Geological and sedimentological characteristics and paleo-environmental conditions of sedimentary - ancient section of Gaskarak in Roudbar county

Introduction
Geoarchaeology is a new and interdisciplinary concept that studies the past of human history using
geological methods. In geoarchaeology, a complete and accurate understanding of ancient records can
be obtained by identifying and examining the sedimentary content and stratigraphy of archeological
layers and materials. Geology has been available to archaeologists over the past two decades as a basic
tool for reconstructing ancient environments and understanding the long-term climatic and
anthropogenic conditions and interactions of pre-historic human and surrounding environments. These
studies are particularly influential in understanding the Pleistocene and Holocene archaeological and
geological backgrounds and materials. Meanwhile, geoarchaeological studies of Iran are practically
linked to Paleolithic observations and exploration and the use of geological methods such as
sedimentology to describe the details of ancient records in caves and rock shelters. Geoarchaeology is
an interdisciplinary specialty between geological sciences and archeology that examines the role of
geological factors in the formation, continuation, and weakening of ancient settlements. In this field,
techniques and methods common in earth science such as aerial photography and satellite imagery,
sampling, microscopic studies, chemical analysis, etc. are used to solve archaeological problems. On
the other hand, often referred to as archaeo-geology, archaeological data are used to solve geological
problems, particularly in relation to dating of Quaternary deposits, ancient seismological studies, and
ancient mining. Large-scale archaeologists study most of the natural landforms and anthropogenic
structures and small-scale archaeologists study the soil, natural sediments, and anthropogenic deposits.
Archaeo-geology also occasionally covers other interdisciplinary studies such as pottery petrography,
ancient mineralogy, ancient metallurgy, dating, etc. and so-called archaeometry.
Materials and Methods
In order to determine the frequency of sediment grains based on their size and to investigate the pattern
of particle size distribution to determine the textural properties, the sorting of grains and the sedimentary
abundances of two natural sediment layers I and II were extracted from Gaskarak section and analyzed
by laser particle size analyzer. Organic matter and carbonate bind the sediment particles to each other
and therefore, to determine the true distribution of particle size, they must be removed prior to analysis
of the samples to separate the sediment particles. The mixing and rotating system of the device will
cause the particle to move and be exposed to the laser beam.
So after the laser light from a high voltage source is exposed to the sample, the laser beam will reflect
from the sample surface and then pass through it. The size of the deposited particles is directly
proportional to the magnitude of the reflected laser light and to the angle of refraction of the laser beam
to the surface of the particle, so that with increasing diameter of the deposited particles, the intensity of
the reflected laser light increases, but its angle of failure decreases.
Results and Discussion
Gaskarak section consists of two layers of fine-grained alluvial-debris sediment, with distinct color
which their lower boundary is confined to the sedimentary bedrock and their upper boundary is covered
by more recent deposits. The natural sediments that form these two layers are silty particles that show
high density. The lower layer (I) is mainly composed of silty fine-grained sediments with calcareous
fragments. Layer II, covered at its upper bound by the natural and cultural deposits of the Bronze Age
is characterized by a charcoal inclination that distinguishes it from its lower layer. Study of the recent
samples suggests dry to semi-arid climatic conditions for the formation of carbonated horizons in the
soil. Low carbonate content, reddish-brown color, as well as the presence of charcoal fragments in layer
II, which is distinct from layer I, suggest a different environmental condition for layer II that seems it
has been facing simultaneous climate changes, such as increasing rainfall and humidity and expanding
forest vegetation in the region.
Conclusion
Field evidence and results of laboratory studies indicate that the natural deposits of the Gaskarak section
probably formed as abnormal sediments on ancient hillslope surfaces of sedimentary bedrock. As the
morphology and slope of the sedimentary layers show a valley-like position, the eroded material
accumulates from sections with higher topography within it. The natural sediments of the Gaskarak
section are distinguished by two distinct layers (I and II) that show distinct differences in color,
carbonate content, and mineral and organic fragments.