CHAPTER 9 . Geomorphological and Physical Characteristics of the Bahia Blanca Estuary
197
sponded to a large delta. However, analysis made by Perillo (1989) and Gonzalez
Uriarte (1984) based on LANDSAT imagery, aerial photography and topographic
charts suggest that the area was dominated by a complex and wide system formed by
the Colorado and Desaguadero Rivers. Perillo (1989) considered that the Principal
Channel was formed by the latter, being the Colorado itself or one of its arms, that
acted as tributary of the Desaguadero. Clear evidences were found of a later southward migration of this tributary which carved consecutively the other major channels (Falsa, Verde and Brightman Bays). Presently the Desaguadero has an endorheic
basin about 300 km west of the estuary; however, its valley is well preserved in today
morphology.
The whole original delta span from the northern coast of Bahia Blanca to Bahia
San Blas, corresponded to almost 200 km of the southern Buenos Aires Province coast.
Both, the northern portion known as Bahia Blanca and the southern portion named
Bahia Anegada, have similar characteristics. They are composed by extensive tidal flats
with patches oflow salt marshes (mostly Spartina grass) and islands. The islands have
full continental characteristics both in soil and plant and animal diversity. The only
differences between them are that Bahia Anegada islands have a larger proportion of
gravel sediments on their surface that is lacking on the northern counterpart, and
Bahia Blanca receives some freshwater input whereas Bahia Anegada has no river runoff into its system except for one of the tributaries of Colorado River that flows at the
northeast corner of the bay.
Between both bays there is a stretch of the coast of about 60 km almost linear and
oriented N -So It corresponds to the present location of the Colorado River delta. Even
though some geomorphologic (Gomez et al. 1994) and seismic (Aliotta 1994) studies
were made on the inner shelf in front of this delta, there are no clear indications of
where its outer boundary was located. However, all the area have suffered a marine
transgression that reached its maximum level about 7000 years B.P.
In fact, Codignotto and Weiler (1980) have shown for the area between Punta
Laberinto and Olga Is. (Fig. 9.1) that the maximum ingression occurred about
6630 years B.P. in deposits that are between 7.5 and 10 m above present sea level. They
also show that the sea has retired reaching the younger ages of 1300 years B.P. at 2.5 m
above present sea level. Similar results were obtained by Gonzalez (1989) and Aliotta
and Farinatti (1990) for the inner Bahia Blanca Estuary. They determined that during
the maximum transgressive cycle (Flandrian transgression) relative sea level reached
also 7-10 m over the present one. For instance, Gonzalez (1989) provided a sequence
of five transgressive stages between 5 990 and 3560 years B.P. Although there are other
beach ridges at lower levels that may indicate a regression sequence similar to the one
observed by Codignotto and Weiler (1980), no radiocarbon dates were obtained from
them.
Studies of submarine outcrops underneath some shoreface-connected sand ridges
located on the outer estuary made by Gomez and Perillo (1995) encountered tidal flat
deposits at about 15 m below present sea level. Wave-cut terraces were identified by
Aliotta and Perillo (1990) and Perillo and Cuadrado (1990) on different parts of the
mouth of the Principal Channel also at depths between 13 and 16 m. All these materials and erosional forms were assigned an age of the order of 8 000-9 000 years B.P.
Based on this information, Gomez and Perillo (1995) constructed a minimum sealevel curve for the last 9 000 years and estimated the different rates of variation in that
197
sponded to a large delta. However, analysis made by Perillo (1989) and Gonzalez
Uriarte (1984) based on LANDSAT imagery, aerial photography and topographic
charts suggest that the area was dominated by a complex and wide system formed by
the Colorado and Desaguadero Rivers. Perillo (1989) considered that the Principal
Channel was formed by the latter, being the Colorado itself or one of its arms, that
acted as tributary of the Desaguadero. Clear evidences were found of a later southward migration of this tributary which carved consecutively the other major channels (Falsa, Verde and Brightman Bays). Presently the Desaguadero has an endorheic
basin about 300 km west of the estuary; however, its valley is well preserved in today
morphology.
The whole original delta span from the northern coast of Bahia Blanca to Bahia
San Blas, corresponded to almost 200 km of the southern Buenos Aires Province coast.
Both, the northern portion known as Bahia Blanca and the southern portion named
Bahia Anegada, have similar characteristics. They are composed by extensive tidal flats
with patches oflow salt marshes (mostly Spartina grass) and islands. The islands have
full continental characteristics both in soil and plant and animal diversity. The only
differences between them are that Bahia Anegada islands have a larger proportion of
gravel sediments on their surface that is lacking on the northern counterpart, and
Bahia Blanca receives some freshwater input whereas Bahia Anegada has no river runoff into its system except for one of the tributaries of Colorado River that flows at the
northeast corner of the bay.
Between both bays there is a stretch of the coast of about 60 km almost linear and
oriented N -So It corresponds to the present location of the Colorado River delta. Even
though some geomorphologic (Gomez et al. 1994) and seismic (Aliotta 1994) studies
were made on the inner shelf in front of this delta, there are no clear indications of
where its outer boundary was located. However, all the area have suffered a marine
transgression that reached its maximum level about 7000 years B.P.
In fact, Codignotto and Weiler (1980) have shown for the area between Punta
Laberinto and Olga Is. (Fig. 9.1) that the maximum ingression occurred about
6630 years B.P. in deposits that are between 7.5 and 10 m above present sea level. They
also show that the sea has retired reaching the younger ages of 1300 years B.P. at 2.5 m
above present sea level. Similar results were obtained by Gonzalez (1989) and Aliotta
and Farinatti (1990) for the inner Bahia Blanca Estuary. They determined that during
the maximum transgressive cycle (Flandrian transgression) relative sea level reached
also 7-10 m over the present one. For instance, Gonzalez (1989) provided a sequence
of five transgressive stages between 5 990 and 3560 years B.P. Although there are other
beach ridges at lower levels that may indicate a regression sequence similar to the one
observed by Codignotto and Weiler (1980), no radiocarbon dates were obtained from
them.
Studies of submarine outcrops underneath some shoreface-connected sand ridges
located on the outer estuary made by Gomez and Perillo (1995) encountered tidal flat
deposits at about 15 m below present sea level. Wave-cut terraces were identified by
Aliotta and Perillo (1990) and Perillo and Cuadrado (1990) on different parts of the
mouth of the Principal Channel also at depths between 13 and 16 m. All these materials and erosional forms were assigned an age of the order of 8 000-9 000 years B.P.
Based on this information, Gomez and Perillo (1995) constructed a minimum sealevel curve for the last 9 000 years and estimated the different rates of variation in that