126
M. Cintia Piccolo . Gerardo M.E. Perillo
River (Fig. 6.15). Seaward of the confluence, a deep channel crosses the mouth of the
estuary between Punta Loyola and Punta Bustamante, with a depth of over 23 m.
Between Punta Loyola and the mouth of the Chico River an extensive intertidal zone
fills Loyola Bay. The area is backed by a raised shingle beach, while shingle spits and
shoals are also observed on the upper intertidal salt marsh levels. The intertidal system compress a tidal flat and a salt marsh.
Although some areas of the Argentine coastline, south of Rio Gallegos, are still scientifically unexplored, it is believed that the Loyola Bay salt marsh is the southernmost
within continental South America. There are at least two other salt marshes in San Sebastian Bay and in the Rio Grande Estuary (190 and 270 km south from Rio Gallegos, respectively) and also along the southern coast of Magellan Strait, all in Tierra del Fuego.
The tidal flat occupies the lower intertidal zone. It varies in width from a few metres near Punta Loyola (Fig. 6.15) up to around 700 m during low tide, spring conditions, towards the mouth of the Rio Chico. The sedimentology of the tidal flat is rather
complex. The upper 0.5 m sediment layer (no data from below) is formed by rounded
gravels (mean diameter 20-150 mm) with a matrix of clayey silt. The gravels are
glacifluvial materials derived from the Rodados Tehuelches formation. This formation covers practically all of Patagonia, and the gravels form most of the beaches on
the Argentine coastline to the south of Peninsula Valdes. In sectors of the flat, gravel
shoals, 0.15-0.30 m relief, occur devoid of fine material (Perillo et al. 1996).
A gravel beach extends from Punta Loyola into the Loyola Bay with a local relief of
about 3 m and a steep (8-15°) slope. The beach terminates in three spits that are encroaching upon the marsh, suggesting the dominant sediment transport in this area
was from Punta Loyola inward due to ocean wave diffraction around it. The lower
portion of the flat has an undulating surface, being entirely covered with minor creeks.
These grooves are generally 0.5-1.0 m wide and 0.10-0.40 m deep, separated by relatively flat inter-channel areas ca. 0.5-2.0 m wide. Only a few larger channels, both in
depth and width, cross the flats and these are directly branched from the largest channels on the marsh.
The salt marsh covers the upper portion of the intertidal environment. It displays
the three typical levels of a salt marsh: lower, middle and upper. Salicornia antigua is
the dominant plant species throughout the marsh, and Franchicenia cf. microphylla,
Festuca sp., and Griceda cf. ayantinensis and are also present. At least three other plants
were sampled, identification was not possible because the plants were not at suitable
growth stage. Salicornia was more abundant throughout the lower and middle marsh
areas and within the channel flanks of the upper marsh area, whereas Festuca and
Griceda became progressively more abundant on moving from the middle to the upper-marsh areas (Perillo et al. 1996).
Within the marsh tidal channels of various dimensions are present, but larger channels are dominant. Channels within the tidal flat have little sinuosity, but as they penetrate further into the marsh, channel characteristics change dramatically. The lower
marsh has the highest density (number of creeks/per unit area) of tidal creeks, but
with the middle and upper segments containing only a slightly lower creek density.
The drainage system is dendritic. Backing the salt marsh there is an area considered
to be a former salt marsh identified only by the drainage pattern because the vegetation there is indicative of a supratidal environment and estuarine water may only inundate these upper reaches during less than once a year or less.
M. Cintia Piccolo . Gerardo M.E. Perillo
River (Fig. 6.15). Seaward of the confluence, a deep channel crosses the mouth of the
estuary between Punta Loyola and Punta Bustamante, with a depth of over 23 m.
Between Punta Loyola and the mouth of the Chico River an extensive intertidal zone
fills Loyola Bay. The area is backed by a raised shingle beach, while shingle spits and
shoals are also observed on the upper intertidal salt marsh levels. The intertidal system compress a tidal flat and a salt marsh.
Although some areas of the Argentine coastline, south of Rio Gallegos, are still scientifically unexplored, it is believed that the Loyola Bay salt marsh is the southernmost
within continental South America. There are at least two other salt marshes in San Sebastian Bay and in the Rio Grande Estuary (190 and 270 km south from Rio Gallegos, respectively) and also along the southern coast of Magellan Strait, all in Tierra del Fuego.
The tidal flat occupies the lower intertidal zone. It varies in width from a few metres near Punta Loyola (Fig. 6.15) up to around 700 m during low tide, spring conditions, towards the mouth of the Rio Chico. The sedimentology of the tidal flat is rather
complex. The upper 0.5 m sediment layer (no data from below) is formed by rounded
gravels (mean diameter 20-150 mm) with a matrix of clayey silt. The gravels are
glacifluvial materials derived from the Rodados Tehuelches formation. This formation covers practically all of Patagonia, and the gravels form most of the beaches on
the Argentine coastline to the south of Peninsula Valdes. In sectors of the flat, gravel
shoals, 0.15-0.30 m relief, occur devoid of fine material (Perillo et al. 1996).
A gravel beach extends from Punta Loyola into the Loyola Bay with a local relief of
about 3 m and a steep (8-15°) slope. The beach terminates in three spits that are encroaching upon the marsh, suggesting the dominant sediment transport in this area
was from Punta Loyola inward due to ocean wave diffraction around it. The lower
portion of the flat has an undulating surface, being entirely covered with minor creeks.
These grooves are generally 0.5-1.0 m wide and 0.10-0.40 m deep, separated by relatively flat inter-channel areas ca. 0.5-2.0 m wide. Only a few larger channels, both in
depth and width, cross the flats and these are directly branched from the largest channels on the marsh.
The salt marsh covers the upper portion of the intertidal environment. It displays
the three typical levels of a salt marsh: lower, middle and upper. Salicornia antigua is
the dominant plant species throughout the marsh, and Franchicenia cf. microphylla,
Festuca sp., and Griceda cf. ayantinensis and are also present. At least three other plants
were sampled, identification was not possible because the plants were not at suitable
growth stage. Salicornia was more abundant throughout the lower and middle marsh
areas and within the channel flanks of the upper marsh area, whereas Festuca and
Griceda became progressively more abundant on moving from the middle to the upper-marsh areas (Perillo et al. 1996).
Within the marsh tidal channels of various dimensions are present, but larger channels are dominant. Channels within the tidal flat have little sinuosity, but as they penetrate further into the marsh, channel characteristics change dramatically. The lower
marsh has the highest density (number of creeks/per unit area) of tidal creeks, but
with the middle and upper segments containing only a slightly lower creek density.
The drainage system is dendritic. Backing the salt marsh there is an area considered
to be a former salt marsh identified only by the drainage pattern because the vegetation there is indicative of a supratidal environment and estuarine water may only inundate these upper reaches during less than once a year or less.