Guest editorial

We are delighted to present the IJG readers with a fresh collection of papers on a classical subject in the peri-Medi- terranean geology: the fate of Mesozoic carbonate platforms through the tough times of plate breakup, opening of new oceanic seaways, and global environmental change. While the benthic carbonate factories tried hard not to succumb to the potentially lethal turmoil caused by the action of those forces, many of them did drown and were converted into pelagic carbonate platforms (PCP), giving rise to depositional systems (PCP/Basin systems) which have been challenging regional geologists, keeping stratigraphers, paleontologists and sedimentologists busy during the past two centuries.

Exhumed ancient PCP/Basin depositional systems occupy vast areas on land, being the essential building blocks of the Mesozoic stratigraphy of Tertiary orogens all across the Mediterranean region. Our understanding of their facies and geometries has progressed steadily in the last decades, centered in their “type area”, meant here as the region where comprehensive facies/structural models for these depositional systems were first proposed, also thanks to the wealth of exceptionally instructive natural exposures: the Apennines of Central Italy. Besides the Apennines, valuable information on the syn- and post-drowning deposits, which typically mark the late stages of carbonate platform development, is being constantly extracted from classic localities in Southern Alps, the Maghrebian Chains in Sicily, the Calabria-Peloritani Arc, the Carpathians, only to make mention of those regions which provided the case studies (Triassic and Jurassic) treated in this Special Section. These same regions have a long-established tradition of providing a priceless flow of stratigraph- ic and sedimentological information on condensed pelagic facies, through the seminal studies of noted paleontologists (mostly ammonitologists) and sedimentologists/stratigraphers since the 19th century (e.g. Zittel, Canavari, G.G. Gemmellaro, Fucini) and in the second half of the 20th century (Sturani, Wendt, Bernoulli, Jenkyns, Farinacci, to name a few).

Summarizing the “state of the art” of our knowledge on this subject would be impossible here, due to space con- straints. Suffice it to say that, following publication of the papers by SANTANTONIO (1993, 1994), a number of missing tesserae were added to the original mosaic. Crucial information was derived from an identification and analysis of the marginal escarpments of PCPs. Over the years, perhaps surprisingly due to the generally high degree of post-Mesozoic deformation (orogenic and post-orogenic extensional), numerous instances of beautifully preserved exhumed paleo- escarpment tracts were detected and mapped in the field, these being the key elements for reconstructing the regional architecture of the rift. We therefore gained precious insight on a number of related themes: the escarpment morphology at different scales; the diagenetic history of the footwall-block pre-rift peritidal limestone exhumed along synsedimentary faults; the existence of an aborted progradational phase at the inception of synsedimentary extension; the distribution of megaclastic belts proximal to marginal PCP escarpments; the existence of isolated megablocks having their longer axis in excess of several hundred meters; the geometries of onlapping beds of the hangingwall-basin successions, as ruled over by differential compaction; the slip rates of the faults which gave rise to the PCPs, based on the ages of patches of condensed pelagites draping the paleoescarpments; the variable density of faults across the rift basin, and dimensions of basins and highs; the existence of highs devoid of a flat top, wherever the pattern of faults was extremely dense and antithetic faults met within the pre-rift limestone. Research on PCP tops evidenced: the general geometry of condensed successions, which become thinner toward the platform edges (“panettone geometry”); the three-dimensional distribu- tion of syn-drowning deposits flooring the condensed pelagic (post-drowning) succession, which often document the ex- istence, with variable relative importance, of a microbial factory; the geochemical signature of certain key turning points in the platform history and key lithostratigraphic boundaries; the existence of rare “overbank deposits” embedded in the condensed succession, a result of gravity flows, sourced by productive benthic factories, impacting - and then climbing - the submarine escarpments. The reader is referred to the bibliographies in SANTANTONIO & CARMINATI (2011) and in SANTANTONIO et alii (2017) for the subjects listed above. Given the impossibility of presenting a thorough review of PCP/ Basin systems here, as we mentioned, we took these introductory pages as an opportunity for stimulating a discussion on the conceptual challenges this fascinating area of research is going to pose in the future, for describing the difficulties one typically encounters in the field when tackling these depositional systems...and for balancing our enthusiasm for the wealth of fresh data, which keep emerging steadily, with a healthy dose of doubt an uncertainty.

We must start with emphasizing the irreplaceable role of field mapping, as over the years researchers have perfected specific field tools, in the form of a dedicated set of field observations, in order to disentangle the complex structure and history of PCP/Basin systems. The last couple of decades unfortunately has seen a progressive decline of geological mapping as a distinct discipline in the earth sciences, along with a growing difficulty in recruiting candidates for any ambitious field mapping project. This is mostly due to the fact that field mapping is time-consuming and often does not guarantee a result in the short term, in terms of number of papers published in high-impact journals, which is the rage of these years. Not even PhD students have the luxury of taking their time to spend months in the field, make mistakes, pretend they have understood what they have seen, then admit they are lying to themselves, and consequently go back to the field only to be suddenly caught by the snow and waste a week and lots of car fuel money, then return and eventually indeed start understanding what they are seeing, only to realize this requires conducting additional research in an area nearby which had not been considered previously... This is not possible anymore, in this world. Or perhaps it still is, instead? Also based on papers in this Special Issue, we believe there is still hope!
 

As PCP/Basin systems are found in mountain chains, their degree of deformation can be severe to extreme. While this is valid for all the rocks exposed in mountain chains, these systems have an added level of difficulty, due to the fact that their birth and primary geometries were typically a product of synsedimentary faulting. One must therefore be at ease with both structural and sedimentary geology - ideally geological mapping is indeed the field where the two disciplines (should) merge. One of the risks here is that of mapping as orogenic contacts also those primary contacts (unconformities) which instead represent preserved basin-margin tracts (and viceversa of course). This is just an example. The dis- tinctive trend towards extreme compartmentalization we are experiencing in our time is clearly an “enemy” of geological mapping (which requires an holistic approach by definition), as the latter works best when the field geologist knows not only how to tell a fault from an unconformity, but also a gastropod from an ammonite, or a cryptalgal laminite from a laminated turbidite.
Then there is the “pelagic issue”. What does that mean? Pelagic carbonates can be mysterious: with peritidal carbon- ates we can often tell water paleodepth within a range of error of few meters. Even from a random sample of limestone with birdseyes, a lot can be said in a paleoenvironmental key. Pelagic carbonates in comparison are rather discomforting. Not one carbonate sedimentologist can reasonably tell from a sample of pelagic mudstone if that mud was originally deposited at, say, 100 or 2000 m water depth. This is one of the reasons why field mapping is so important because it can ultimately provide a physical link with deposits of known paleodepth, or at least it can serve to build a three-dimensional puzzle/model of paleostructural compartments with their relative paleodepths. But this takes us straight into the realms of paleontology and paleoecology...

An actualistic model for a condensed cephalopod limestone on a Jurassic PCP is risky. Where are pelagic platforms today, hosting such a style of sedimentation? Or, sometimes we receive answers we would have preferred not to receive: for instance, zooxanthellate corals on PCPs. No, please no!! These corals have been reported extensively, mostly from the Upper Jurassic of Northern Apennines, following the original findings by NICOSIA & PALLINI (1977). How do we know they were zooxanthellate, one may ask? Well, paleontology is exactly about this (among many other things): get the most out of the hard parts which are preserved in the fossil record, for example by means of functional morphology analysis. Now imagine this. You have a T. rex tooth: do you really need a perfect T. rex specimen, with a preserved stomach, and inside it the preserved remains of a poor herbivorous dinosaur, in order to conclude that the T. rex was not on a vegan diet? Coral skeletons tell their story in a similar fashion, in the absence of preserved soft tissues. Coral paleontologists have developed qualitative and quantitative methods for assessing the paleoecology of fossil forms, based on key parameters of the skeletal structure, uniformitarianism, and obviously facies models in modern and ancient well-constrained reef and deeper-water depositional systems. Knowing that an in situ, genuine reef coral lived on top of a PCP while a typical pelagic facies (base of the Maiolica Formation) was being sedimented certainly poses problems, to put it mildly (CIPRIANI et alii, 2019). Unsettling as it may seem, however, a photic environment is the most conservative conclusion we can reach today, reflecting the state of the art in coral paleontology. This is an enigma, but we have to live with it, at least until something entirely new emerges, which we cannot foresee today.
Yes, we are well aware that accommodation space problems arise when one considers the volumes and facies of the post-Tithonian succession. Still, we cannot pretend those corals do not exist: they do. Plus, if there is one thing we have learned regarding Tethyan pelagic successions, is that the chief “facies equalizer” in this game was paleoceanographic change, and not quite paleodepth. This is demonstrated worldwide by coeval, widespread same-facies development and synchronous lithostratigraphic boundaries across different paleogeographic domains: pelagic formations of like composition and general facies must necessarily have been deposited across an array of paleodepths. Uniformitarianism fails here, as there are for example no modern counterparts of “pelagic shelf” conditions like those beautifully documented in the Chalk basins of Northern Europe, with reconstructed depths as shallow as few tens of meters (STOW et alii, 1996). With that said, however, we can now play the devil’s advocate (actually the advocates of several different devils): had these corals been independent from light, and thus depth, why are they not found everywhere across the whole depositional system (including structural lows)? More: if their occurrence were instead related to nutrients availability, why do these corals possess the morphology of light-dependent corals? Aphotic corals inhabiting deep-water environments display quite different morphologies, as photon capture in not their priority. PCP tops had to be swept by currents (or internal waves?) - this is generally inferred as the cause for stratigraphic condensation of Pliensbachian to Tithonian condensed successions on Apenninic PCPs. Don’t these currents or waves notoriously also bring nutrients? Why then not link the two phenomena? Sure, one might say, but then why don’t we find corals (azoo-xanthellate but disguised as zooxanthellate corals...) throughout the Jurassic condensed succession, instead of select levels in the Upper Jurassic? (Even) more: if, again, the cause for condensed successions is the sweeping of PCP-tops by “currents”, what kind of currents were they? “Generic” contour currents? Tidal waves/solitons? The latter imply the pycnocline meeting a submarine obstacle, so a PCP/Basin system would be perfect, right? Problem: condensed suc- cessions can cover the greater part of the Jurassic (~45 Myr). So the regional pycnocline should have been stationary, always hitting the same obstacle(s) over and over for tens of millions years.
Another example. We did not feel too good when we first discovered a textbook example of hummocky cross strat- ification in the Saccocoma limestone onlapping a PCP (CECCA et alii, 1990; SANTANTONIO, 1993): how much deep could storm wave base have been? But thirty years ago HCS was a synonym of storm deposit, especially when forming large (metre) scale structures, period. We felt therefore relieved to learn recently that hummocky cross stratification could also occur in deep water due to solitons (MORSILLI & POMAR, 2012). Great, because submarine escarpments were the perfect site for the breakup of internal waves. Yes, but solitons would have had to hit the escarpment at some given height (above the basin bottom), then climb it to eventually sweep the platform top. So how could the basin-bottom sediment (the crinoidal sands) be instead reworked to form HCS? Plus, one might also wonder, why is there only one (or two) hummocky cross beds, if breakup of internal waves had to occur on a daily basis (tidal waves) for tens of mil- lions years (in the hypothesis that this was indeed the cause for condensation of PCP-top deposits)? So we have come full circle, which often happens with many things in life, including science: those above are just a handful of puzzling themes associated with research in PCP/Basin systems.

Contributions to this Special Section cover a diverse array of topics and paleogeographic compartments, at different stratigraphic levels, with emphasis ranging from regional geology to sedimentology, paleontology to paleogeography.
Cipriani, Zuccari, Innamorati, Marino and Petti discuss the occurrence of Toarcian mass flow deposits detected in several localities in the Umbria-Marche-Sabina Apennines, adjacent to PCPs. Based on a description of their sedimentology, and an interpretation of their causative mechanisms, the Authors postulate the existence of a Toarcian pulse of synsedimentary tectonic activity, with reactivation of the late Hettangian rift faults.
Santantonio and Fabbi describe, also through a new geological map, the complex stratigraphic/paleotectonic history of a structural high in the Longobucco Basin (Northern Calabria). The peculiar association of igneous and metamorphic Paleozoic basement rocks with typical Tethyan pelagic units is seen through the occurrence of different types of unconformities, and the production of unusual instances of carbonate/siliciclastic mixtures. Facies models derived from research in the Apennines are applied, with differences, in this sector of the Calabrian Arc, which documents the stepwise evolution, time-constrained through the use of ammonite biostratigraphy, of the “Iberian” continental margin in the Jurassic.
Basilone describes the tectonic and paleogeographic evolution of Western Sicily, a classic region for the stratigraphy of Jurassic pelagic deposits which displays a complex pattern of pelagic carbonate platforms and basins. The Author interprets the peculiar geometries of condensed carbonates as the result of repeated phases of faulting during the Jurassic, as parts of this region also saw the emplacement of thick volcanites, and proposes a subsidence curve for the “Trapanese margin”.
Tomašových, Schlögl, Michalík and Donovalová describe the sedimentology, taphonomy and diagenesis of Bositra shell-beds in the Western Carpathians. By means of an analysis of shell-size distribution across the depositional profile, the Authors identify the area of environmental optimum as being that allowing for complete growth of the specimens (larger size=low juvenile mortality), their survivorship being due to the input of nutrients brought by bottom currents or internal waves.
Franceschi, Preto, Caggiati, Gattolin, Riva and Gianolla study two microbial mounds developed at the flanks of the Middle Triassic (Late Anisian) Latemar platform in the Dolomites of Northeastern Italy. By means of a detailed reconstruction of the submarine topography of the platform/slope system, the Authors derive the range of absolute paleodepths at which the mounds were able to grow actively, concluding that their drowning was due to sinking below a certain threshold (-300-350m depth), and was followed by burial by slope clinoforms.
Two additional papers should have been part of this Special Section, but had no luck with the reviewing process and/ or the final assessments by the Editorial Board. The readers interested in a more complete picture of the original vision behind this Section can contact the Guest Editors for a link to the two international journals which have subsequently accepted these papers for publication.