian vegetation. Three types of lamination in the
stromatolites and oncoids reflect distinct morphological types of cyanobacterial
communities. The textural pattern of lamination parallels d13C and
d18O changes, suggesting short-term cycles of precipitation and temperature.
A moderately to strongly contrasted seasonal and/or pluriannual precipitation
regime is inferred from the cyclic d13C pattern of the lamination and
from the discontinuous and asymmetrical growth of oncoids. Thus, the isotopic
and sedimentological attributes of the carbonate deposits were linked
to short-term climate changes associated with semi-arid conditions, consistent
with the studied climatic zone

The Pleistocene and Holocene tufas of the Añamaza valley (stepped buildups,
up to 70 m thick, along the valley) consist of several depositional stages
separated by erosional surfaces. Eight associations of tufa and related carbonate
facies, plus minor polygenic detrital facies, represent the processes that
occurred in different fluvial and related environments. The bedrock lithology
and structure controlled the location of the knickpoints along the valley and
allowed separation of two stepped stretches with distinct conceptual facies
models. The moderate-slope model includes extensive standing-water areas
dammed by barrage-cascades. In the lakes, bioclastic silts, sands and limestones
along with phytoclastic and marly, at places peaty, sediments formed.
Abundant stem phytoherms account for extensive palustrine areas. The
high-slope model consists of smaller dammed areas between close-up cascades
and barrage-cascades, which were composed primarily of moss phytoherms
and phytoclastic tufas. An outstanding feature is the extensive steep
reach with phytoclastic and polygenic detrital sediments, and stepped cascades
consisting of stromatolitic and moss phytoherms. There, the steep
slope limited the preservation of stem phytoherms and favoured erosion.
The geometry and thickness of the sedimentary fill (wedge-shaped units
composed of cascade and barrage-cascade deposits downstream, and
dammed and gentle-sloped channel deposits upstream) are therefore different
for each model. Multi-storey wedges are a distinctive feature of the highslope
model. The initial knickpoint geometry and the tufa aggradation/progradation
ratio on such steep surfaces (for example, related to changes in
discharge) controlled the growth style of the cascades or barrage-cascades
and, hence, the extent, thickness and vertical evolution of the upstream
deposits. The sedimentological attributes and stable-isotope composition of
the carbonate facies suggest a higher and more variable precipitation/evaporation
ratio during the Pleistocene than during the Holocene, consistent with
an overall decrease in the river discharge. This evolution was coupled with
warm conditions, which prevailed during the stages of tufa formation. These
results may help to assess architectural patterns in interpreting other basins,
and underscore the significance of tufas as records of past hydrology and
climate.
Keywords Fluvial tufa depositional architecture, hydrology and climate,
Pleistocene and Holocene, river slope changes, sedimentary facies models.

Stratigraphic and sedimentologic analyses and geochemical data from the middle and late Miocene record in the
southwestern-central sector of the Ebro Basin have allowed the evolution of its Neogene final fill stage to be characterized and its
controlling factors to be discerned. The focus of these analyses was the distinct depositional environments that occurred over time
(tecto-sedimentary units T6, T7, and T8). Twelve main associations of lacustrine, palustrine, fluvial, and alluvial facies revealed
that the sedimentary processes leading to the formation of the units occurred in different environments in units T6 and T7 (middle
and late Aragonian, i.e., Langhian–Serravallian) and in unit T8 (Vallesian–Turolian?, i.e., Tortonian). The depositional
environments of units T6 and T7 encompassed a closed, shallow carbonate lake system with wide palustrine fringes. The lake was
fed by small sandy and gravelly channels flowing from the southwest (Iberian Range) and a sandy fluvial system from the north
(Pyrenees). Inputs were also received from a southeastward-flowing axial fluvial system (AS). The mouths of these systems
commonly formed small deltaic sandy lobes on the northern lake margins. The evolution of unit T6 and in particular that of unit T7
was greatly influenced by Pyrenean tectonics. The depositional environments of unit T8 included the occurrence of the AS, but this
system was soon replaced by an open, shallow carbonate lake surrounded by wide and shallow oncoid-bearing channels and
palustrine tufaceous areas on its west-southwest margin. Distinct depositional events from the south and west, which involved intra-
T8 erosion, resulted in minor coarse alluvial deposits. The carbonate system of unit T8 occurred in an open depression created after
local erosion of the pre-T8 deposits and subsequent northward shifting of the AS since early deposition of unit T8, the origin of
which was related to late contractile deformation in the northwestern Iberian Range. Bicarbonate- and calcium-rich water from the
Mesozoic carbonate rocks in the Iberian Range fed the T8 system. Sedimentologic, d13C, d18O, and trace-element data support the
change from closed to open drainage, and strongly suggest a general climate trend toward wetter and cooler conditions, which is
consistent with the middle–late Miocene climate evolution of other European regions. Despite the general timing coincidence
between unit T8 deposition and the opening process of the Ebro Basin, the attributes of the T8 deposits are inconclusive to support
the previous hypothesis that the fluvial–lacustrine–palustrine system could be directly connected to the emptying drainage of the
basin into the Mediterranean Sea. Instead, the results of this study demonstrate the combined influence of tectonics and climate on
the evolution of the final fill stage of this part of the Ebro Basin. These results may be useful for assessing the influence of such
allocyclic factors on the sedimentologic, hydrologic, and geochemical evolution of other intermontane lake basins.

A 12-year study (2000–2012) of the stable isotopes of recent fluvial tufas has been performed in the Monasterio de
Piedra Natural Park (River Piedra, NE Iberian Peninsula), an area of Mediterranean climate with intense tufa deposition since
the Pleistocene. The biannual monitoring of the tufa calcite d13C and d18O signatures and of the water d18O value
has demonstrated a clear seasonal pattern for the tufa-calcite oxygen isotope composition, with less negative values for cool
periods, which is consistent with regional temperature oscillations. The tufa calcite d13C values only occasionally exhibit a
pattern, which is inconclusive and opposite to the d18O clear pattern. The water d18O signature has a distinct seasonal pattern,
also opposite to the tufa-calcite d18O clear pattern. The temperature calculations from the tufa-calcite d18O values and average
water d18O values agree with the seasonal pattern of measured water temperatures, with a mean difference of 2.3 uC between
the calculated and measured temperatures.
The anomalous values recorded for both the water and the tufa-calcite oxygen isotope composition during a two-year period
are characterized by smoothing of the seasonal variations of the tufa calcite and by reversal of the water pattern. There is also
an increase in the differences between the measured and estimated temperatures. The detected anomaly is roughly synchronous
with a change in the isotopic signature of the regional precipitation d18O values.
This contribution demonstrates that tufas in this depositional and climatic context are good indicators of seasonal
temperature oscillations, and that tufas can also record other interannual environmental changes such as variations in the
isotopic composition of precipitation. These results can be extrapolated to ancient tufa systems developed in similar contexts,
which would expand the potential of tufas as high-resolution records of paleoenvironmental conditions.

The tufa record and hydrochemical characteristics
of the River Piedra in the Monasterio de Piedra Natural
Park (NE Spain) were studied for 6 years. The mean
discharge of this river was 1.22 m3/s. The water was
supersaturated with calcium carbonate. The HCO3
-, Ca2?
and TDIC concentrations decreased along the 0.5-km-long
studied stretch, whereas the calcite SI showed no systematic
downstream or seasonal variation over the same stretch.
Several sedimentary subenvironments exist in which four
broad types of tufa facies form: (1) Dense laminated tufa
(stromatolites), (2) Dense to porous, massive tufa, (3) Porous,
coarsely laminated tufa with bryophytes and algae, and
(4) Dense, hard, laminated deposits in caves. The halfyearly
period thickness and weight of sediment accumulated
on 14 tablets installed in several subenvironments
showed that the deposition rate was greater in fast flowing
river areas and in stepped waterfalls, and lower in slow
flowing or standing river areas and in spray and splash
areas. Mechanical CO2 outgassing is the main factor controlling
calcite precipitation on the river bed and in waterfalls,
but this process does not explain the seasonal changes
in depositional rates. The deposition rates showed a halfyearly
period pattern recorded in all fluvial subenvironments
persistent over time (5.26 mm, 0.86 g/cm2 in warm
periods; 2.26 mm, 0.13 g/cm2 in cool periods). Mass balance
calculations showed higher calcite mass values in
warm (21.58 mg/L) than in cool (13.68 mg/L) periods. This
biannual variation is mainly attributed to the seasonal differences
in temperature that caused changes in inorganic
calcite precipitation rate and in biomass and the correlative
photosynthetic activity. Tufa sedimentation was therefore
controlled by both physicochemical and biological processes.
The results of this study may help test depositional
rates and their environmental controls and thus assess the
climatic and hydrological significance of ancient tufas in
semi-arid conditions, in particular in the Quaternary.

High-gradient, stepped fluvial tufa systems with dammed areas existed in the River Añamaza valley
(NW Iberian Ranges, Spain) during Quaternary times. Single deposits range from a few meters to about 70
m thick, in which prograding-aggrading wedges separated by erosional surfaces exist. Several episodes of
tufa formation have been distinguished by means of U-series, Amino-acid racemization and radiocarbon
techniques. These correlate to MIS 8, 7, 5 and 1. The presence of MIS 9 is uncertain, as chronological data
may also correspond to older stages. Most tufas in this area formed in MIS 5. Distinct tufa episodes can
also be distinguished in the Holocene. These are the first chronological data presented for one of the
northernmost Quaternary tufa systems in the Iberian Ranges.

"Physical and hydrochemical parameters and sedimentation rates were monitored twice
a year from August 1999 to March 2003 at the Monasterio de Piedra area (NE Spain). Different tufa
facies related to distinct fluvial subenvironments were characterized and the isotopic composition
of water was analysed seasonally. Sedimentary features (thickness, texture and structure) and
stable isotope composition of the seasonal record on tablets were analysed. The seasonal intervals
were identified from six-monthly thickness measurements on tablets.
Sedimentation rates had a strong seasonal pattern with higher values in warm periods than in
cool ones, although erosive events and sporadic, warmer-than-normal climate conditions altered
it. Three main types of fluvial facies were studied in detail: dense, stromatolitic tufa; dense to
porous, massive tufa; and spongy, moss- and alga-bearing, crudely laminated tufa. Textural features
of deposits from warm and cool periods had a variable pattern.
The sediment d18O composition showed a rhythmic variation, with higher values in cool periods
and lower in warm ones, caused by the fractionation due to seasonal temperature variations. The
calculated temperatures for a theoretical equilibrium precipitation accord with the actual measured
temperatures. The sediment d13C composition had an irregular pattern, indicating that other
parameters than temperature intervened in the fluvial system."

The Santanyí Limestone, a 30–35-m thick upper Miocene limestone succession cropping out in Mallorca,
contains abundant microbialite deposits, the shape, microstructure and texture of which was controlled by
environmental factors: depth, energy and salinity. Three main types of microbialites are distinguished: (1)
domed (DNOS) and stratiform, mostly undulate (UNOS) non-oolitic stromatolites, (2) undulate oolitic laminites
(UOL) and (3) domed-oolitic thrombolites (DOTs). Based on lithofacies associations and occurrence of
microbialite types, the Santanyí Limestone succession is subdivided into five stratigraphic units (I to V)
separated by sharp surfaces. Within units II, III and V, the vertical evolution of microbialites was induced by
changes in accommodation space/depth: (1) intertidal/very-shallow subtidal conditions at the base were
induced by flooding over a wide area, (2) continued sea-level rise caused submergence to subtidal conditions,
and (3) a significant bathymetric decrease created the sharp surface bounding these units.
In units II and III, NOS accumulated in variable energy and depth conditions, as buildups with thick, somewhat
discontinuous and mostly non-isopachous lamination, surrounded by oolitic grainstones with wave and
current structures and oolitic intraclasts. In contrast, thin and generally regular and smooth lamination of NOS
in unit V suggests, along with the absence of oolite grainstones and macrobiota, calm and restricted, maybe
more saline, conditions.
UOL, consisting of oolitic layers separated by thin micritic laminae, developed adjacent to NOS in units II and
III and to DOT at the lower part of unit III, in shallow-water and low-energy conditions. Both ooids andmicrite
laminae have evidence for biogenesis. Micritized ooids containing microbial remains are common. Micritic
laminae in UOL and the dark micritic laminae in NOS are thought to represent bacterially enhanced calcite
precipitation and lithification during periods of low sedimentation.
Oolitic thrombolites containing macrobiota are only present in unit III. They represent deeper and openmarine
conditions affected by high-energy events, in which microbially mediated precipitation favoured
microbialite accretion and lithification.

A variety of meteoric diagenetic features reveal the development of a syngenetic karst on lacustrine deposits of the
Ebro Basin. Diagenetic processes that operated on lacustrine laminated and stromatolitic carbonates include the following.
(1) A first syndepositional stage with processes such as dolomitization, desiccation and related breccia formation and
sulphate precipitation, either as lenticular gypsum crystals or nodules. This stage took place under progressive evaporation
due to lake level fall, when the previous carbonate deposits became exposed as a supra-littoral fringe surrounding saline
mud flats of adjacent sulphate depositional environments. (2) A second early diagenetic stage in which processes such
as sulphate dissolution and collapse brecciation, dedolomitization, calcite spar cementation and silicification occurred as
a result of meteoric water input that caused a progressive rise in lake level. Light isotopic compositions (d13C and d18O)
of diagenetic calcites, versus heavier compositions in primary laminated and stromatolitic limestones, confirm a meteoric
influence. The syngenetic karst is best developed at the boundary between two allostratigraphic units and coincided with
one of the extensive stages of sulphate deposition at the end of the Early Miocene. The karst facies occurred in an area
that was a low-relief barrier that separated two sites of sulphate deposition during low lake levels. This indicates that
the karst development was controlled by topographic changes within the basin and record a shift from arid to wetter
climatic conditions, as suggested by the overlying freshwater carbonate deposits. The presence of diagenetic features such
as those described in the central Ebro Basin affecting saline lacustrine carbonates is relevant because they can be used as
indicators of subaerial exposure periods in terrestrial environments and they also reveal important palaeogeographic and
palaeoclimatic events of basinal extent

In the central part of the Ebro Basin (Spain), north of the Ebro River, three allostratigraphic or tectosedimentary
units differentiated within the latest Oligocene–Late Miocene (upper Agenian–Vallesian) interval are sedimentologically
characterized. These deposits represent carbonate and sulphate lacustrine environments fed by Pyrenean alluvial systems
(Luna system and Huesca system). In each unit, several facies associations or lithofacies were mapped. These interfinger
through time: gypsum lithofacies occupy the southernmost part of the study area and are surrounded by carbonate and=or
detrital lithofacies to the north. Each lithofacies consists of different carbonate, sulphate and=or detrital facies, arranged
in simple sequences, decimetres to metres thick. The four types of sequences characterized reflect a complex shallowing
evolution involving carbonate or sulphate contexts. Thus, each lithofacies represents the sedimentation in a particular
lacustrine or alluvial subenvironment. A single lacustrine system is proposed for the central part of the basin. Its evolution
followed cycles of water level variations, which caused kilometre-scale migrations of the lake shoreline. In this model,
two distinct situations or lakes alternated through time. (a) One represents high levels associated with a single body of
dilute carbonate water, where massive carbonate facies formed influenced by organism proliferation. These lakes had wide
palustrine vegetated margins, in which bioturbated facies developed. (b) The other represents low levels that correspond
to a playa-lake model. Laminated sulphate deposits formed in the lake waters, while nodular gypsum originated in the
surrounding saline mud flats and within the exposed, previously deposited carbonate sediments. Transitions from one
situation to the other, including minor water level fluctuations within carbonate sedimentation conditions, were marked
by the development of laminated and stromatolitic facies. Lake level variations were caused by climatic cycles and
gave rise to the simple sequences and to the lithofacies lateral relationships recognized in this study. Correlation among
tectosedimentary units in the studied Pyrenean domain and in the Iberian domain of the Ebro Basin shows that for units
N1 and N2 the facies that resulted from dilute carbonate waters are rare in the lacustrine areas of the Iberian margin. This
indicates that a strong hydrological contrast existed between both regions. On this basis, a theoretical asymmetrical facies
model is proposed for the single lacustrine basin that extended over these regions.

"The lacustrine carbonate facies from three allostratigraphic units (N1, N2 and N3) of Miocene age in the Los
Monegros region (Ebro Basin) are characterized isotopically. The dominant facies are marls, laminated limestones,
stromatolitic limestones, massive limestones and bioturbated limestones. These are associated with lacustrine sulphate
deposits and distal alluvial facies. Palaeohydrological reconstructions of the study area are supported by the isotopic
and the mineralogical composition of the carbonates. Massive and bioturbated limestones occupy a very similar
compositional domain and have the lowest isotopic compositions of all the facies (--9%o<818OpDB<--4.5%0 and
--6.4%0 < 813CpDB < --0.4%o), reflecting a short residence time of the water and variable, commonly high influence of
biogenic CO2. Laminated limestones and stromatolites define a comparatively enriched domain with filso values
ranging from-6 to 0%0 and fi13C values ranging from-3.5 to 0%0. These values indicate stronger evaporation and
enhanced a2C assimilation due to intense biological activity during periods of longer water residence time. Marls
have isotopic values intermediate between these two domains and correspond to periods of lacustrine dilution.
Dolomite-bearing samples, mainly laminated and stromatolitic facies, show a A18ODoL.CAL=7.5%0 with respect to
calcite from the same facies, while 8~3C values show very little enrichment (up to 1%0). The positive correlation (r=
0.85) between 180 and dolomite contents suggests that dolomite resulted from the progressive evaporative concentration
of a single water mass and not from the mixing of waters of different compositions. The dolomite is thought to be
primary or, at most, the product of very early diagenetic processes. The isotopic composition of the facies and their
spatial and temporal variations depend on the depositional environment and were constrained by changing regional
paleogeography. Isotopic trends from units N~ to N 3 (Upper Agenian-Vallesian period) display a regular depletion
in heavy isotopes that indicates a climatic change towards wetter conditions."