Publicaciones Geolo_gicas Especiales Del lngeominas

ZONATION OF THE CRETACEOUS OF CENTRAL COLOMBIA BY AMMtlNITES 1
Cretaceous rocks ranging in age from
. Upper Aptian to Lower Santonian llve been
carefully mapped, measured and colleoted bed
by bed in two regions (it Central Colombia namely_:
Villa de Leiva (Department Boyaca) and
Apulo (formerly Rafael Reyes, Department
·
Cundinamarca).
The first mentioned sequence comprises
about 2.000m of locally highly fossiliferous
shales, although limestones, sandstones and
chert beds are interspersed and sporadically
yielded fossils. At Apulo the sequence consists
dominantly of shales with locally abundant
fossiliferous calcareous "concretions" and one
intercalcation of fossiliferous graded-beddedsandstones
(turbidites) with a total thickness of
about l.OOOm.
The lithostratigraphic terminology utilized
is that previously proposed and published by
the writer, the old terminology is reevaluated
and some new units are introduced and their
type sections and reference sections described.
Similarly the occurrence of ammonites is indicated.
The core of the investigation is however
the study and description of ammonite taxa:
102 species assigned to 52 genera were thoroughly
investigated, of them 85% of the species
and 21% of the genera are considered to represent
new taxa. A zonal scheme consisting of
7"A'iSemblage zones" is proposed, based on the
succession of ammonites in these two sections,
hopefully it will be a yardstick for the Cretaceous
biostratigraphy of the· northern part of
South America. No close correspondence between
lithostratigraphic boundaries and bios-
Caminante no hay camino ,
se hace camino a/ andar.
ABSTRACT
* * *
Yo u ask me the secret
It only has a word,again.
tratigraphic units has been found, however
some taxa of ammonites witeculiar morphologic
characteristics seem to-'associated with
definite lithofacies.
The diverse taxa of ammonites were studied
through dissection of their conchs in order
to determine their ontogenetic changes and all
morphological characteristics were evaluated in
an attempt to understand each taxon independently.
From the taxonomic investigations and
the known stratigraphical distribution of the
faunas it is shown once more that similar--although
not 'identical' -morphologic types of
ammonites recurred during Cretaceous times.
The taxonomy resulting from· the-criteria
utilized herein indicate that the described taxa
have proportionally a distinctly South American
character, however some ubiquitous elements,
several of them previously unknown from the
Cretaceous of South America are recorded; as a
possible biological explanation for this fact it is
hypothesized, that as in the case of modem cephalopods
the ubiquitous elements of the ammonoid
faunas may correspond to those hatched
from small eggs (with small protoconchs) and
vice versa.
All the species described are illustrated by
drawings of their whorl cross-section and suture
line, and at least one photograph of each.
Conch measurements are expressed in
mm. The symbol D represents the diameter
of a volution. Euw is the external umbilical
width. H is the maximum height and W the
maximum width of the whorl.
Me habeis preguntado que hila el
crustacea entre sus patas de oro
y os re spondo: e/ mar lo sabe.
(MACHADO, HOLUB, NERUDA, poets)
But all this is unripe fruit. Our scholars will some day know more than their masters do
now; so let us patiently continue our work and rema in friends.
E. Suess.