Evaluation of contribution sources for the sediments of the La Paz Lagoon, based on statistical treatment of the mineralogy of their heavy fraction and surrounding rock and drainage basin characteristics

Resumen

To identify the main
sources of terrigenous sediments of the La Paz Lagoon, a description
of the hydrology of the drainage basins and the mineralogy of the
heavy fraction of the lagoon sediments is needed. In this work, 14
terrigenous sediment samples from the arroyos (dry streams) and 55
superficial marine sediment samples were collected from the study
area. The drainage basins of the principal arroyos discharging into
the lagoon were defined using geomorphologic analyses on Geographic
Information Systems (GIS). The sediment samples’ grain size fraction
of 3.25 – 3.00 phi was obtained by sieving. The heavy minerals were
separated using bromoform and the mineralogical composition was
determined using a petrographic polarization microscope. Three main
sedimentary provinces in the lagoon were distinguished based on
principal component analysis of the heavy mineral data and the study
of the surrounding regional streams. The eastern sedimentary
province is characterized by the predominance of orthopyroxene,
especially hypersthene, as well as a high hornblende presence. This
is due to the erosion of volcanoclastic sequences of the Comondú
Formation and intrusive granitic complexes of Sierra de Las Cruces.
The southeastern province displays a high abundance of hornblendes
(up to 81% in some stations), micas, apatite, sphene, anatase,
tourmaline, chlorite, clinopyroxene and orthopyroxene. Lower
contents of amphibole (up to 27.2% in some stations) and resistant
garnet and zircon minerals were found, as were some soft-pink
piedmontite and pale-green hypersthene. These minerals were supplied
by the El Cajoncito, Los Bledales, La Palma, Cardonal and El Novillo
arroyos eroding the intrusive rocks of “Granito Sierra de Las
Cruces”, “Tonalita La Buena Mujer” and in a smaller extent the
extrusive and non-marine rocks (tuffs, riodacites, conglomerates and
sandstones) of the Comondú Formation. The predominant minerals of
the northwestern province are clinopyroxene, orthopyroxene and
phosphatic ooids. Relatively high abundance of olivine, hornblende,
zircon, epidote and garnet were also found. These minerals are
presumably supplied by the arroyos La Ardilla and other streams to
the north of the El Centenario and El Comitán villages, as well as
by eolian and littoral transport of the El Mogote sandbar dunes,
which accumulates eroded sediments from the marine sedimentary rocks
of El Cien Formation.