A new stratigraphic profile of Punta Hermengo (Buenos Aires Province, Argentina)

SOIBELZON E.; TONNI, E. P.; BIDEGAIN, J. C.

Current Researches in the Pleistocene

Center for the Study of the First Americans

Año: 2010 vol. 27 p. 151 - 154

8755-898X

Punta Hermengo (38° 17′ 13.8″ S, 57° 50′ 14.9″ W) is located near Miramarcity on the coast of the Buenos Aires province (General Alvarado County).The first reference to the geology and paleontology of this locality is by F.Ameghino (1908), who studied the sea cliffs exposed in Miramar, particularlyfrom a paleontological point of view. Subsequently, this locality was the subjectof numerous scientific contributions (for a summary see Soibelzon et al. 2006,2009).Kraglievich (1952) established a lithostratigraphic scheme for the sedimentsexposed along coastal cliffs between Mar del Plata and Miramar. In hisscheme, Punta Hermengo is categorized as Plio-Pleistocene sediments(Vorohué, San Andres, Miramar, and Arroyo Seco geological formations).The natural evolution of the landscape in the area is characterized byerosion of the cliff ledge and filling in of the bays, causing change in thecoastline and finally a regular retreat of the coastline. This dynamic is highlymodified by anthropic alteration (extraction of sand, urbanization, artificiallyfixed dunes, etc.) (Cenizo et al., in press).The geological and magnetostratigraphic profile presented here is exposedat the “Vieja Farola” (Old Lighthouse) of Punta Hermengo and consists ofeight units, but unfortunately the basal units are currently impossible to access(see below). Two discontinuities (d1 and d2) divide the profile into threemajor sections (Figure 1: I, II, III), and this sequence can be followed along the coast for at least 2000 m. The base of unit A is only exposed during very lowtide, and it constitutes the abrasion plains (Figure 1). A brief lithologicaldescription of the Punta Hermengo profile was provided by Soibelzon et al.(2009).The lower half of the profile (section II, units B–E) contains Ensenadanfauna (lower to middle Pleistocene, Cione and Tonni 2005; Soibelzon et al.2008). Overlying this is unit G, which includes Bonaerian fauna (middlePleistocene), and unit H, which contains Lujanian fauna (late Pleistocene/early Holocene). Biostratigraphically these units are characterized byMesotherium cristatum and Arctotherium angustidens (Ensenadan), Ctenomyskraglievichi (Bonaerian) and Equus (Amerhippus) neogaeus (Lujanian). These finds partially correspond to those of Tonni and Fidalgo (1982) and Soibelzonet al. (2006) because here we recognize one more lithostratigraphic unit andone biostratigraphic stage (Bonaerian).From a magnetostratigraphic point of view, units A–C are of reversedpolarity and correspond to Chron C1r1r (c. 0.90 to 0.78 million years ago).Units D-H are of normal polarity and correspond to Chron C1n (< 0.78million years ago).In addition, it is important to note that the southern Punta Hermengogeological sequence is interrupted by numerous palaeochannels that hinderhorizontal and regional correlations. Consequently, identifying the eightunits becomes challenging. While units A, B, and H and the twodiscontinuities (d1 and d2) are clearly identifiable throughout this region,units D to G are individually indistinguishable to the south of “Vieja Farola” ofPunta Hermengo. There they are integrated into two major units as unit D+Eand unit F+G. It is also important to mention that in the southern area ofPunta Hermengo, unit B contains M. cristatum remains, which allow us todetermine the Ensenadan age for these unit.The Punta Hermengo profile was used for comparison and correlation withother associations, not only at a regional level but also at a continental level.Unfortunately, the authorities of General Alvarado County placed quartziteblocks parallel to the cliffs to protect against erosion and retreat; consequentlyall sampling efforts and collection of new paleontological specimens werefrustrated due to the impossibility of accessing to section I (those containingEnsenadan fauna) (more details in Cenizo et al., in press). With the loss of thisimportant historical and natural place, the cultural heritage, part of thebulwark that gives a sense of identity and pertinence to the community, willdisappear.