Muhammad Nadeem A Dissertation Mississippi State, Mississippi
Late Holocenepalaeohydrographical evolutionin the...
Transcript of Late Holocenepalaeohydrographical evolutionin the...
Silvia E. PiovanUniversity of PadovaDepartment of Historical Geographical and Antiquity ScienceGeography Sectione‐mail: [email protected]
15th IP Seminar on “Geography of Water”Munich and Upper Bavaria
15th‐26th July, 2012
Late Holocene palaeohydrographicalevolution in the southern Venetian Plain
ADRIATIC SEA
VENICE
Po
ADRIAROVIGO
PADOVA
Adige
Brenta
BacchiglioneCHIOGGIA
Euganei Hills
Berici Hills
Prealpine relief
Tartaro
• Reconstruct the paleohydrographical evolution of the southern Venetian plain.
• Study the evolution of the coastline in relation with the palaeohydrographical changes during the Late Holocene (the last 5000 years ca).
• Project Arcus “Via Annia”, for the study of the Roman Age road.• Project CARG “Sheet 147 – Monselice”, for the geological mapping of Italy at the scale of 1:50,000.
FIRST PARTOF THE
PRESENTATION
SECOND PARTOF THE
PRESENTATION
Remote sensing• DTM• LIDAR• Aerial photos, satellite images
Literature review• Historical cartography analysis• Historical documentation analysis
Field survey and chrono‐stratigraphical analysis• More than 100 manual boreholes organized in transects across the major
alluvial ridges (depth from 4 to 9 m).• Open sections from building and archaeological excavations
Laboratory analysis• Radiocarbon datings• Petrographical analysis
• Quantitative compositional point counting of the sand samples , following the standard Gazzi‐Dickinson technique (Gazzi, 1966; Dickinson, 1970; Zuffa, 1980).
• 300 grains/sample
• An alluvial ridge is an elevated feature of a floodplain, formed by the aggradation of fluvial sediments deposited at the side of the channel.
• During high discharge periods when the river floods, coarse sediment settles out near the stream channel and grades to finer material further away.
2‐3 meters higher thanthe floodbasin
1.
2.
3.
4.
Photo by A. Ninfo
• To prevent floodings, artificial banks are often built close to the channel, typically higher than natural levees.
• Confining the flood discharge to a small area increases the velocity of flow.
Mississippi River delta in Louisiana
• For example, the levees of the Mississippi River increase the flow velocity near the mouth as it enters the Gulf of Mexico.
• As a result, sediment is shot into the Gulf rather than being deposited near the mouth building the famous 'bird's foot' delta.
ADRIA
ADRIATIC SEA
PADOVA
Po
ROVIGO
• Many published works attributed the major alluvial ridges to a particular fluvial system (Adige and Po) only by mean of remote sensing analysis, archaeological evidences and storiographical information.
ROVIGO
Adria
Saline
Conselve
ConaPettorazzaGrimani
N
Montagnana
EsteMonselice
Badia Polesine
28 m a.s.l.
-4 m a.s.l.
PROBLEM: to distinguish alluvial ridges formed by Adige from those formed by Po alluvial system (and, for the southermost area, by the Tartaro one???).
11 transects of boreholes and open sections across the major alluvial ridges have been performed, in order to:• study their stratigraphical architecture, also in relation with archaeological sites.• collect sand samples in order to perform a petrographical analysis and correlate the palaeochannels to the right fluvial system.
Sand petrography sample
Final Bronze to Roman Age archaeologicalstratigraphy
1
Bronze Age(Piovan et al. 2010)
Bronze to Roman Age(Piovan et al., submitted)
Bonavigo
Present-day Adige alluvial ridge
Montagnana-Este-Conselve alluvial ridge
In the High Middle Ages, an important avulsion occurred in Bonavigo, about 30 km western fromthe study area, and Adige River abandoned the Montagna‐Este‐ Conselve course in favour of the present‐day one, about 20 km to the south.
ConselveEste
Montagnana
DTM of the Bonavigo avulsion area.
Montagnana
Bonavigo
Legnago
N
Present-d
ay Adige
alluvial ri
dge Montagnana-Este-
Conselve alluvial ridge
28 m a.s.l.
1 m a.s.l.
Sand petrography sample
Adige - Present-day(Piovan et al., submitted)
Po - Iron Age?(radiocarbon datings still in
progress)
Modified from Piovan et alii (2010)
Petrographical sand analysis: the sampling
Microcline - NA01 Dolostone - TRE01
Siltstone - NA01Volcanite - TRE01Glaucophane - NA01
Inosilicate - TA01
PETROFACIES B: Quarzolithic petrofacies,with 52.4% average quartz content and a relatively more abundant feldspar content
PETROFACIES A: Large abundance of dolostone and of acidic volcanic rock fragments
Quartz
Lithic + Extrabasinal CarbonatesFeldspar
(Gazzi et al. 1973)
PETROFACIES B, belonging to Po sands is also characterized by abundant heavy minerals with the diagnostic presence of glaucophane (Gazzi et al., 1973; Stefani, 2002).
• Adige and Po sands are well distinguishable by mean of the triangular plot Q‐F‐L+CE.
• Po sands are distinguishable from the Adige ones also by the presence of glaucophane.
• Petrographical sand analysis performed on sand samples confirm that Fratta and Saline‐Cona alluvial ridge belonging to Po fluvial system.
Zecchin et al. (2009) - Continental Shelf Research
Zecchin et al. (2009), thanks to a stratigraphical investigation (seismic profiles and cross‐section) recognized a late Holocene
delta mouth sedimentary system in the southern Venice Lagoon.
Buried DELTA recognized byZecchin et al.
(2009)
OBJECTIVE: To reconstruct the coastline evolutionin relation with the Adige and Po palaeohydrography.
DELTAAdige + Po(Piovan et al., submitted)
• On the basis of the geomorphological and stratigraphicdata, it is possible to assign this delta system to the Saline‐Cona Po branch, active during 4‐3 ka BP.
• In that time, the Adige River did not directly reach the sea, as it was a tributary of the Po River through the Conselve ridge.
• The northernmost lobe of the Po delta was thus fed by the sedimentary input of both the Po and Adige Rivers.
• This implies that the Po delta was extending as far as 30 km north of the present‐day river position.
Bronze Age (3000-1000 BC ca.)
AdigeBronze to Roman Age
(Piovan et al., submitted)
Po - Bronze Age
(Piovan et al. 2010)
Iron to Roman Age (10th - 4th BC century)
AdigeDELTA
(Piovan et al., submitted)
Po DELTA(Bondesan et al., 1995;
Stefani & Vincenzi, 2005)
AdigeBronze to Roman Age
(Piovan et al., submitted)
Po - Iron Age?(radiocarbon datings still on work)
• The deactivation of the Saline‐ConaPo branch through avulsion just upstream of Rovigo, around 3 ka BP (at the beginnig of the Iron Age), led to a southwards shift of the Po delta system.
• The Adige River kept its mouth in the same area and, from 3 ka BP to Roman times, built its own delta in the southern Venice Lagoon, prograding on the previous northern Po delta lobe.
X
Middle Ages (6th BC century) - now
AdigeDELTA(Middleand
ModernAge)
• The Adige avulsion at Bonavigo during the early Middle Ages led to the abandonment of the Montagnana‐Este‐Conselve‐Santa Margherita path. As a consequence, the delta in the southern Venice Lagoon was also definitely abandoned and the Adige River started to build another delta in its present‐day position, about 15 km further to the south.
PoDELTA(Middleand
ModernAge)
X
Piovan et al. (submitted to Geomorphologie)• Accelerated growing of the Po delta during the Middle and Modern Ages is due to...
Po River artificial embankment
…the construction of very high artificial embankments since XIV century (by the Venetian Government), that do not allow the rivers to deliver any sediment discharge to its lateral alluvial plain (Cremonini, 2007).
• The general seawards migration of the coastline since the mid‐Holocene indicates that the Po and Adige sediment supply overhelmed the Holocene relative sea‐level rise. The results show that major avulsive events in the upstream tracts of the Adige and Po Rivers (in Bonavigo and Rovigo respectively) forced the migration of delta lobes.
• The detailed reconstruction of the directions of the major Adige and Po River branches in the alluvial plain allows a better understanding of deltaic evolution along the Adriatic coast.
• A general southward shift of the river systems and related delta lobes is recorded during the last 4 ka, suggesting a migration of the depocenter towards the present Po delta area.