THE DISCOVERY OF A PROBABLE WELL-PRESERVED IMPACT CRATER FIELD IN CENTRAL ITALY
Synopsis
This investigation presents what we propose
to be the first discovered impact craters in Italy.
They form a crater field in the Sirente plain within the
mountains of the Abruzzo region, central Italy. The
craters were studied morphologically by detailed leveling,
and geologically by drilling and trenching. We
used magnetometry to localize possible meteoritic
material in the smaller, non-explosion craters in the
field. The field has about 17 craters in the size-range of
2-20 m, and a droplet-shaped main crater that is 140 m
long and 115 m wide. It has a well developed rim wall
about 2.2 m high and 15 m wide. The target consists of
loose sediments and the age of the impact event is late
Holocene.
Introduction
The Sirente crater field is located in
the western part of the Prato del Sirente plain. The
plain is about 1750 m long and 750 m wide, and is
bordered on its southern side by a NW-SE striking
fault. Several of the Abruzzo high plains hosted
ephemeral lakes during the Late Pleistocene and Holocene
with deposition of lacustrine sediments 1. During
the late Holocene many of these lakes dried up and
eolian deposition became stronger.
No bedrock is outcropping within the Sirente plain.
The valley being a half-graben on the northern side of
the high Sirente massif indicates that the valley infill is
of great thickness.
The drilling and trenching conducted in this study
showed that the upper ten meters or more of the valley
infill is fine sand and silt with sporadic limestone
blocks. The Sirente crater field is located within the
Sirente-Velino Regional Park and is protected by the
park regulations. All our activities have been done in
agreement with the park authorities.
Description:
The crater field is 450 m in length
and 400 m wide and has at least 17 smaller craters distributed
close to a main crater. (Fig. 1) The main crater
is located in the southern end of the crater field and is
an order of magnitude greater than the other craters in
the field (Fig. 2). All the craters, including the main
crater, are developed entirely in yellowish, silty-sandy
unconsolidated eolian and lacustrine sediments.
Only the most prominent of the craters were
mapped. The surface of the plain has numerous undulations
and shallow depressions possibly representing
additional crater structures. The diameters of the
mapped craters vary from 2 m to 20 m. Some have an
apparent depth of 1.5-2 m, whereas others are only
shallow circular depressions. Only one of the small
craters has a low rim wall on one side. A magnetic
survey with a Geometrics Cesium G858 magnetometer
coupled with DGPS showed distinct magnetic anomalies
(20-30 nT) linked to most of the prominent small
craters (Fig. 3). The plain in general and the main crater
were magnetically calm. The magnetic anomalies
can be ascribed to objects characterized by a remanent
magnetization with different direction with respect to
the present geomagnetic field. Excavation of an approximately
10 m wide crater to 4.5 m depth did not
reach the object causing the magnetic anomaly. However,
the excavation gave valuable information on the
subsurface structure of the crater. At the rim the pedogenesis
reaches to about 1 m depth, which is the
normal thickness in the plain, whereas at the center of
the crater a black organic soil dominates to at least 4
m. Between the rim and this about 3 m wide central
column of soil is a section dominated by slump structures
and lenses of deformed organic-rich soil. The
slumped section is interpreted to have formed by collapse
of the crater somewhat like the breccia lens in
craters formed in solid targets. Water may have percolated
down in the disturbed sediments of the central
part of the crater causing pedogenesis to great depth.
The excavation will be resumed in the near future and
continued until the object causing the anomaly is
found.
The main crater has a slight droplet shape and is
140 m long and 115 m wide rim-to-rim. It has a well-
developed, saddle shaped rim wall that rises at a
maximum of 2.2 m above the surrounding plain.
Drilling through the rim showed that the target surface
at the rim is positioned about 1 m lower than today’s
plain surface. The true height may therefore be about 3
m. A shallow moat outside the western part of the rim,
and compressed sediments in one drill core, give further
indications for a downwarping of strata at the rim
rather than the uplift that could be expected. Downwarped
and compressed strata have been noticed at
experimental TNT explosion craters in loose sediments 2,
as well as at the impact craters formed in loess at
Campo del Cielo, Argentina 3. This circumstance
indicates that the Sirente crater field can serve as a
well-preserved example for cratering mechanics studies
of small craters formed in loose sediments. We will
further investigate the implications that this may have
for the understanding of small craters visible in MOC
images of Mars.
Preliminary radiocarbon dating of the target surface
preserved below the rim wall indicates a formation in
the late Holocene. The young age is consistent with the
apparent little modification of the rim wall and the
distinct surface expression of some of the smaller craters.
The morphology of the main crater and its relation
to a crater field with buried objects with remanent
magnetization strongly points to an impact origin. The
main crater is in the size range of explosion craters and
has many features comparable to craters formed in
loose sediments, both experimental and meteoritic impact
craters. This crater represents a rare example of
well-preserved, small explosion craters from impacts
into unconsolidated target materials.
References:
- Giraudi (1997) Il Quaternario, 10, 191-200.
- Jones (1977) In: Roddy, Pepin, and Merrill (Eds.)
Impact and Explosion Cratering, Pergamon Press: NY, 163-183.
- Cassidy and Renard (1996) Meteoritics & Planetary Science,
31, 433-448.
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