Date of Award
Geomorphology, Santa Marta, Colombia, South America
Doctor of Philosophy (PhD)
William D. MacDonald
James R. Beerbower
Thomas W. Donnelly
Science and Mathematics
The Santa Marta area, occupying the extreme NW corner of the Sierra Nevada de Santa Marta is formed by rocks comprising two distinct structural and metamorphic provinces. These correspond to the Concha Pornation to the north and the Gaira Group to the south. The Concha Formation is composed of fine-grained quartzo-feldspathic schists and massive chloritic feldspathic greenschists, the former characteristically displaying a crenulation foliation in varying degrees of development. The Concha Formation has suffered retrograde metamorphism under high PCO2 conditions which account for the general absence of Ca-amphibole and the abundance of chlorite + carbonate assemblages. Local occurrences of coarse grained amphibole schists radiometrically determined to be Cretaceous in age apparently survived the high PCO2 conditions.
The Gaira Group is composed of three formations which are from north to south: the Punta Betin Formation: massive basic schists composed of chlorite, carbonate, feldspar and abundant acicular Ca-amphibole; the Cinto Formation: a dark gray graphitic phyllite; and the Rodadero Formation: two mica schists and banded amphibolites. The Gaira Group is intruded along its entire southern contact by the Santa Marta Batholith, a quartz-diorite of Eocene age.
The contact between the Concha Formation and the Gaira Group is approximated near the town of Santa Marta by the Punta Betin fault. To the east this fault is truncated by the concha thrust fault which defines the contact east of the Quebrada de Concha alluvial plain.
Structures in rocks north of the Concha thrust fault are characterized by shallow dip (≤ 40°). Structures between the Concha Fault and the Punta Betin Fault are characterized by steep southerly dips.
The Gaira Group is comprised of largely NE and ENE striking foliations which are slightly discordant to the schist-batholith contact. Two NNW trending steeply dipping faults (the Palangana and Naguange Faults) which offset metamorphic structures and isograds, form a graben structure east of Palangana. These faults appear to predate the movement of the Concha Fault to the north which apparently overrides the Palangana Fault. Also the east-west trending Jordan Fault cuts the batholith as well as the NNW trending faults near the schist batholith contact.
Two distinct metamorphic zones are established: Zone I and Zone II which approximately correspond to the Concha Formation and Gaira Group respectively. Metamorphic grade increases in both zones from north to south resulting in five subzones containing the following characteristic phases: Zone IA: rutile + calcite; Zone IB, sphene + calcite, epidote; Zone IIA: pale green-blue green Ca-amphibole; Zone IIB: green-brown Ca-amphibole, diopside; Zone IIC: deep brown Ca-amphibole. A systematic change in optical properties and lattice parameters of Ca-amphibole with increasing metamorphism is noted south of the town of Santa Marta.
According to mineralogical, structural and radiometric evidence, at least three metamorphic events have occurred since mid-Cretaceous time. Evidence of the earliest event in mid to late Cretaceous is the coarse-grained amphibole schist of the Concha Formation. The second metamorphic event (Paleocene?) resulted in a “retrograde” metamorphism of the Concha Formation under high PCO2 conditions associated with movement on the Punta Betin fault. The third metamorphism was associated with deep seated contact metamorphism in Eocene time which resulted in an overprinting of the earlier mid-Cretaceous event.
Intrusion was followed by formation of the graben structure near Palangana, the overriding of part of the Gaira Group by the Concha thrust fault and formation of the east-west trending Jordan Fault. The movement on the later faults may be associated with uplift of the Sierra Nevada de Santa Marta in Miocene or younger time.
Doolan, Barry Lee, "The structure and metamorphism of the Santa Marta area, Colombia, South America" (1970). Graduate Dissertations and Theses. 154.