SIRENTE: A POSSIBLE SMALL IMPACT CRATER IN  ITALY
J. Ormö,  A. P. Rossi and G. Komatsu

International  Research  School  of  Planetary  Sciences
Universita  d'Annunzio
Viale  Pindaro  42
65127  Pescara,  Italy
(ormo@irsps.unich.it).

Introduction:  The  Sirente  structure  (UTM
33T0384094  4670350)  is  located  in  the  Prato  del  Sirente 
karstic  plain,  Abruzzo,  on  an  altitude  of  about
1100 m. Our study of the structure has revealed a morphology 
inconsistent  with  a  karst  formation.  The
structure  contains  a  lake  encircled  by  a  prominent
ridge. The slope of the inner side of the ridge and the
flat floor of the shallow lake generates a bowl  shaped
morphology.  The  ridge  is  about  10 m wide  and  rises
prominently  3 m  above  the  flat  plain. The  rim-to-rim
diameter of the structure is about 100 m. The structure
is subcircular droplet shaped with the apex towards the
WNW. The ridge is in this part only half a meter high.
The ridge rises rapidly towards the ESE and is highest
and most volumous on the northern and southern sides
of the lake. The ESE part is lower, maximum height is
about 1.5 m, but  forms  a  low  and wide bulge  sloping
with a low angle towards the plain. The plain and  the
ridge are covered by an up to 20 cm thick humus layer.
The structure is formed entirely in coarse clastic valley
fill. Limestone boulders are outcropping on the rim.
Discussion: We  see  only  three  possible  causes  of
formation  for  the  Sirente  structure;  (i)  an  impact  of  a
small cosmic body, (ii) a periglacial pingo, or (iii) the
result  of  human  activities,  present  or  ancient.  Pingo
can  likely  be  excluded  due  to  inconsistency  between
the necessary high age (>10 Ky) and the apparent mi-
nor erosion of the structure. It is also the only structure
of its kind in the area. The volume of the ridge requires
extensive manpower or machinery for its construction.
Neither  the  Sirente  park  authorities,  nor  the  local
community  know  of  any  use  of machines  at  the  lake
for  this  purpose.  Human  activity  is  indicated  by  the
presence  of  low  stone  rows  on  the  surrounding  plain,
as well as a circle of 1.5 m boulders on a small hill 200
m from the structure. The low stone rows are property
borders  indicated  on  the  local  land  register  map [1],
which shows the lake resided in the historical property
outline established over centuries. In the map the lake
is mentioned  as  a  natural  lake  .lago  naturale., which
tells  against  the  lake  as  created  by  human  activity  in
recent historical time.
At  present  the morphology  of  the  structure  is  the
strongest evidence for the impact hypothesis. The ridge
may represent a rim wall formed by excavation of the
loose plain material. The size of the structure is in the
lower  range  of  explosion  craters  formed  by  hypervelocity 
impacts.  Impacts  of  this  size  occur  on  Earth
about once per a couple of hundred years [2]. Craters
of  the  size  of  Sirente  are  known  from many  locations
on  Earth.  In  Europe  the most well  known  example  is
the Kaali crater field in Estonia [3].
The slightly asymmetric rim wall connected to the
droplet shape of the Sirente  structure is in accordance
with  the morphology  of  craters  from  oblique  impacts.
If the structure is an impact  crater  it  is very well  preserved,
which  makes  it  likely  not  older  than  a  few
thousands  of  years.  Our  ongoing  research  activities
include magnetic separation of soil to find traces of an
impactor,  a  detailed  levelling  of  the  morphology,  as
well as a sediment coring of the infill, which hopefully
will give  the age  of  formation,  and  thus,  assist  in  
understanding the origin of the structure.



References: 


 
 Land  Register  Map  (Catasto  map), Commune  di  Secinaro 1977.
 
 Chapman C.R. & Morrison D. (1994) Nature, 367, 33-39.
 
 Czegka, W. &  Tiirmaa, R. (1998) Meteoritics & Planet. Sci, 33, A37.