The Messinian salinity crisis in the Adriatic foredeep: Evolution of the largest evaporitic marginal basin in the Mediterranean

6 The recent release of a large number of subsurface geological data by the Italian 7 Minister of Economic Development, including boreholes and seismic profiles, provided the 8 occasion for a new assessment of the deposits associated with the Messinian salinity 9 crisis (MSC) in the Adriatic foreland basin system and a new integration with the 10 outcropping successions of the Apennines. In particular, the study of the Messinian 11 evaporites allowed to reconstruct a new detailed palaeogeographic and palaeobathymetric 12 framework for all the stages of the crisis. 13 We identified the largest evaporitic marginal basin ever described for the Mediterranean 14 hosting the precipitation of the primary shallow-water gypsum deposits (PLG, Primary 15 Lower Gypsum) during the first stage of the crisis. During the second and third stages of 16 the crisis, the PLG basin underwent uplift and erosion and the evaporite accumulation 17 moved to the deeper part of the basin and was characterized by the deposition of the 18 Resedimented Lower Gypsum unit including clastic evaporites, recycling the PLG ones, 19 primary halite and terrigenous deposits. 20 The distribution of the different evaporitic facies, was the basis for an improved 21 reconstruction of the upper Miocene tectonic evolution of the Apennines thrust belt. Our 22 results show a clear separation between shallower depocenters, located in the wedge-top 23 and in the Adriatic foreland basins and characterized by MSC stage 1 PLG deposition, and 24 deeper-water ones, located in the Adriatic foredeep and close to the Calabrian Arc, where 25 MSC stage 2 terrigenous and gypsum-bearing clastic deposits and primary halite 26 accumulated. 27


INTRODUCTION 29
The distribution of the Messinian salinity crisis (MSC) related deposits in the Apennines 30 and in the Adriatic foredeep basin has been matter of several studies during the last 31 decades, mostly based on outcrop data (Roveri et al., 2001(Roveri et al., , 2004(Roveri et al., , 2014b. Gibraltar area and glacio-eustatic changes . The largest part of the 52 evaporites deposited during the MSC is now buried below the deep Mediterranean 53 seafloor but the large number of outcrops has allowed the reconstruction of a very-high 54 resolution stratigraphic framework through the integration of bio-, magneto-and 55 cyclostratigraphic data (Clauzon et al., 1996;Krijgsman et al., 1999;Hilgen et al., 2007;56 CIESM, 2008). Although a detailed distribution of the MSC deposits in the offshore area 57 has been made available (Lofi et al., 2011;Lofi, 2018), the lack of continuous cores and 58 logs in the deeper settings together with insufficient seismic resolution leave the 59 interpretation of the different seismic facies still a problematic issue (see discussion in 60 -336 cross the MSC; in the supplementary documents (tab. S1 and klm file), these 298 boreholes are grouped according to the crossed evaporite deposits. 299 We focused of the late Tortonian-early Pliocene stratigraphic interval in order to 300 reconstruct the distribution of the deposits associated with the Messinian salinity crisis 301 along the Adriatic foredeep. 302 303

THE OUTCROPPING MSC UNITS 304
Here we will briefly describe the main physical and sedimentological characters of the 305 different evaporitic units as they appear in outcrop; these features can be useful in the 306 interpretation of the borehole logs. 307

Primary bottom-grown gypsum (PLG unit; stage 1) 308
Due to its peculiar characters the PLG unit is easily recognizable in the field. The 309 complete succession forms large-scale tabular bodies with a thickness of 200 m or more 310 (Fig. 2a,b) and includes up to 16 gypsum beds separated by thin (typically 1-3 m) intervals 311 of dark euxinic shales (Fig. 3a). The internal organization, which is maintained over large 312 distances, is characterized by (Lugli et  facies containing a larger amount of limestone and/or shale (Fig. 3b). 328 The PLG deposits rest conformably on hemipelagic or shelf shale and are erosionally 329 cut at the top by the MES.

Gypsum and hybrid clastic deposits (RLG unit; stage 2) 331
The RLG unit is floored by the MES; it rests unconformably on pre-crisis deposits but 332 locally, in the basinal areas where the MES pass down basin to its correlative conformity 333 surface (MES-cc), a barren organic-rich shale interval (FBI) is present below the unit 334 . The RLG evaporites form tens of m-thick lenticular or tabular bodies 335 ( Fig. 2c)

gypsum-bearing turbidites (RLG2) 348
This group includes the gypsum-bearing gravity flow deposits (granular flows and high-to 349 low-density turbidity currents; facies R2 to R7 of Manzi et al., 2005) commonly consisting 350 of m-thick composite graded beds showing a lower coarser-grained (rudite or arenite) 351 gypsum-bearing division capped by a finer-grained one mostly composed by gypsiltite or 352 shale (Fig. 3c). Commonly these beds show a good lateral persistency and limited 353 thickness (Fig. 2c). Carbonate and terrigenous sandstone clasts recycled from older 354 deposits may be found in the coarser-grained interval. The base of these beds is 355 commonly sharp and the top is smooth due to the normal gradation and the transition to 356 the shale interval. 357

Primary halite and gypsum deposits (RLG unit; stage 2) 358
These deposits can only be observed where diapirs crop out or in mines in Calabria 359 (Crotone basin), Sicily (Caltanissetta basin) and Tuscany (Volterra basin) otherwise they 360 are absent in the rest of the Apennines. Halite forms lenticular bodies with local thickness 361 up to 600 m due to intense halotectonics. Internally they consist of dm-thick beds 362 separated by thin anhydrite or shale horizons; thin K-Mg rich salt beds are locally found in 363 the middle part of the halite bodies (Lugli et al., 1999;Manzi et al., 2012).

post evaporitic deposits (Lago-Mare unit; stage 3) 365
The primary gypsum deposits of the Upper Gypsum unit (UG; Manzi et al., 2009) occur 366 only in the Caltanissetta basin (Sicily), capping the RLG unit. In the Calabrian arc and in 367 the rest of the Apennines the RLG is capped by thick terrigenous fine-grained deposits 368 including a rhyolitc volcaniclastic key-bed described in paragraph 2 and showing a 369 coarser-grained upper portion (p-ev 2 unit) including conglomerates (Cusercoli Fm, 370 Romagna, Roveri et al., 1998; Carvane unit, Crotone basin, Calabria, Roda, 1964), 371 sandstones and thin limestone layers (so called colombacci). The Lago-Mare biota are 372 mostly distributed in the p-ev 2 unit and in its time equivalent, upper half, portion of the 373 Upper Gypsum unit. The end of the crisis is marked everywhere by the sudden transition 374 to fully marine deposits, commonly preceded by a dark shale horizon . 375

SUBSURFACE 377
The 1341 boreholes drilled in the study area can be grouped as follows on the basis of 378 the crossed deposits (see tab. S1): 379 Boreholes not crossing the MSC -642 boreholes did not cross the salinity crisis

Primary Lower Gypsum intervals (PLG) 415
The PLG unit is characterized by a peculiar blocky pattern obtained by thin spikes of 416 low resistivity/high gamma ray that punctuate a high resistivity/low gamma ray base line,

Resedimented Lower Gypsum intervals (RLG) 427
The RLG unit (e.g. Thurio_001 and Dalila_001 boreholes; Fig. S1) is characterized by a 428 (finely) spiky pattern obtained from a thin alternation of spikes with high resistivity/low 429 gamma ray (gypsum) and spikes with low resistivity/high gamma ray (clays). As shown in 430 the previous paragraph the clastic gypsum beds are thinner with respect to the PLG beds.

Salt-rich intervals 432
A very high resistivity (~10000 Ohm.m) identifies the salt-rich interval (e.g. Thurio_001 433 borehole; Fig. S1). The alternation of thin halite, gypsum and clay may result in a spikey 434 pattern whereas massive halite may produce a blocky one. Halite is commonly 435 characterized by low gamma ray values (0-10 API units) whereas K-salts can be 436 Molise-Lagonegro nappes and ii) to the Adriatic foreland basins (Fig. 1). barren shale unit can be found in the area where the PLG unit is absent (Fig. 4b). 508 The peculiar pattern of the PLG successions observed in outcrops and described in the an almost horizontal bedding. It follows that the thickness obtained from the boreholes can 545 be used for the reconstruction of the isopach maps (Fig. 6b, c, d). The PLG1+2 beds are 546 relatively thin and have been grouped together. PLG3 and PLG4 have been considered in 547 separate maps. No map has been reconstructed for the overlying beds because they are 548 not continuous all along the study area due to erosion at the top. 549 The preservation of the complete succession in the southwestern area between the Gran 550 Sasso and the Gargano can be explained in terms of evolution of the foredeep. During the 551 pre-MSC this area was shallow, and shelf carbonate deposits accumulated, while 552 hemipelagic deposits were deposited more to the north. During stage 2 and later this area 553 experienced a rapid subsidence that can be related to the flexure of the foreland ramp due 554 to the load of the eastward migrating Apennine chain; thus, the present-day depth of the 555 PLG unit has been reached long after their deposition.  . 4d). The only exception is found in a small area in the Basilicata region, described 576

below. 577
Evaporite-free deposits containing typical hypohaline biological association are 578 comprised between the clastic evaporites, below, and the Pliocene, above, and can be 579 assigned to stage 3. The Lago-Mare biota could be present also in the stage 2 deposits 580 but become more abundant in stage 3 (Roveri et al., 2008c); the direct recognition in 581 boreholes indicate a relatively high abundance of biota, suggesting an assignment to 582 stage 3 rather than to stage 2. 583

The Messinian Apennines: distribution of the MSC deposits 584
The distribution of the different evaporitic facies in the Adriatic foredeep led to depict 585 more clearly the geological evolution of the Apennines during and after the MSC. The 586 integration of outcrop and borehole data has been the base for the reconstruction of two 587 borehole-based regional-scale geological sections (Fig. 7) extending from the Tyrrhenian 588 to the Adriatic sides of the Apennines. Gargano high (G fig. 6a) and confined the Adriatic PLG basin to the south. To the north of 605 the sill a large Adriatic evaporitic basin hosted the deposition of the PLG (Fig. 4c) The W-E section (Fig. 7b), perpendicular to the previous one, shows more clearly the A slightly different situation can be described for the Basilicata area (Fig. 8) Molise-Lagonegro Nappe as they have been deposited more to the west and at a greater 630 depth than their present-day location. 631 We have also reconstructed two regional-scale seismic sections in the northern and 632 central Apennines (Fig.9) in order to better show the distribution of the evaporites in the 633 Adriatic foreland. In the northern Apennines (Fig. 9a)

Implication for tectonic reconstructions 668
The distribution of the evaporites provides some important constraints that can be used 669 for the restoration of the Apennines in the Messinian. shales Fms) and, locally, shelf carbonate deposits (Fig. 4). where the unit crops out, the Adriatic basin is much larger (see comparison in tab. S1). 712 Additional smaller occurrences of PLG deposits above the foreland, are found in basins 713 located both onshore (EV basin) and offshore (between Ravenna and the Po river delta) 714 All these basins containing PLG can be considered to have an average paleo water- Mediterranean (Roveri et al, 2014c), fits well with the observations in the study area. It is worth noting, infact, that halite has not been found in situ above the Apulian Platform, but it 733 is present in some sectors of the Calabrian Arc (e.g., the Crotone basin), in those area that 734 were deeper during the pre-MSC and stage 1 (Figs. 4, 10). Following this interpretation, a 735 thick halite unit was deposited in the deep Ionian basin and in its western sector. The 736 halite unit was subsequently accreted at the front of the Calabrian Arc during the south-737 eastward migration of the arc that occurred in the Plio-Quaternary (e.g. Gutsher et al., 738

2017). 739
In the Apennine outcrops the RLG unit is overlain by thick terrigenous deposits 740 Platform. Our analysis of the ViDEPI dataset allows to define a large area (pink area in 750 Fig. 04 d) where the evaporites are buried below the Molise-Lagonegro Nappe (Fig. 7). On 751 the basis of log patterns, the unit can be interpreted as clastic evaporites (RLG), similar to 752 those extending the Romagna to the Laga basin (Manzi et al., 2005). Conversely, the 753 evaporites found above the Molise-Lagonegro Nappe belong to the PLG, deposited during 754 stage 1. 755 Therefore, it can be inferred that the Molise-Lagonegro Nappe during stage 1was close to 756 sea level, because of its thrusting onto the Apulian Platform. On the contrary, the stage 2 757 deposits in the Crotone basin, resting above the allochthonous units, indicate a deep water 758 depositional environment. 759 The black arrows in Fig. 10 are intended to illustrate the inferred routing of clastic 760 sediments, without implying a precise path. The same applies to the red arrows that depict 761 possible flow paths of salt brine feeding the deep basin, coming from the adjacent shallow 762 marine areas where the brine factories are inferred to be located. The slopes of the halite 763 basins are studded by canyons (e.g. Lofi, 2018) which can act as potential fairways for the 764 clastic gypsum turbidites and the salt brines.
It is worth noting that borehole data do not allow to reconstruct the high-resolution 766 stratigraphic framework for the stage 3 which was obtained from the outcropping 767 successions. It follows that, the distribution of the Lago-Mare sediments below the Molise-768 Lagonegro Nappe can not be defined.