Growth process of a long-lived Late Pleistocene spit along the Adriatic coast (Brindisi, southern Italy)

Abstract

Loiacono F., Growth process of a long-lived Late Pleistocene spit along the Adriatic coast (Brindisi, southern Italy). (IT
ISSN 0394-3356, 2010).
The Quaternary succession cropping out along the cliff at NW and SE of Brindisi town (Adriatic coast, Southern Italy), is mainly composed of silty sands and is referred to Lower-Middle Pleistocene by biostratigraphic integrated analyses. Moreover the benthic
assemblage of the cropping out sequence indicates a shallowing upward trend. A coarse grained calcarenite body (Punta Penne
Calcarenites), overlies unconformably the Middle Pleistocene fine-grained sediments cropping out along the coast for more than 9 km
and for up to ten meters thick, thus making a good shelter for the Brindisi port and town. The sedimentological analysis carried out
through this well exposed sedimentary body allows to recognize its growth process and make some hypotheses on the control factors
acting during its development. The sedimentary body is composed of bioclastic grainstones and rudstones arranged in well bedded
succession made up of about 30° SE dipping beds and bed sets in a cyclic alternation of coarse-grained planar units (Facies A),
small- to medium-scale trough cross-bedded units (Facies B), and Bichordites-bearing beds (Facies C). Facies analyses point out a
storm-dominated, foreshore-to-upper shoreface environment, characterized by migrating bedforms such as ripples, megaripples and
low-angle or quasi-parallel flat beds. In this scenario, the aforementioned units’ alternation is the result of the SE-trending lateral
accretion of a coastal spit, which developed accordingly to the dominant longshore currents, similar to the present day ones. The spit
sedimentary body becomes thinner toward SE; its upper boundary is sharp, being covered by recent soil; locally, Holocene aeolian
sands overlay the spit deposits, recording the last highstand. The alignment of this coastal spit results from two interactive factors: the
abrupt change in the coastline profile and a long balance between material supply (uplift-erosion) and alongshore movement. The first
control factor was a differential subsidence rate of the coast in this area that caused different wave refraction and high erosion of the
bend point. The SE-directed dominant longshore currents were also highly efficient in the transport and down-current deposition of
the sediment load. The SEward accretion of the coastal spit followed a long balancing of local subsidence, sediment production and
lateral accumulation. In time the decreasing efficiency of the longshore transport and lateral accretion caused facies changes with
increase in depth. Finally in the SE direction the stratigraphic record is regressive according to the regional trend. Grain size local
trends and measurements of foreset and festoon dips allow the distinguishing of frontal, sea- and land sides of the spit, as well as the
likely changes in the growth stages and its dying out. The duration of the spit construction could be limited to a few thousand years in
a time interval corresponding to Tirrenian maximux highstand (MIS 5 e).