The North-Betic Strait
José M. Martín, Juan C. Braga, Julio Aguirre
& Ángel Puga-Bernabéu
Departamento de Estratigrafía y Paleontología
Universidad de Granada
Significance
- The North-Betic Strait was considered
for a long time as the main way of
communication between the Atlantic
Ocean and the Mediterranean Sea during
the Miocene (see Hsü et al.,1973,1977).
Precise location and age
Nevertheless, its precise location and age were unknown. It was thought to have
been located somewhere in the Prebetic Zone (outermost part of the Betic Cordillera)
and to have been differentiated as such sometime during the Miocene.
How to recognize ancient straits?
- Huge, cross-bedded structures are
characteristic features in the sedimentary record
of ancient straits.
- Two representative examples of ancient straits
are found in the Betic Cordillera: the “Dehesas
de Guadix” and the “Guadalhorce” Straits. In
both cases bottom currents flowing from the
Mediterranean Sea to the Atlantic Ocean moved
giant dunes on the sea floor generating largescale (giant) cross bedding.
The “Dehesas de Guadix” Strait
The “Dehesas de Guadix” Corridor
was a Late-Miocene (LateTortonian) strait connecting the
Atlantic Ocean and the
Mediterranean Sea via the
Guadalquivir and Guadix-Baza
basins (Betzler et al., 2006).
The “Guadalhorce” Strait
The “Guadalhorce” Corridor was a Late Miocene (Early
Messinian) strait connecting the Atlantic Ocean and the
Mediterranean Sea via the Guadalquivir and Málaga
basins (Martín et al., 2001).
GUADALHORCE STRAIT: MAJOR
SEDIMENTARY FEATURES
The Miocene record of the western Prebetic
- In the western Prebetic the Miocene record comprises several
marine units separated by unconformities. The topmost unit exhibits
giant, cross-bedded structures.This unit is thought to represent the
North-Betic Strait deposits.
The North-Betic Strait unit.
Outcrop disposition
- The cross bedded unit can be traced along a narrow east-west (ENE/W-SW) trending band for almost one hundred kilometres.
The North-Betic Strait Unit.
Its erosional base
- The crossbedded unit lies
unconformably
on top of deeplyeroded materials
ranging in age
from Triassic to
Middle Miocene
and belonging to
both the Prebetic
and Subbetic
Zones of the
Betic Cordillera.
The North-Betic Strait Unit.
Lithologies
The predominant lithologies are bioclastic carbonates and mixed
siliciclastics-carbonates. Siliciclastic content may reach up to
30%. Carbonate bioclasts are mainly from coralline algae,
bryozoans and bivalves. Carbonates range from calcarenites to
medium-grained calcirudites. Terrigeneous clasts are of
quartzites (derived from the fluviatile-deposits from the Prebetic
Oligocene) and carbonate clasts ( both limestones and
dolomites) derived from Prebetic and Subbetic,Triassic- to
Middle Miocene-units.
Lithologies (II)
Conglomerates may also appear
locally. They correspond to fandelta deposits which, in most
places, have been intensively
reworked and reincorporated
into the giant cross-bedding
structures. Only in one locality
(Cerro de la Covacha) one of
these delta bodies is preserved.
Fan delta sediments, grading in
size from coarse conglomerates
to sands and up to 80 m thick,
concentrate there at the base of
the sequence. Upward they
interfinger with and/or are
covered by the giant crossbedded sediments.
Selected sections
For the detailed study of the NorthBetic Strait unit four sections (Los
Olmos, El Bajil, Nerpio and Cortijos
Nuevos sections) have been selected
along a northeast-southwest transect.
Further observations refer to the
westernmost outcrops (Santiago de la
Espada area).
Eastern outcrops: Los Olmos section
Eastern outcrops of the North-Betic Strait
Unit: Los Olmos section
In the eastern
outcrops the giant
cross-bedded
structures are mainly
pointing to the East.
Single troughs are up
to 5 m high and a few
tens of metres in
lenght.
Eastern outcrops of the North-Betic Unit.
Field views of Los Olmos section
Central outcrops: El Bajil and Nerpio sections
The North-Betic Unit.
The central outcrops
In the central outcrops the giant
cross-bedded structures point both to
the East and to the West. Single
troughs are up to 15 m high and
some tens of metres in lenght.
Central
outcrops of
the NorthBetic Unit:
El Bajil
stratigraphic
section
Central outcrops of the North-Betic Strait Unit.
Field view of the El Bajil section
Detailed internal structure
Central
outcrops of
the NorthBetic Strait
Unit: Nerpio
stratigraphic
section
Central outcrops of the North-Betic Strait Unit.
Field view of the Nerpio area
Western outcrops: Cortijos Nuevos section
Western outcrops of the
North-Betic Strait Unit:
Cortijos Nuevos
stratigraphic section.
All cross-bedded
structures point to the
W-NW
Western outcrops of the North-Betic Strait Unit.
Field views of the Cortijos Nuevos section
Westernmost outcrops: Santiago de la Espada area
The westernmost outcrops of
the North-Betic Strait unit
In the westernmost outcrops
(Santiago de la Espada area) the
giant cross-bedded structures are
pointing to the W-SW. Single troughs
are up to 20 m high and a few
hundreds of metres in lenght.
Westernmost outcrops of the North-Betic Strait Unit. Field
views in the Santiago de la Espada area
Genesis of the giant cross-bedding
- The giant cross-bedding observed in the North-Betic
Strait deposits is thought to be linked to the migration of
large-scale dunes moved by tides. Similar structures in
size and composition have been described in Presentday tide-dominated channels (Berné, 1991).
- The existence of composite structures with reverse
cross-bedding direction and abundant reactivation
surfaces are strong arguments in favour of tide action.
- For the formation of tidal dunes, comparable in size and
with similar grain-sizes, current velocities of up to 1.5
m/s have been reported from some Present-day
environments (Harris, 1989; Berné, 1991, ZhenXia et al.,
1998).
Depositional depth
- There is general agreement that in modern settings a
relationship exists between height of dunes and
depositional depth. This relationship is expressed in
several ways. According to Rubin and McCulloch (1980)
and Dalrymple and Rhodes (1995), H = 0.17 D (being H
= height of the dune and D = depositional depth).
Applying this formula to our example it yields a
depositional depth of approximatally 30 m for the 5 m
high dunes, 90 m for the 15 m high, and 120 m for the 20
m high respectively. It means that the North Betic Strait
shallowed to the East and deepened to the West.
Depositional depth (II)
Following Berné’s (1991) proposed equation (H = 0.50 D – 10.2),
significantly smaller values would result for the highest dunes (30 m
for the 5 m high dunes as before, but 50 m only for the 15 m high,
and 60 m for the 20 m high respectively). These latter figures seem
to be more in accordance with what is observed in recent tidal
environments (Belderson and Stride, 1966; Twichell, 1983).
Cross-bedding distribution
The observed cross-bedding distribution is similar to that
described by Thauront and Berné (1996) in some
present-day tidal channels which show a flooddominated shallower area, an intermediate area with
flood- and ebb-dominated structures and a deeper zone
with a predominance of ebb-dominated structures. In
modern environments, the asymmetry in cross-bedding
distribution seems to be related with changes in the
velocity and intensity of the flood and ebb currents at
different depths (Kranck, 1972; Bartholdy et al., 2002; Le
Bot and Trentesaux, 2004; van der Vegt et al., in press).
The ebb current dominated the North-Betic Strait
generating the largest, west-orientated cross-bedding.
BASIN EVOLUTION AND
DIFFERENTIATION OF THE
NORTH-BETIC STRAIT
The Miocene record of the western Prebetic
Zone revisited
Three main sedimentary units can be differentiated. The lowermost
one (Langhian in age), with platform deposits to the N-NW and
basinal deposits to the S-SE, represent the last Prebetic deposits,
linked to a northern emerged area. The next unit, (Serravalian in
age) is made up of carbonates and detrital rocks linked to a
southern relief formed as a result of the Subbetic-nappes uplifting.
The last unit (presumably lower Tortonian in age), represented by
the last marine deposits found in the area, accumulated within the
North-Betic Strait.
Miocene evolution of the Prebetic Zone
The evolution towards the formation of the North-Betic Strait can be
divided into a series of steps. The palaeogeography evolved from a
southern-facing platform, marginal to a northern relief, to a wideopen marine passage limited by a southern platform and, finally, to a
tidal-dominated strait. From the outcrop-distribution map it can be
deduced that this Strait was at least 85 km long and reached up to
18 km at its widest point.
The closing of the North-Betic Strait
The closing of the North-Betic Strait is presumably
recorded in the area by the presence of some lagoonal,
silty deposits, covered by a stromatolite layer and
crowned by a red soil found in the La Muela outcrop,
located to the northeast of Santiago de la Espada.
“La Muela” major sedimentary features
Evolution of the AtlanticMediterranean connections (I)
The history of Miocene Atlantic-Mediterranean connections can be
now fully reconstructed. In the early Tortonian a narrow passage,
the North-Betic Strait, existed in the southern part of the Prebetic
area, limited by an emerged southern relief.
N
Alicante
Murcia
Jaén
Guadalquivir
Basin
100 km
Atlantic Ocean
present-day coast
Granada
Almería
Mediterranean Sea
emergent land
Evolution of the Atlantic-Mediterranean connections (II)
In the late Tortonian the Atlantic-Mediterranean connections were
through the Dehesas de Guadix Corridor and the Granada basin in
the Betic Cordillera and the Rifean Corridors, in northern Morocco.
In the early Messinian through the Guadalhorce and Rifean
Corridors at the beginning and later only through the Rifean
Corridors and, in the Pliocene, through the Gibraltar Straits.