12.1 Rift Basins
Ameriea (in the regions of the former Appalachian
orogeny) belong to this group (Fig. l2.7b). They represent the western end of a wide detachment zone
and experienced no post-rift subsidence. The syn-rift
phase of these basins was characterized by rapid subsidence and fluvial-laeustrine sediments (Manspeizer
1988b; Schlische and Olsen 1990; cf. Sect. 7.7.1).
Sediment supp1y eould not always keep pace with
subsidence, but the water depth of these basins
(mostly lakes) was limited and subjeeted to c1imatie
change. The Tucano basin in Brazil is another exampIe of this basin type (cf. Fig. 12.9d; Karner et al.
1992).
An example of a partly emerged rift basin on the
European side of the Atlantie is the Lusitanian basin
in western Portugal (Fig. 12.7a). From the late Triassie to the Cretaeeous this basin experienced four
phases of rifting, beginning in the central and southern part as symmetrieal grabens and half-grabens
(Rasmussen et al. 1998). Early Jurassie evaporites
were later transformed into salt structures. Rifting
ceased during the early Cretaeeous and was foHowed
by regional subsidenee and the deposition of relatively uniform marine muds.
The third (Oxfordian) rifting phase is weil documented on
land because the eastern part of the Lusitanian basin was
uplifted and exhumed during the Miocene (basin inversion). Here, the syn-rift sediments consist of continental
siliciclastics (alluvial fans, fluvial sandstones and red beds)
and marine muds and sands inc\uding turbidites (Ravnäs et
al. 1997). Increasing rates of subsidence and block rotation
caused backstepping and deepening-upward facies in the
basin; slowing tectonic activity was followed by
prograding facies, return to a fluvial environment, and erosIOn.
Drowned rift basins below present-day shelves.
Most of the ancient rift basins of the North Atlantie
are buried under late Mesozoie and Cenozoic postrift shelf sediments (Fig. 12.7c). They experienced
longer and more complicated depositional histories
than the emerged basins caused by initial rifting:
- First phase of rifting in the late Triassic. The fiHing
of these basins started with continental siliciclastics,
mostly red beds, but most of them lay in regions of
low relief, were inundated by the sea, and accumulated evaporites of varying thickness. This phase was
terminated before any oceanic crust was generated.
In the middle Jurassic, slow, thermal subsidence
(Fig. 12.8a) led to the formation of a wide epeiric sea
and predominant deposition of shallow-marine shales
and limestones.
- The second phase of rifting in the North Atlantic
began in the late Jurassie and lasted approximately
50 Ma until the Aptian. After an initial episode of
uplift aeeompanied by erosion and valley cutting,
extension of a wide zone of crust was accomplished
555
along a low-angle detachment (cf. Figs. 12.2d and
12.8a,b). This mechanism reactivated pre-existing
basins, for example the Jeanne d'Arc basin on the
Grand Banks of Newfoundland (Fig. 12.8e), and
generated a number of new basins. Fault-eontrolled
synsedimentary subsidence ereated special depo-eenters separated by sills. The sediment wedges in these
basins consists predominantly of shallow-water limestones and shales with interbedded sandstones and
organic-rich layers.
- At the c1imax of rifting in the 1ate Jurassic, basins
near the continent or adjacent to uplifted blocks were
partially fi1led with fluvial and deltaic deposits.
- The late-rift evolution is charaeterized by decreasing local subsidence and a transition into regional
thermal subsidence (cf. Fig. 8.14). However, further
adjustments of the broken, brittle upper crust to continuing extension and aehanging stress field (cf. Fig.
12.2d,e) led to local variations in the sedimentary fill
of the basins. A basal Cretaceous transgression created a widespread uneonformity. The subsequent
sediments inc1ude shallow-water limestones and are
affeeted by additional unconformities.
- Breakup uneonformity. The last and weH developed uneonformity during the Aptian is associilted
with the breakup of the stretched continental erust
and the separation of Eurasia with Galicia Bank and
the Iberian peninsula from the North American plate
(Fig. 12.8b). This breakup uneonformity indicates
that up to this time the individual blocks of the eontinental erust had rotated, subsided differentially, or
were uplifted and eroded.
- Rifting and breakup in the North Atlantic
prograded from south to north with time. In the
Norwegian-Greenland Sea rifting started in the latest
Cretaceous and early Tertiary and was followed by
extensive basaltic magmatism in the PaleocenelEocene. This was eaused by the presence of a
large mantle plume generating the North Atlantic
Igneous Province (Fig. 12.9a,b). Sea-floor spreading
began in the Eoeene (e.g. Price et al. 1997). Continued magmatism on. the Greenland side and some
compression led to the uplift of East Greenland and
minor deformations on the Norwegian side of the
ocean.
In spite of the great differences in their subsidence histories
and sedimentary fills, the Mesozoic North Atlantic rift basins share a number of "sedimentological events" (Fig.
l2.8a) These features include more or less coeval hiatuses
and periods of condensation. Unconformities are associated
with the deposition of coarse-grained clastics. The rift
phases in the Triassic and upper Jurassic are characterized
by high rates of subsidence (50-100 mlMa, up to 250 mlMa
in some basins). "Pulses" of extension and similarities in
the evolution of these basins result from their common
origin by intracontinental rifting and subsequent separation
ofEurope and North America.
For further information see, e.g., Manspeizer (l988b),
Tankard al'.d We\sink (1988), Ziegler (1988) Welsink et al.
Ameriea (in the regions of the former Appalachian
orogeny) belong to this group (Fig. l2.7b). They represent the western end of a wide detachment zone
and experienced no post-rift subsidence. The syn-rift
phase of these basins was characterized by rapid subsidence and fluvial-laeustrine sediments (Manspeizer
1988b; Schlische and Olsen 1990; cf. Sect. 7.7.1).
Sediment supp1y eould not always keep pace with
subsidence, but the water depth of these basins
(mostly lakes) was limited and subjeeted to c1imatie
change. The Tucano basin in Brazil is another exampIe of this basin type (cf. Fig. 12.9d; Karner et al.
1992).
An example of a partly emerged rift basin on the
European side of the Atlantie is the Lusitanian basin
in western Portugal (Fig. 12.7a). From the late Triassie to the Cretaeeous this basin experienced four
phases of rifting, beginning in the central and southern part as symmetrieal grabens and half-grabens
(Rasmussen et al. 1998). Early Jurassie evaporites
were later transformed into salt structures. Rifting
ceased during the early Cretaeeous and was foHowed
by regional subsidenee and the deposition of relatively uniform marine muds.
The third (Oxfordian) rifting phase is weil documented on
land because the eastern part of the Lusitanian basin was
uplifted and exhumed during the Miocene (basin inversion). Here, the syn-rift sediments consist of continental
siliciclastics (alluvial fans, fluvial sandstones and red beds)
and marine muds and sands inc\uding turbidites (Ravnäs et
al. 1997). Increasing rates of subsidence and block rotation
caused backstepping and deepening-upward facies in the
basin; slowing tectonic activity was followed by
prograding facies, return to a fluvial environment, and erosIOn.
Drowned rift basins below present-day shelves.
Most of the ancient rift basins of the North Atlantie
are buried under late Mesozoie and Cenozoic postrift shelf sediments (Fig. 12.7c). They experienced
longer and more complicated depositional histories
than the emerged basins caused by initial rifting:
- First phase of rifting in the late Triassic. The fiHing
of these basins started with continental siliciclastics,
mostly red beds, but most of them lay in regions of
low relief, were inundated by the sea, and accumulated evaporites of varying thickness. This phase was
terminated before any oceanic crust was generated.
In the middle Jurassic, slow, thermal subsidence
(Fig. 12.8a) led to the formation of a wide epeiric sea
and predominant deposition of shallow-marine shales
and limestones.
- The second phase of rifting in the North Atlantic
began in the late Jurassie and lasted approximately
50 Ma until the Aptian. After an initial episode of
uplift aeeompanied by erosion and valley cutting,
extension of a wide zone of crust was accomplished
555
along a low-angle detachment (cf. Figs. 12.2d and
12.8a,b). This mechanism reactivated pre-existing
basins, for example the Jeanne d'Arc basin on the
Grand Banks of Newfoundland (Fig. 12.8e), and
generated a number of new basins. Fault-eontrolled
synsedimentary subsidence ereated special depo-eenters separated by sills. The sediment wedges in these
basins consists predominantly of shallow-water limestones and shales with interbedded sandstones and
organic-rich layers.
- At the c1imax of rifting in the 1ate Jurassic, basins
near the continent or adjacent to uplifted blocks were
partially fi1led with fluvial and deltaic deposits.
- The late-rift evolution is charaeterized by decreasing local subsidence and a transition into regional
thermal subsidence (cf. Fig. 8.14). However, further
adjustments of the broken, brittle upper crust to continuing extension and aehanging stress field (cf. Fig.
12.2d,e) led to local variations in the sedimentary fill
of the basins. A basal Cretaceous transgression created a widespread uneonformity. The subsequent
sediments inc1ude shallow-water limestones and are
affeeted by additional unconformities.
- Breakup uneonformity. The last and weH developed uneonformity during the Aptian is associilted
with the breakup of the stretched continental erust
and the separation of Eurasia with Galicia Bank and
the Iberian peninsula from the North American plate
(Fig. 12.8b). This breakup uneonformity indicates
that up to this time the individual blocks of the eontinental erust had rotated, subsided differentially, or
were uplifted and eroded.
- Rifting and breakup in the North Atlantic
prograded from south to north with time. In the
Norwegian-Greenland Sea rifting started in the latest
Cretaceous and early Tertiary and was followed by
extensive basaltic magmatism in the PaleocenelEocene. This was eaused by the presence of a
large mantle plume generating the North Atlantic
Igneous Province (Fig. 12.9a,b). Sea-floor spreading
began in the Eoeene (e.g. Price et al. 1997). Continued magmatism on. the Greenland side and some
compression led to the uplift of East Greenland and
minor deformations on the Norwegian side of the
ocean.
In spite of the great differences in their subsidence histories
and sedimentary fills, the Mesozoic North Atlantic rift basins share a number of "sedimentological events" (Fig.
l2.8a) These features include more or less coeval hiatuses
and periods of condensation. Unconformities are associated
with the deposition of coarse-grained clastics. The rift
phases in the Triassic and upper Jurassic are characterized
by high rates of subsidence (50-100 mlMa, up to 250 mlMa
in some basins). "Pulses" of extension and similarities in
the evolution of these basins result from their common
origin by intracontinental rifting and subsequent separation
ofEurope and North America.
For further information see, e.g., Manspeizer (l988b),
Tankard al'.d We\sink (1988), Ziegler (1988) Welsink et al.