Lowstand carbonates, highstand sandstones?

ExpresSed

Lowstand carbonates, highstand sandstones?

T.C. Bracherta,*, M.H. Forsta, J.J. Paisb, P. Legoinhab, J.J.G. Reijmerc

a Institut fur Geowissenschaften, Johannes Gutenberg Universitat, Becherweg 21, D-55099 Mainz, GermanybFaculdade de Ciencias e Tecnologia, Centro de Estratigrafia e Paleobiologia, Universidade Nova de Lisboa, Quinta da Torre,

2825 Monte de Caparica, PortugalcGEOMAR, Wischhofstr. 1-3, 24148 Kiel, Germany

Received 15 March 2002; accepted 11 September 2002

Abstract

The sedimentary facies, sediment dynamics and sequence architecture of modern high-energy shelves in the mid and high

latitudes are largely governed by wave abrasion processes. Cool-water carbonates may form there, if the influx and/or net

accretion of siliciclastics is kept at a minimum. Little dilution of the carbonate produced in situ is generally promoted by a

wide ‘‘epicontinental’’ shelf, subdued topography of the adjacent mainland, the predominance of limestone outcrops, and an

arid climate. The aforementioned requirements are rarely met, and thus will automatically lead to the formation of mixed

siliciclastic–cool-water carbonates. Such an example is found in the Early to Mid-Miocene Lagos-Portimao Formation

(Algarve, S-Portugal), which formed on a narrow high-energy shelf of the Atlantic Ocean that was bounded by a mountain

range. The sediments of the formation consist of fossiliferous sandstone (FS), shell beds, and rhodolith blankets. Along strike,

the stratification of the formation is monotonous for tens of kilometres and well exposed in coastal cliffs, whereas no outcrops

of dip sections exist. The bulk skeletal composition of the sediments is typical for the warm-temperate climatic zone: various

endo- and epibenthic bivalves, bryozoans, coralline algae, echinoderms, gastropods, and large foraminifers (Heterostegina). In

some very rare beds, a few isolated, not framework-forming specimens of zooxanthellate corals (Porites, Tarbellastrea)

indicate temporally elevated surface water temperatures close to the lower threshold of the coral reef ecosystem. In

sandstones, the fauna is well preserved and burrowing bivalves are commonly found in life position. In limestone beds, the

state of preservation of the grains ranges from intact to disintegrated and abraded specimens. We infer an accumulation of the

shell beds through winnowing of fine materials (siliciclastic sand and carbonate mud) at wave abrasion depth and

concentration of calcareous skeletons associated with the subsequent attraction of new epibiota in a complex shell bed. The

vertical alternation of fossiliferous sandstone and shell beds, and in-phase variations of the ‘‘Photo Index’’ (photic biota vs.

bryozoans) and ‘‘Bryozoan Index’’ (bivalves vs. bryozoans) is envisaged to document variations of water depth (and sea

level). Sandstone units built up when wave abrasion depth (WAD) rose above the sea floor during TST (and early HST),

whereas the shell beds formed during LST when the WAD for sand intersected with the sea floor. Clastic sediments were

probably brought on the outer shelf during early transgression, and by longshore currents. Sea-level signatures inferred in the

mixed siliciclastic–cool-water carbonate shelf setting of S-Portugal therefore significantly deviate from conventional concepts

of carbonate sequence stratigraphy, which were developed for flat-topped platforms. Successful interpretations of ancient

0037-0738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.

PII: S0037 -0738 (02 )00329 -9

* Corresponding author.

E-mail address: [email protected] (T.C. Brachert).

www.elsevier.com/locate/sedgeo

Sedimentary Geology 155 (2003) 1–12

mixed sequences must therefore take into consideration the processes of production, concentration and accretion of the

carbonate sediments.

D 2002 Elsevier Science B.V. All rights reserved.

Keywords: Temperate carbonates; Shell beds; Sea-level signatures; Miocene; Portugal

1. Introduction

When inundated, the carbonate factory of flat-

topped platforms runs on its maximum efficiency

and rates of production commonly exceed rates of

creation of new accommodation space. Hence, during

sea-level highstands, large volumes of carbonate are

preserved and exported into the surrounding periplat-

form realm (Mullins, 1983a,b; Droxler et al., 1983;

Droxler and Schlager, 1985; Reijmer et al., 1988,

1991; Haak and Schlager, 1989; Glaser and Droxler,

1991). This concept is now widely accepted (Schlager,

1992, 1999; Schlager et al., 1994). In contrast, during

lowstands or falling base level, carbonate sediment

production is halted and large areas of the platforms

may emerge above sea level leading to subaerial

diagenesis related with rapid cementation and karsti-

fication (Dravis, 1979, 1996). Consequently, the

periplatform realm becomes relatively carbonate sedi-

ment starved. Within attached carbonate platform

systems, a lowering of base level encourages erosion

on the mainland, recycling of coastal materials and

dumping of clastic sediment into the ocean (Davies et

al., 1989). This new sediment dispersal system might

also hamper the maximum potential carbonate pro-

duction and reduce carbonate production (Esker et

al., 1998; Ferro et al., 1999). The geological record of

this process commonly is a pronounced clastics–

carbonate cyclicity even on the platform top. System-

atically, this sequence model has only been tested

within tropical carbonate systems made up of photo-

zoan-dominated biological systems. Beyond the

tropics, within the cool-water carbonate realm of

James (1997), previous sedimentological studies con-

centrated on the ‘‘nice’’ and pure carbonate examples,

whereas the dirty mixed carbonate–siliciclastic suc-

cessions were largely neglected (Alexander, 1996;

Carannante and Simone, 1996; Carter et al., 1998;

Martın et al., 1996). In this paper, we present the

study of a mixed cool-water carbonate–siliciclastic

depositional system of the Lagos-Portimao Formation

of southern Portugal. The skeletal component com-

position fits the formation within the warm-temperate

segment (Betzler et al., 1997) of the cool-water

realm. Water temperatures always stayed below the

reef-threshold (Brachert et al., 1996), although the

very rare occurrence of isolated (not frame-building)

zooxanthellate corals (Porites, Tarbellastrea) implies

short periods with increased sea surface temperatures.

The detailed analysis of biogenic facies and tapho-

nomic attributes led us to present a model for mixed

cool-water carbonates, which significantly deviates

from the expected patterns of flat-topped platforms:

lowstand deposition of carbonate and highstand

development of sandstone.

1.1. Methods

The outcrops along sea cliffs allow a bed-by-bed

correlation of individual sections. Field descriptions of

sedimentary facies were complemented by measure-

ments of natural gamma-radiation at an equal distant

10 cm spacing using a portable Exploranium Gamma-

Ray Spectrometer. The point counts of thin sections

(>300 points, grain bulk method) followed the stand-

ard procedures described by Flugel (1982). The com-

positional data were normalized to 100% for skeletal

grains, and used to calculate proxies of water depth.

The ‘‘Bryozoan Index’’ (%bryozoans/%bryozoans +

%bivalves) adopted from James et al. (2001) covers

the upper slope and mid-shelf environment, whereas a

new ‘‘Photo Index’’ (%coralline algae +%large fora-

minifers +%zooxanthellate corals/%coralline algae +

%large foraminifers +%zooxanthellate corals +%bry-

ozoans) is supposed to cover the mid to inner shelf en-

vironment.

2. Geological setting

The Atlantic coast of southern Portugal (Algarve

area) is widely known for its spectacular yellowish-

T.C. Brachert et al. / Sedimentary Geology 155 (2003) 1–122

white sea cliffs and beaches. The cliffs are 30–80 m

high and form the abrupt end of an elevated coastal

terrace, f 10 km wide and f 100 m high. To the

north, the terrace is attached to the Serra do Caldeirao, a

mountain range 900 m high and dominated by Late

Palaeozoic metasedimentary rocks (flysch facies). The

coastal terrace consists of shallow-marine carbonates

and evaporites of Mesozoic age, which were tilted prior

to the formation of a wedge of Neogene to Quaternary

open shelf carbonates and clastics. The Lagos-Porti-

mao Formation is made of fossiliferous sandstone and

coarse skeletal limestone, and is Early–Mid-Miocene

in age (Burdigalian to Serravallian). The Burdigalian–

Early Serravallian segment described in this paper is

well exposed in central Algarve between the towns of

Portimao and Albufeira (Fig. 1). The Tortonian Fine

Sands (no formal lithostratigraphic name) and Cacela

Fm. are Early and Late Tortonian in age, and covered

by the red Plio-Pleistocene Olhos de Agua sands

(Antunes et al., 1997, 2000; Antunes and Pais, 1993;

Breisig, 2001; Forst et al., 2000). The stratal geo-

metries of the shelf deposits are not known because

of poor outcrop conditions, however, the stacking

patterns within the Miocene timeframe strongly sug-

gest a 3rd order eustatic control (Figs. 1 and 2).

3. Results

The Burdigalian–Early Serravallian segment of the

Lagos-Portimao Formation is 60 m thick and displays a

regular alternation of decimetric fossiliferous sand-

stone and coarse calcirudstone. Along strike, which

is within the sea cliffs, individual sandstone and lime-

stone beds can be traced for more than 40 km.

Correlations rely on lithologies and gamma-ray signa-

tures (Fig. 3). Within the outcrops studied, no signifi-

cant traces of erosion or nondeposition were observed.

This observation is supported by high-resolution Sr-

isotope datings, which suggest rather continuous sed-

imentation (20 cm/ka) not punctuated by significant

hiatuses (Antunes et al., 2000; Breisig, 2001; Forst et

al., 2000). In a dip direction, the outcrop situation does

not allow the documentation of the stratal geometries

and sequence architectures. Because outer and mid-

shelf depositional systems are only partially controlled

by variations in accommodation space, we used water

depth reconstructions as a proxy of relative sea-level

changes instead. The bathymetric reconstructions are

Fig. 1. Distribution of the Lagos-Portimao Formation along the southern Atlantic coast of Portugal. Inundated area and paleo-coastline modified

from (Pais, 1982).

Fig. 2. Miocene stratigraphy of the central Algarve area (Lagos-

Portimao Formation). Third order eustatic sea-level curve from

(Hardenbol et al., 1998). Brick signature shows predominance of

limestone, grey shading predominance of sandstone.

T.C. Brachert et al. / Sedimentary Geology 155 (2003) 1–12 3

based on taphonomy and biofacies. In this paper, we

present the section Leixao do Ladrao as a representa-

tive example for central Algarve (Fig. 3).

Three lithologies are present: (1) fossiliferous

sandstone (FS; Fig. 4), (2) coarse skeletal rudstone

(SR; Fig. 5), and (3) rhodolith rudstone (RR).

3.1. Fossiliferous sandstone (FS)

This lithology consists of medium-sized partially

cemented sandstone with mottled structure and no

visible stratification. In an outcrop scale, endobenthic

and epibenthic bivalves, bryozoans, irregular echi-

noids (including large Clypeaster), and large benthic

foraminifers (Heterostegina) predominate, with minor

proportions of gastropods and asteroideans (starfish).

The matrix consists of a micritic carbonate with lesser

proportions of noncalcareous fines. Except for scat-

tered asteroidean ossicles, the skeletal material is well

preserved, i.e. the endobenthic bivalves are double

valved and predominantly preserved in life position.

Fragile bryozoan colonies forming fenestrate cups or

foliate shrubs tend to be well preserved. Because of

their typifying bryozoan or large foraminifer faunas

and lateral continuity, some beds of FS do represent

excellent stratigraphic marker beds (Fig. 4; Forst et

al., 2000).

Interpretation: The skeletal content of the FS

implies a fully marine shelf setting. Physical sedi-

mentary structures are not preserved due to intense

burrowing. Skeletal biota are well preserved which

points to a rather quiet depositional setting below

wave abrasion depth (WAD). The ‘‘Bryozoan

Index’’ is commonly high which implies outer shelf

depths; in the southern Australia province, this

index is reached in >75-m water depth (cf. James

et al., 2001). Large benthic foraminifers have a

broad depth range, and their presence fits a deposi-

Fig. 3. Leixao do Ladrao section showing bathymetric indices, matrix content (point-count results) and outcrop gamma ray log. Sandstone and

limestone marker beds have grey shading. Diagram in lower right corner shows bathymetric indices calculated for various modern sediment

facies of the S-Australia shelf plotted against mean water depth (data from Tables 1 and 2 of James et al., 2001). Bathymetric indices: Bryozoan

Index (black dots) is high (f 1) in deep water and low (f 0) in shallow water. Photozoan Index (white boxes) is high (f 1) in shallow water

and low (f 0) in deep water.


tional environment in the lower segment of the

photic zone, in the southern Australia province their

lower limit of life occurrence is at 90 m (James et

al., 2001).

3.2. Coarse skeletal rudstone (SR)

This lithology consists of a partly lithified skel-

etal hash with a low carbonate and siliceous matrix

content (point counting and gamma-ray spectra),

some sand, and no visible stratification nor any

specific orientation of grains. Biofragments originate

from bivalves, bryozoans, echinoids (including large

Clypeaster), balanids, large benthic foraminifers,

gastropods, and coralline algae. These skeletal

grains represent a mixture of epi- and endobenthic

forms, which were never found in life position (or

any articulated bivalve shells), and exhibit various

stages of disintegration (ranging from undamaged to

completely broken). Some reddish shell beds of

disarticulated pectens or oysters with little associated

biota exhibit some stacking of shell beds, which

results from variations in the style of preservation of

the shells, their orientation or packing. The shell

beds are deeply penetrated by bioturbation. The

reddish, stacked shell bed facies may grade laterally

into fossiliferous sandstone (FS).

Interpretation: From the low to intermediate

‘‘Bryozoan Index’’ and ‘‘Photo Index’’, we infer a

mid-shelf environment, which in the southern Aus-

tralia analogue, is reached at 50–75-m water depth.

The fragmentation of the shell material and the low

content of fines, i.e. the carbonate matrix and

siliciclastics, as documented by the gamma-ray

signatures and point-count results (Fig. 3), implies

some higher water energy conditions for the depo-

sition of these sediments in comparison with the FS

facies. The abundance of out-of-place and disarticu-

lated endobenthic biota (burrowing bivalves, irregular

echinoids such as Clypeaster), often concentrated

at lithological contacts, implies the concentration

of fossil hard parts due to exhumation, and

preferential resuspension of fines (Fig. 5). This

conclusion is supported by flume experiments in

which lateral transport of biogenic particles was

minimised on soft substrates, while sand was

preferentially resuspended and exported (Futterer,

1978). The resulting shell lags may attract new

epibenthic faunas and floras, such as secondary

encrusters but may repel burrowers. Kidwell and

Aigner (1985) originally described this increase in

diversity and complexity through allogenic and

autogenic modifications of the original sediment,

and condensation effects.

Fig. 4. Fossiliferous sandstone showing thicket of the well preserved bryozoan Celleporaria palmata. Millimetric holes within the bryozoan

colonies are from the parasitic coral Culicia. LDL-section, upper sandstone marker (Fig. 3). Note the high ‘‘Bryozoan Index’’. Size of pencil is

14 cm.


3.3. Rhodolith rudstone (RR)

This facies is composed of a well cemented, coarse

rudstone formed by densely packed spherical rhodo-

liths (5 cm) and bivalves with various bryozoans and

some siliciclastic sand. Some beds of the RR facies

form blankets of wide lateral continuity.

Interpretation: The RR is deposited in situ within

the photic zone. It is equivalent to the rhodolith facies

of James et al. (2001), which presently forms on the

inner shelf of the Great Australian Bight in areas

protected from massive swell. The high ‘‘Photo

Index’’ implies shallow water depth (35–50 m in

the Australia example). There, it represents a temper-

ate rhodalgal association and is related to weak

seasonal upwelling (James et al., 2001).

4. Discussion

The Lagos-Portimao Formation exhibits a shelf

geometry that is not rimmed by reefs or nonskeletal

sand bodies and has a grain composition, which fits

the characteristics of cool-water carbonates (Lees and

Buller, 1972; Nelson, 1988; James, 1997). The pres-

ence of large foraminifers (Heterostegina) implies

warm-temperate surface temperatures (Betzler et al.,

1997). Zooxanthellate corals of the genera Tarbellas-

tera and Porites are present within a few rare beds.

The corals represent isolated colonies and never exist

in a framework-forming habit, however, some Tarbel-

lastrea form crusts on large oyster shells. Offshore

southwestern Australia, scattered sea floor incrusta-

tions of colonial corals exist at the transition of the

tropical and temperate biogeographic provinces in the

modern ocean, where water temperatures remain close

below the reef-threshold value (Betzler et al., 1997).

We envisage a similar situation for the shell beds with

corals, which implies ambient surface water temper-

atures temporally rose close to 20 jC (Brachert et al.,

1996, 2001; Forst et al., 2000). Additional evidence

for this interpretation is the presence of subtropical

sharks and crocodiles (Antunes et al., 1997, 1981;

Antunes and Pais, 1993). Little is known with respect

to the palaeoceanographic context, however, net out-

flow of Mediterranean surface waters (Hay, 1993),

and the ubiquity of the mixotrophic large foraminifers

suggests the presence of oligotrophic or mesotrophic

surface waters. It thus seems reasonable to assume

that the absence of zooxanthellate corals over most

segments of the formation reflects low water temper-

atures rather than high clastic input and associated

nutrient flux.

In the temperate carbonate systems, heterotrophic

biota play an important role in the bulk carbonate

production. For this reason, production extends far

into the lower photic and upper aphotic zone depend-

ing on the trophic resources (James et al., 2001), and

is, therefore, variably affected by storm reworking.

The open Atlantic exposure of the present southern

Portugal shelf implies a high-energy situation,

affected by deep storm waves and long-periodic

swell. A suitable modern analogue for the facies

model is the southern Australian temperate carbonate

province (James et al., 1994). A major limitation to

the application of the Australia analogue to the

Fig. 5. Shell bed (coarse skeletal rudstone) composed of endobenthic

bivalves interpreted to be exhumed and out-of-place because of

disarticulation and fragmentation. Arrifao, WVof Albufeira, LowerLagos-Portimao Formation. Size of the individual shells is 5 cm.


Lagos-Portimao Formation, however, is the high

content of clastics. There, siliclastics are nowhere

significant, although reworked carbonate grains,

which are not derived from syndepositional carbo-

nate production (‘‘lithoskels’’ of Collins, 1988;

‘‘intraclasts’’, ‘‘relic’’ or ‘‘stranded’’ grains of James

et al., 2001), represent a major sediment component

( < 75%) on the inner shelf. These grains are com-

monly well rounded and well sorted due to frequent

reworking. Intraparticle pores show micritic infill

and in situ growth of clay minerals, and may be

several thousands of years older than the embedding

sediment. Carbonate relic grains have been reported

from ancient temperate limestone sequences of the

Queensland Plateau (NE Australia), which lack the

quartz sand component as well (Brachert et al.,

1993). In the mixed calcareous siliciclastic lower

Lagos-Portimao Formation, however, relic grains

have not yet been identified in thin section. The

only clear pre-depositional (clastic) constituents are

the detrital quartz grains, which are particularly

abundant in the FS. In contrast to the modern

analogue, the clastic facies (FS) represent the outer

shelf. According to our interpretation, this variation

is an expression of (1) the nature and climates of the

clastic source areas, (2) the different topography of

the shelves, and (3) the high-amplitude sea-level

variations of the Pleistocene (Table 1). Not with-

standing the close similarity to the New Zealand

cool-water carbonate province (Nelson, 1988; Nelson

et al., 1988), we use the Australia analogue because

of the overall similarity of the sedimentary processes.

4.1. Clastic source areas and climate

The southern Australia shelf is bordered by young

carbonate deposits, with few outcrops of Precambrian

basement. Due to the arid climate and subdued top-

ography of the adjacent hinterland, little amounts of

detritus sediments enter the shelf (James and von der

Borch, 1991). This is in a clear contrast to the

Miocene situation of southern Portugal. The adjacent

hinterland was warm and temporally humid (Antunes

and Pais, 1984), and a high mountain range (900 m)

composed of Late Palaeozoic flysch deposits bounded

the shelf to the north (Table 1). For these reasons,

siliciclastic input to the shelf should have been high.

4.2. Topography of the shelf

The effects of the differing sediment input is even

enhanced by the shelf topography, because the

extremely wide southern Australia shelf (100–200

km) traps detritus on the inner sector due to an

effective partitioning of sedimentary systems by deep

grounding swell and storm waves on the mid shelf.

The Miocene Algarve shelf was some tens of kilo-

meters wide only, and sediment redistribution might

have been efficient through longshore currents and

specifically during sea-level changes.

Table 1

Main attributes of the shelves in southern Australia (modern) and southern Portugal (Miocene)

Southern Australia shelf

(Great Australian Bight)

Miocene shelf of southern Portugal

Width 100–200 km Narrow (some tens of kilometres)

Topography of the shelf Flat, with Pleistocene terraces

(max. � 120 mbsl), and steep slope

Flat

Topography of the

adjacent hinterland

Subdued, plains bounded by sea cliffs Mountain range (900 m)

Geology of the hinterland Neogene and Quarternary carbonates,

crystalline rocks of Precambrian age

Siliciclastics (Palaeozoic), limestone,

marl, evaporites (Mesozoic); Volcanics

(Mesozoic), minor igneous rocks (Mesozoic)

Substrate Neogene and Quarternary calcarenite Marl and micritic limestone, Cretaceous

Climate of the adjacent

mainland

Arid, subtropical Lower Miocene: humid; Mid-Miocene: arid

(Langhian), semi-arid (Serravallian); Late Miocene:

semi-arid (Tortonian); Miocene cooling trend

From (Antunes and Pais, 1984; James et al., 2001).


4.3. Amplitude of sea-level variations

The sediment starved southern Australia shelf

exhibits a number of Pleistocene terraces, which

testify high-frequency, high-amplitude sea-level fluc-

tuations, the last of which caused emersion of most of

the shelf surface (Boreen and James, 1993). During

early transgression, initial flooding of the shelf caused

a substantial amount of coarse sediment to be swept

outward and deposited on the outer shelf and upper

slope (Boreen and James, 1993; Brooks and Holmes,

1989). This process of clastic sediment dispersal most

likely also operated on the Miocene Lagos-Portimao

Formation shelf, which brought clastic sediments on

the outer shelf, however, amplitudes of the sea-level

fluctuations were comparatively low, and therefore,

lag times may have been too short for the growth of

clay minerals within relict grains.

4.4. Facies analysis and high-frequency cyclicity

The predominance of red algae in the RR facies

documents that sediments formed in the photic zone.

In the Australia analogue, such a facies occurs in 35–

50 m water depth on the inner shelf, in settings

protected from main wave energy. The presence of

bryozoans as the second carbonate producers may

imply some upwelling of nutrient enriched waters

on the inner shelf (cf. James et al., 2001). In the

Lagos-Portimao Formation, the condensed shell beds

(sensu Einsele and Bayer, 1991; Kidwell, 1991;

Kondo et al., 1998) of the SR facies may represent

the mid-shelf affected by grounding storm waves and

long periodic swell although physical sedimentary

structures are not visible, possibly due to bioturbation

of slowly accreting lag deposits. The FS facies may

have formed in deeper water below the zone of wave

abrasion, that is the outer shelf. This taphonomic

interpretation is consistent with the shaved carbonate

shelf model which predicts seaward transport of fines

on the outer shelf and slope (James et al., 1994), and

the shelf equilibrium profile model, which implies a

landward shallowing of the WAD (Pomar, 2001). Fine

matrix occurs therefore preferentially in the FS inter-

mixed with in situ grains. Among the coarse grains,

bivalves predominate on the mid-shelf and bryozoans

on the outer shelf and upper slope, respectively. The

‘‘Bryozoan Index’’ is a suitable first-order proxy for

water depth, although bathymetric zonations may be

strongly modified by temperature and trophic resour-

ces (cf. James et al., 2001; Pomar, 2001). We therefore

do not use the indices as quantitative proxies of sea

level, however, out of phase fluctuations of the two

indices also imply high-frequency bathymetric fluctu-

ations, which are clearly reflected in the gamma-ray

signal (Fig. 3). Values close to 1 indicate deep water

for the ‘‘Bryozoan Index’’ and shallow water for the

‘‘Photo Index’’, respectively (Fig. 3). Because the

large-scale stratigraphy of the shelf implies a eustatic

control on its stratigraphic architecture, the high-

frequency cycles also probably reflect eustatic sea-

level fluctuations.

The sedimentary model presented in this study is

only partially compatible with the shaved shelf con-

cept sensu James et al. (1994), because of the presence

of quartz sand on the outer shelf of the Lagos-Portimao

Formation. The taphonomic observations and water

depth proxies clearly argue against a lowstand position

of the sandstones (here: low water depth). The most

important are the laterally (along strike observation

only) very continuous lithology and biofacies (high

‘‘Bryozoan Index’’, low ‘‘Photo Index’’) of some

sandstone beds (stratigraphic marker beds!) and tapho-

nomic characteristics (fauna in life position, shells

articulated). The inferred sedimentary processes and

model assumptions are visualized in a one-dimen-

sional graphic simulation (Fig. 6). The basic assump-

tion of the simulation is a vertical movement of the

wave abrasion depth (WAD), which occurs in phase

with eustatic sea-level fluctuations. The WAD, there-

fore, moves up and down over the shelf profile during

a sea-level cycle, and the critical shelf segment located

between the minimum and maximum of the WAD

fluctuation is therefore variably affected by hydraulic

energy. For this part of the shelf, the model assumes

constant accumulation of sand, when the water is deep

and the WAD is located within the water column and

does not reach the sea floor. This takes place during a

rise of the WAD relative to the sea floor build-up

causing an increase in water depth. The situation

changes when the WAD intersects with the sea floor

related to a relative fall of sea level. Resuspension of

sand concentrates shell materials in skeletal lags,

whereas sand is exported outward into areas below

the WAD. On the other hand, longer-term accumula-

tion of shell beds takes place, where and when the


relative rise of the WAD is slower than potential sand

accumulation rates (condensed shell bed). The results

of this simulation do not vary principally with a change

of parameters, and imply the formation of sand (-stone)

essentially during TST and HST, that of the shell beds

during LST (Fig. 6). The shell lags do not exhibit any

specific patterns of infestation or fouling of the

exhumed shell materials, a pattern which has also been

observed in modern sediments (Meldahl et al., 1997).

Although the simulations do not imply any scale, they

clearly suggest that resuspensional shell beds (model

1) must reflect short-term cyclicity, whereas stacking

of complex shell beds reflects a longer-term, slow

relative rise of WAB, modulated by high-frequency

fluctuations (model 1). Alternatively, thick sandstone

beds with thin intervening shell beds (model 2) corre-

spond to the TST and early HST of a significant long-

term rise of WAD (Fig. 6). Although hard to identify in

the field, shell concentrations formed through sedi-

ment bypass (model 3) imply subsidence rates on the

order of magnitude of WAD fall (Fig. 6). The results of

the simulation demonstrate the validity of water depth

as a proxy for sea level in this shelf environment. Some

horizontally continuous stratigraphic sandstone

marker beds exhibit an exceptionally well-preserved

bryozoan fauna, which shows clusters and thickets of

largely in situ bryozoans with a branching growth form

(e.g. Celleporaria). These units are interpreted to

reflect very deep water and, therefore, to represent

the maximum flooding surface (mfs) of a longer term

WAD cyclicity (Fig. 3).

The facies reconstructions and model simulation

imply a differentiation of the shelf into shoreline

parallel facies belts. The rhodolith beds represent the

inner shelf, shell beds formed on the mid shelf, and

sandstone beds on the outer shelf. Apart from small

isolated outcrops, no data of the stratigraphic geo-

metries in a dip section exist. Nonetheless, the present

reconstruction of sea-level fluctuations based on litho-

logic criteria alone provides a plausible explanation

for the formation of limestone beds during lowstands

and fossiliferous sandstone during highstands of sea

level. It offers an alternative model for some mixed

sequences that needs further testing.

5. Conclusions

The Early–Mid-Miocene Lagos-Portimao Forma-

tion (S-Portugal) is predominantly a mixed siliciclas-

tic–cool-water carbonate sequence, which formed on

a high-energy Atlantic shelf. The Miocene timeframe

Fig. 6. One-dimensional simulation model of LP sandstone– limestone cycles (models 1–3). The sinusoidal curve shows wave abrasion depth

fluctuations. A long-term eustatic fall (or very slow relative rise) modulated by high-frequency fluctuations of the wave abrasion depth results in

stacked shell beds (model 1). Long-term relative rise (TST and early HST) promotes deposition of thick sandstone and thin shell beds (model 2).

Sediment bypass implies rates of short-term WAD fall f subsidence (model 3). Not to scale.


of the units suggests a 3rd order eustatic control on

the Miocene stratigraphy of the shelf. We therefore

assume an eustatic origin of the meter-scale high-

frequency cyclicity. The sediment dynamics, biotic

distributions and taphonomic signatures agree with

the shaved shelf model (S-Australia shelf, Holocene),

although the shelf was narrow and the Miocene

climate was generally humid. A consequent addition

to the model, therefore, is the rhythmic intercalation

of fossiliferous sandstone, which reflects high silici-

clastic influx, and the differentiation of the Miocene

shelf into shoreline parallel facies belts: On the inner

and mid-shelf affected by wave abrasion (above

WAD), winnowing and export of sand concentrated

skeletal materials were subsequently colonised by

specialised benthos. The limestones therefore repre-

sent a mixture of exhumed endofauna and various

epibenthos, including coralline algae. The sands

which were exported to the outer shelf (below

WAD) were colonised by burrowing infauna and

clusters/thickets of rooted or epizoic bryozoans.

Rhythmic fluctuations of the ratio of photic biota

(coralline algae, large foraminifers) vs. bryozoans

(‘‘Photo Index’’) and bryozoans vs. bivalves

(‘‘Bryozoan Index’’) within the sandstone/limestone

couplets reflects water depth variations. Using the

extended shaved shelf model for an one-dimensional

simulation of the limestone–sandstone cycles visu-

alizes, that the limestones formed during lowstands,

the sandstones during highstands in sea level. Rapid

clastic sediment remobilisation on the inner shelf

during transgression may have promoted sandstone

accretion below WAD in deeper water. Because

processes of carbonate production, concentration

and accretion detected in the Miocene Lagos-Porti-

mao Formation are so fundamentally different from

those of flat-topped carbonate platforms, care should

be taken when interpreting ancient mixed carbo-

nate–siliciclastic sequences.

Acknowledgements

R. Haude (Gottingen) determined the asteroideans.

We thank J.C. Braga (Granada) for stimulating

discussions in the field. Oliver Stuckrad (Mainz)

kindly helped with the preparation of thin sections.

Funding by the Deutsche Forschungsgemeinschaft

(German Science Foundation; Br 1153/6) is gratefully

acknowledged.

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Lowstand carbonates, highstand sandstones?