AGE OF THE DESEADO FORMATION
The locality worked by the Amherst party is situated about three miles
east of the Chico River, just across the line of the homestead of D.
J. Venter as plotted on the Plano de la Gobernation del Chubut, 1910,
by A. Lefrançois. This would be 45° 10ʹ S., and 67° 32ʹ W. (or as on
the map 9° 15ʹ W. of the meridian of Buenos Aires). The exposure is on
all sides of an elongated hill about a sixth of a mile long, averaging
200 feet wide, and constricted in the middle to a narrow neck. Figure 2
shows a section of the hill, made along the north side, and indicates
the varied character of the stratified deposits.
The material varies from brown sandy clay shales, to yellow sandy
clay with concretions, and is capped with a varying layer of greenish
sand, which, in some places, is coarse and irregular, in others fine
and uniform, and in still other places is mixed with considerable
quantities of volcanic ash. In it are many mud balls and also bits of
bone which have been worn round, others but slightly worn, and finally
bones and skeletons which apparently have been buried where they fell.
This green sand is mostly covered with a layer of two feet of hard
sandstone of the same composition as the rest of the bed, but cemented
into a dense layer. Above the green sand is a layer of fine grey sand,
prettily crossbedded, and of varying thickness, but without fossils.
Remains of vertebrate animals occurred in the brown clay, the yellow
clay and the green sand, and in all the cases fossils were of unusual
abundance so that in this limited locality we collected over 300
specimens.
[Illustration: Fig. 2. Section of Deseado exposure showing character of
the various materials.]
Above the Deseado (layers 2 to 5) lies the Patagonian in its typical
development, filled with Ostrea ingens, Turritellas, Brachiopoda,
sharks’ teeth, etc. It is separated from the Deseado by a marked
unconformity, one of the finest examples of unconformity I have ever
seen. Evidently the upper surface of the Deseado was fairly new
at the time of the transgression, or it is much disturbed by the
transgression, the upper layers in places being broken up into sort of
blocks and the crevices filled with Patagonian sands with the contained
shells; just as I saw the beds on the seashore being disturbed by the
waves of today. Then too in the basal foot of the Patagonian I found
material which without question came from the underlying Deseado beds,
various fragments of mammal bones bored by seashells, and with the
Patagonian barnacles on them, but these were never more than a few
inches up in the Patagonian. The contact was not horizontal, but in the
middle of the hill dipped down so that it came there onto the yellow
beds of clays, and it was at this point only where we found bones had
been washed out by the Patagonian sea.
In the section the Deseado consists of layers 2 to 5, the white sandy
clay below belonging to the St. George series and being Cretaceous.
The contact below was also an unconformity, clearly marked for the
white sandy clays were all horizontally bedded, while the Deseado is
crossbedded in every direction, and has a distinct color. These white
sandy clays of the St. George series are similar to the same beds
as shown in sections A and B (figures 3 and 4), and extend in all
directions for miles. Going down toward the Chico River one passes into
the green shales that make up the upper part of the Salamanca and had
similar invertebrate fossils. About ten miles to the north was another
bed of fossil trees similar to the one to be described on the Puerto
Visser side of the pampa.
The character of the material making up the Deseado deposit, its
variations in size and material, the presence of worn pebbles and bits
of bone, show these layers to be a water deposit. The absence of any
marine fossil in a bed otherwise rich in fossils indicated that it was
a fresh water formation. The crossbedding, the irregularity of the
deposits and the mud balls, prove that it was the work of a river. As
there are no aquatic forms in the fauna I further conclude that it was
the deposit of a temporary or intermittent stream, such as occur in
arid and semiarid countries. The layer could hardly be interpreted as
a part of a flood plain; for it is very limited in extent, there being
bluffs on three sides of our exposure, but in them no trace of the
Deseado was found, nor was I able to pick up the formation again across
the Chico River. Then the bedding is very irregular, much more so than
is typical of flood plain deposits. The conclusion I reach then is that
this Deseado pocket represents the bottom of an ancient stream, which
flowed over a land surface made up of the white sandy clays of the St.
George age.
The age then of the Deseado beds must be older than the Patagonian, and
younger than the white sandy clays of the St. George.
As to the age of the Patagonian two very divergent positions have been
taken, which may be best indicated by the following table.
----------+----------------------------+-----------------+----------
| AMEGHINO, | WILCKENS, | ORTMANN,
| 1906[3] | 1906[4] | 1901[5]
----------+-----------------+----------+-----------------+----------
Lower | terrestrial | marine | |
Miocene +-----------------+----------+ |
| | |Patagonian |Patagonian
| | |(transgression) |
----------+-----------------+----------+-----------------+----------
Oligocene | | |Deseado |
| | |(regression) |
----------+-----------------+----------+-----------------+----------
Eocene |Santa Cruz | | |
|Notohippus | | |
|Astrapothericulus|Patagonian|Casamayor |
|Colpodon | |(regression) |
----------+-----------------+----------+-----------------+----------
Upper |Deseado |Sehuen | |
Cretaceous|Astraponotus | |St. George |
|Casamayor |Salamanca |(transgression) |
| |Roca | |
| |Luisa | |
----------+-----------------+----------+-----------------+----------
[Footnote 3: Formations Sedimentaires, p. 498, Anal. Mus. Nac. Buenos
Aires, ser. 3, t. 8.]
[Footnote 4: Neues Jarhbuch fur Mineralogie. bd. 21, p. 193.]
[Footnote 5: Princeton Expeditions Reports, vol. 4, p. 303.]
Without going into the history of the various positions which
different authors have taken, and which will be found given in detail
in Wilckens’ paper, or in less detail in Ortmann’s, we will consider
the positions of the most recent students of the question. Ameghino
postulates a marine and a continental series of deposits being laid
down more or less simultaneously. In the marine series below the
Deseado, which is grouped as Guarantic, he places the Luisa, the Roca
and the Salamanca, followed by a hiatus, then the Sehuen, which in turn
is followed by another hiatus and the end of the Cretaceous is reached.
The Patagonian is his Eocene. Parallel to the marine series is the
terrestrial, where the Casamayor (= Notostylops) is contemporaneous
with the Salamanca, the Deseado with the Sehuen, and the Colpodon,
the Notohippus and Astrapothericulus with the Patagonian, thus making
the Deseado of Cretaceous age. After a very detailed study of a large
series of Patagonian fossils, Ortmann concludes that the Patagonian
is of Lower Miocene age. This is the most detailed study which has
been made. Wilckens coincides with this view, though feeling that
the Patagonian may have extended down a trifle into the last of the
Oligocene. This latter author finds a long gap between the Upper
Cretaceous and the Patagonian, a period when Patagonia was above
water. It was during this interval that the Casamayor, the Deseado
and possibly other beds were deposited on the continent. I have gone
over Ortmann’s argument, and studied a large collection of Patagonian
fossils, both vertebrate and invertebrate, of my own; and while there
are some places where we would like further data, I can come to no
other conclusion but that these Patagonian beds are Lower Miocene, the
exact relationship with beds in North America and Europe, being as yet
not definitely settled, nor will this be possible until a study of the
migrations of the elements of the Patagonian fauna has been made.
As to the beds underlying the Patagonian, I am sure that a considerable
study of the marine series is still requisite to determine the
relationships of the beds in different parts of Argentine, and their
relative positions as compared with beds in other countries. Ameghino
appended to his paper on the Formations Sedimentaires a section
of the strata exposed on the coast of Patagonia from Rio Negro to
Cape Virgenes, on which from above Punta Atlas south to below Pico
Salamanca, the Casamayor (= Notostylops) beds fill the interval from
the Salamanca formation up to the Patagonian. On the strength of this
map I followed these beds the whole distance looking for vertebrate
fossils of Casamayor age. Nowhere did we find a Casamayor fossil.
Instead at several points we did find marine fossils. I can not but
feel that these beds are plotted as Casamayor, because of their
resemblance in color and general texture to the beds carrying the
Notostylops fauna at Casamayor.
Of several sections of these beds I pick out two as typical, and also
because they are near the locality which we worked for the Deseado
fauna. On the map they are indicated as A and B. The former passes
through a bed of green sands which is, I think, the locality indicated
as his northern locality for the Pyrotherium fauna.
[Illustration: Fig. 3. Section at _A_ on map page 5, showing strata
from sea level up to the Patagonian.]
[Illustration: Fig. 4. Section _B_ on map, page 5, showing strata from
sea level up to the Patagonian.]
From Punta Atlas to Pico Salamanca, Ameghino plots at or just below sea
level a bed known as the Salamanca, being typically developed opposite
Pico Salamanca. In this in the neighborhood of Pico Salamanca we found
the fauna typical of this horizon.
Ostrea rostigera v. Ih.
Ostrea riongrensis v. Ih.
Ostrea ameghinoi v. Ih.
Chlamys salamanca v. Ih.
Rostellaria striatissima v. Ih.
Rostellaria sp.
Cytherea calcedonica H.
Discinia sp.
Diplodon sp.
This Salamanca formation is considered by Wilckens as the equivalent of
the Roca as exposed on the Rio Negro, and to the Luisa as exposed on
the Rio Coyle. All agree that the Salamanca is Upper Cretaceous and a
period when Patagonia was covered by the ocean.
In section B we found the above fauna in layer 1 which is just
above sea level here. In layer 2 we found casts of delicate marine
shells (30 to 40 in number), representing four or five species and
as yet undescribed. They seem to represent a deeper water facies of
the Salamanca. In fact all the shales represented by layers 1 to 5
evidently belong to the Salamanca. Layer 5 was distinguished by having
in it at a point some 200 yards north of the section line a quantity of
turtle shell fragments.
Layer 7, consisting of coarser sandstones, was at the point of the
section, simply filled with a vast quantity of fossil wood, most of
it agatized, though some was carbonized, and representing some eight
species, mostly pines and palms, the latter much scarcer. The tree
trunks, hundreds in number, lay scattered in all directions; but all
were lying horizontal, and there was no indication of stumps in place;
so I consider that the wood was driftwood. It is common in the series
of beds of this general horizon along the Gulf of St. George. In the
other layers up to the Patagonian we found no fossils. The contact with
the Patagonian was unconformable, in some places being 50 feet higher
than in others near by.
In section A the typical Salamanca is below sea level, and the lower
parts of the section are made up of the white sandy clay shales,
so typical all along the Gulf of St. George. In the midst of these
clays at the level indicated as 2 occurred a layer of concretions.
On breaking these we found two specimens of Nautilus valencienni H.,
clear evidence that they were of marine origin. Layer 5 was filled
with hundreds of the very characteristic oyster, described as Ostrea
(Gryphaea) pyrotheriorum. Though in earlier papers suggesting that O.
pyrotheriorum represented a horizon of marine sediments corresponding
in age to the Deseado (= Pyrotherium) formation, in his Formations
Sedimentaires, Ameghino places this fossil in the Salamanca fauna,
though it here occurs at least 275 feet above the typical Salamanca
fauna. I believe the layer should be distinguished. It is later than
the typical Salamanca, though belonging to the same transgression of
the sea over Patagonia. In layer 7 we found still another marine fauna
consisting of
Ostrea guarantica H.
Venericardia sp.
Corbula sp.
Aporrhais.
Patomides.
Oxyrhinca.
Milobates.
Fragments of the limbs of a crab in abundance.
This seems to be the same fauna as that described by Ameghino as the
Sehuen developed on the Rio Sehuen.
In layer 8 we found large quantities of gypsum, occurring mostly in
balls of radiate structure. Layer 11 was a coarse green sand, and in
it we found some fragments of some sort of a bone. I think this layer
is what Ameghino designated as a Deseado exposure; and it has the same
general appearance and color which is found in the green sands of the
Deseado pocket on the Rio Chico. However it is conformable interbedded
with the underlying and overlying marine beds and I consider it a part
of the marine series. Above it come more white sandy clays that are
characteristic of the most of the section.
Wilckens takes all of this series, from the base of the Salamanca,
up to the unconformity below the Patagonian, and makes of it his St.
George Period, a transgression epoch, lasting to the end of the Upper
Cretaceous. I believe it is all marine, and is all a part of the
Upper Cretaceous transgression of the sea over Patagonia. However the
Salamanca is a clear cut deposit and I feel it should be retained as
a distinct horizon. The overlying light colored (white, grey, brown,
yellow, or green) sandy clay shales represent a deeper water and
later facies, which is characteristically developed on the Gulf of
St. George, and may well be distinguished as the St. George epoch or
series, but I should use the term only in this more limited way. It is
the same series which Ameghino has plotted as the Notostylops beds on
his section of the coast of Patagonia. This last it certainly is not.
The unconformity between these white (or light) sandy clays and the
Patagonian represents a regression period, during which Patagonia was
not only above water, but extended an unknown distance further to the
East.
It was during this interval of time between the Upper Cretaceous and
the Lower Miocene (Patagonian) that the limited and local land deposits
known as Casamayor (= Notostylops), the Astraponotus, and the Deseado
(= Pyrotherium) and probably other beds were laid down. In each case
the age must be determined for the individual bed by its contents
mostly; for as far as I know none of them overlap anywhere.
In regard to the discussion as to whether Dinosaurs were
contemporaneous in South America with the fauna of the Deseado, I can
only say, we found no trace of a dinosaur or any other Cretaceous
animal in the Deseado beds which we worked. As the Cretaceous beds
lie as high as the Deseado and are also practically horizontally
striated, dinosaur remains might be found on the same level. I think
the assigning of any such material to these beds was due to failing
to recognize the unconformity under the Deseado beds. As to the
Notostylops fauna and dinosaurs being contemporaneous, I only worked
the Notostylops beds at Mazaredo, but there I found nothing to indicate
the contemporaneousness of these two groups. As I have shown above,
Ameghino’s idea of the extent of the Notostylops or Casamayor beds was
mostly at fault, and very much of that which he has designated as of
Notostylops age is Upper Cretaceous. It is in these Upper Cretaceous
beds that dinosaurs do occur and this seems to me to be the basis of
the confusion.
This Upper Cretaceous series is a field where considerable work may
profitably be done, in straightening out the relationships of the
various layers to each other, their extent, and their relationship to
the Salamanca and other Upper Cretaceous formations in other parts of
Argentine.
As to the age of the Deseado deposit which we worked. It is under the
Patagonian, and therefore must be as old as the Oligocene. On the
other hand it must be as young as the Eocene, lying as it does above
the Upper Cretaceous. Of the three general faunas described it is
clearly more advanced than either the Casamayor, or the Astraponotus;
so should be put as the youngest of these three. The Colpodon,
the Astrapothericulus and the Notohippus, faunas are said to be
interstratified with the Patagonian and therefore of the same age. The
amount of advancement from the Casamayor to the Deseado is considerable
and the relationships of the Deseado are fairly close with the various
genera of the Santa Cruz; so that I should put the Deseado as far up as
possible toward the Santa Cruz. The Santa Cruz is above the Patagonian,
and I think that the Deseado should be put just before the Patagonian;
that is in the Oligocene, but just what part of the Oligocene can only
be determined when the other faunas have been further studied.