Seis mi cally in duced soft-sed i ment de for ma tion in cre vasse-splay microdelta de pos its (Mid dle Mio cene, cen tral Po land) – com ment
A.J. (Tom) van LOON1
1 Shandong Uni ver sity of Sci ence and Tech nol ogy, Col lege of Earth Sci ence and En gi neer ing, 579 Qianwangjang Road, Huangdao dis trict, Qingdao 266690, China
Van Loon, A.J.T., 2019. Seis mi cally in duced soft-sed i ment de for ma tion in cre vasse-splay microdelta de pos its (Mid dle Mio - cene, cen tral Po land) – com ment. Geo log i cal Quar terly, 63 (2): 424–428, doi: 10.7306/gq.1476
As so ci ate ed i tor: Anna Wysocka
The Mio cene suc ces sion of cre vasse-splay microdelta de pos its in the JóŸwin IIB lig nite opencast mine con tains some as - pects that are more in ter est ing than Chomiak et al. (2019) seem to re al ize in their anal y sis of the sed i ments and the soft-sed i - ment de for ma tion struc tures that they con tain. More over, the au thors use a ter mi nol ogy that is not com pletely ad e quate, leav ing some ques tions about the pre cise seis mic pro cess that in duced the de for ma tion struc tures. Both as pects are de - tailed in this com ment. The in ter pre ta tion of the de for ma tion struc tures pre sented here may change the in sight into the tec - tonic his tory of the graben, in which the study area is lo cated.
Key words: soft-sed i ment de for ma tion struc tures, dom ino boudinage, seismites.
INTRODUCTION
The con tri bu tion by Chomiak et al. (2019) is a most valu able con tri bu tion to sedimentology, as it de scribes and in ter prets cre vasse-splay de pos its, a type of sed i ment that seems largely un der val ued in sedimentology. Rel a tively lit tle is still known about these de pos its if com pared to most other types of sed i - ment, and par tic u larly the de tailed fa cies anal y sis pro vided by Chomiak c.s. is a most wel come ad di tion to our sedimento - logical knowl edge. Ac tu ally, I am not aware of an other fa cies anal y sis of cre vasse-splay de pos its that is equally thor ough and de tailed. I read the pa per there fore with great in ter est, also be cause soft-sed i ment de for ma tion struc tures (SSDS) from these de pos its are de scribed in quite some de tail, and be cause SSDS have my in ter est al ready for many years.
The Poznañ For ma tion, to which the sed i ments un der study be long, are lo cated in a brown coal mine sit u ated in the north - ern most part of the Kleczew Graben, a fault-bounded shal low tec tonic de pres sion sev eral kilo metres north of Konin in cen tral Po land. This set ting is of in ter est, as tec tonic ac tiv ity must have been pres ent both dur ing and af ter de po si tion of the ex am ined sed i ments. It is well known from nu mer ous stud ies in com pa ra - ble set tings, such as the Kleszczów Graben near Be³chatów,
that par tic u larly the synsedimentary tec ton ics can cause abun - dant soft-sed i ment de for ma tion struc tures (SSDS), al though they can not al ways with cer tainty be at trib uted to seis mic ac tiv - ity (e.g., Brodzikowski et al., 1987; Van Loon, 2002; Gruszka and van Loon, 2007).
The SSDS de scribed by Chomiak c.s. seem, in deed, caused by tec tonic ac tiv ity (this holds, at least, for most of them). This ac tiv ity must have af fected the sed i ments shortly af - ter de po si tion, as it seems that they were con sol i dated but not yet lithified. It is note wor thy in this con text that nu mer ous anal y - ses of SSDS have con vinc ingly made clear that sed i ments need not be lithified to show brit tle be hav iour. On the con trary, even wa ter-sat u rated, com pletely (geo log i cally) fresh sed i - ments can show sig nif i cant fault ing (Van Loon and Wiggers, 1976). The be hav iour of the sed i ment de pends not only on the state of the sed i ment, but also on the en ergy of the de form ing pro cess and the ve loc ity of the changes in the stress field (Rodríguez-Pascua et al., 2000; Gladkov et al., 2016; Ko et al., 2017).
Most of the SSDS an a lysed and de picted by Chomiak c.s.
seem “nor mal” for sed i ments that were af fected by seis mic ac - tiv ity. There is, how ever, one type that they men tion spe cif i cally and that they de pict clearly, but that most prob a bly did not orig i - nate due to the pas sage of seis mic shock waves. The re spon si - ble tec tonic con di tions were ap par ently not rec og nized by the au thors. This con cerns what they call “dom ino-type de for ma - tions” in the cap tion of their figure 9C (sim i lar, also fairly well-de - vel oped ex am ples are vis i ble in their figure 7, where they only men tion “brit tle de for ma tion struc tures”). These struc tures con - se quently de serve some more at ten tion.
* E-mail: Geocom.VanLoon@gmail.com; tom.van.loon@wxs.nl Re ceived: April 23, 2019; ac cepted: May 15, 2019; first pub lished on line: June 3, 2019
DOMINO STRUCTURES
It is re mark able that the au thors give the name “dom - ino-type de for ma tions” to the struc ture that they show in their fig ure 9C, as this struc ture is known in struc tural ge ol ogy as
“dom ino boudins”. Ap par ently, the con fig u ra tion of the bro - ken-up layer into tilted frag ments with a more or less imbricated po si tion is so much alike a line of fall ing dom ino stones that all re search ers come to a com pa ra ble name. We pre sume, at least, that Chomiak c.s. “in vented” the name “dom ino-type de - for ma tions” them selves, as they do not come to the same con - clu sion that struc tural ge ol o gists tend to come to when they find such a struc ture.
In this con text, it should be re al ized that the au thors can not be blamed for not be ing aware of the lit er a ture of these SSDS, be cause dom ino boudins have al most ex clu sively been de - scribed from rocks that were al ready lithified when the boudinage oc curred (e.g., Goscombe et al., 2004; D¹browski and Grasemann, 2014). They have also been men tioned from slumped masses in the mainly cal car e ous Late Pleis to cene Lisan For ma tion in Is rael (Alsop and Marco, 2011), but it should be no ticed that these rocks now are more com monly con sid - ered to rep re sent seis mi cally de formed sed i ments, orig i nated in the Dead Sea Graben, and thus in a set ting that is com pa ra ble with that of the sed i ments de scribed by Chomiak et al. (2019).
As far as known, only one sin gle study (Yang and van Loon, 2016) has men tioned dom ino boudins from a Cre ta ceous (hard-rock) suc ces sion that must have been unlithified when the de for ma tion oc curred (Figs. 1 and 2). It is in ter est ing, how - ever, that ex per i ments have in di cated that boudins can also form in un con sol i dated sed i ments (Zulauf et al., 2011; Marques et al., 2012).
In all the above field ex am ples, the dom ino boudins are pres ent in lay ers that were bro ken up by small nor mal faults at fairly equal dis tances of some centi metres, re sult ing – in a cross-sec tion – in more or less rect an gu lar blocks of a few centi - metres wide and up to about a decimetre high. These un der - went a slight ro ta tion due to the fault ing pat tern. This is es sen - tially what is also shown by Chomiak et al. (2019). As sim i lar SSDS have not been de scribed and in ter preted as due to other pro cesses, we de duce that the struc tures de scribed by these au thors also rep re sent dom ino boudins.
This is in ter est ing, be cause dom ino boudins are a spe cific (and rare) ex am ple of boudins, which are, rep re sented as pinch-and-swell struc tures in a less-de vel oped form, and, in a well-de vel oped form, as iso lated masses that have been torn apart from their par ent layer. Par tic u larly if diagenetic pro - cesses later af fect such struc tures, they may give the false im - pres sion of nod ules (Fig. 3).
All stud ies of hard-rock boudins in di cate ten sion as the cause of boudinage; there is no rea son to as sume that the tec - tonic con di tions in the study area of Chomiak c.s. were dif fer ent.
More over, in deed, their study site is sit u ated in a graben, which im plies, by def i ni tion, a ten sional set ting. A graben set ting dif - fers, how ever, from many other ten sional set tings, be cause of the dom i nance of nor mal faults. In many ten sional ar eas, on the other hand, lay ers be come lat er ally stretched and con se quently thin ner. It might be in ter est ing to find out whether this stretch ing and thin ning pro cess took also place in the Kleczew Graben, as this might shed a new light on the tec tonic his tory of this area.
TECTONICS VERSUS SEISMICS
It is not truly clear from the in ter pre ta tion by Chomiak c.s.
which pro cess they think re spon si ble for the for ma tion of the SSDS, be cause they men tion dif fer ent pro cesses. In the ab - stract they men tion “The oc cur rence in a tec tonic graben and char ac ter is tic mor pho log i cal fea tures sug gest an or i gin of these deformational struc tures with seis mic shocks; thus, they can be called seismites.” This is in it self an un for tu nate state ment:
when Seilacher (1969), ex actly half a cen tury ago, in tro duced the seismite con cept, he made clear that seismites are lay ers that are en tirely de formed by seis mic shocks; the SSDS in these lay ers should there fore not be called seismites, even though this is some times done, even by au thors who cite Seilacher’s orig i nal study (e.g., Anand and Jain, 1987; Alfaro et al., 1997; Rodríguez-Pascua et al., 2000). More over, it is, as a rule, im pos si ble to in ter pret a seis mic or i gin for each in di vid ual SSDS, as ear lier formed SSDS may also be pres ent and as seis mi cally in duced SSDS may be de formed again later by other pro cesses.
More prob lem atic, how ever, is that the pro cess (or pro - cesses) that de formed the sed i ments is de scribed with much am bi gu ity. For in stance, Chomiak et al. (2019) state that “…
one pos si ble ex pla na tion for the dra matic growth in pore-wa ter Fig. 1. Dom ino boudins, formed when the sed i ment was still
unlithified (note the undeformed un der- and over ly ing lay ers), closely re sem bling the dom ino-type de for ma tions de picted by Chomiak et al. (2019) in their fig ures 7B and 9C
There is no in di ca tion for col lapse of the un der ly ing sed i ment; photo adapted from Yang and van Loon (2016)
Fig. 2. De tail of a layer with dom ino boudins in the Cre ta ceous Lingshandao Fm.
Note the sharp fault, in di cat ing sud den rup tur ing of the sed i ment that still must have been unlithified; all “dom i nos” have es sen tially the same sizes, like in the ex am ples de picted by Chomiak et al.
(2019); photo adpted from Yang and van Loon (2016)
pres sure is an abrupt change in the ground wa ter level, caused by strong earth quakes, in the backswamp area. Pore-wa ter pres sure can also in crease by chang ing the grain pack ing dur - ing seis mi cally-in duced liq ue fac tion, when wa ter can be ex - pelled (Obermeier and Pond, 1998). In such con di tions, the wa - ter flows up wards, pierc ing the over ly ing beds or lift ing and bend ing them.” This state ment would im ply that earth quakes are ca pa ble of mo men tary rais ing the ground wa ter ta ble with sev eral decimetres. Such a pro cess is in con trast with what is known: the seis mic waves that pro ceed along the sur face (com - monly Ra leigh waves), se lect ing a path way through the most suit able sed i ments, com monly a few decimetres un der the sed - i men tary sur face, pass a spe cific place rap idly. This leaves in - suf fi cient time, par tic u larly in the of ten fairly fine-grained sed i - ments that are most sus cep ti ble to soft-sed i ment de for ma tion and that house the great ma jor ity of seismites de scribed in sedimentological lit er a ture (e.g., Alfaro et al., 1997), to raise the ground wa ter ta ble sig nif i cantly; the per me abil ity of such sed i - ments is too low. Cer tainly, it will be phys i cally im pos si ble that a shock-in duced sud den rise of the ground wa ter ta ble re sults in lift ing and bend ing of the sed i ments. Pierc ing in the form of clastic dykes is a pro cess that is known to oc cur fre quently if a seis mic wave passes, in deed, and some ma te rial will be trans - ported up wards in such cases, oc ca sion ally re sult ing in the char ac ter is tic sand blows (Cox et al., 2007; Grube, 2019;
Fig. 4), but this pro cess dif fers fun da men tally from pro cesses such as up lift ing and bend ing.
It is true that lo cal bend ing of lay ers, re sult ing in SSDS, oc - curs when seis mic waves pass, but the un der ly ing pro cess is dif fer ent. Chomiak et al. (2019) are ap par ently aware, con sid er - ing what they state in their ab stract: “The de for ma tion takes the form of de formed lam i na tion and load (loadcasts and flame struc tures) struc tures as well (plas tic ally de formed) SSDS are the re sult of liq ue fac tion of a layer due to in creased pore-wa ter pres sure, so that over ly ing seis mic brec cias […]. Duc tile de for - ma tion struc tures were gen er ated first by liq ue fac tion […].”
Here it is clear that the au thors fol low the com monly ad hered to ex pla na tion that most plas tic ally de formed struc tures re sult from fluidization of a layer un der the in flu ence of in creased
pore-wa ter pres sure due to the pas sage of the seis mic wave, and that the over ly ing liq ue fied layer sinks into the fluid-like sed - i ment be low un til the lat ter re gains its sta bil ity when the shock wave has passed. This im plies that sed i ments sink down by seis mic ac tiv ity. The fact that Chomiak c.s. are ap par ently well aware of this char ac ter is tic pro cess raises the ques tion of why they opted for a phys i cally prob a bly im pos si ble pro cess of up lift - ing of sed i ment un der the in flu ence of a sud denly ris ing ground - wa ter ta ble.
The ex pla na tions by Chomiak c.s. be come even stranger when they state, as a con tin u a tion of the sen tence quoted in the pre vi ous para graph: “… and then the brec cia was formed un der brit tle con di tions. The brecciation fol lowed a sud den tec tonic col lapse re sult ing in an in crease in pore pres sure re lated to up - ward wa ter move ment.” Apart from the ques tion how the brit tle con di tions orig i nated, the au thors in tro duce here a ‘tec tonic col - lapse’. It re mains un clear what pro cess they have in mind, how this pro cess was trig gered, and how a col lapse can oc cur if no space un der neath is pres ent. It seems that they can not cope well with their ob ser va tion of what prob a bly con cerns the dom - ino-type brec cia, which we ex plained in the pre vi ous chap ter.
There is no need for a col lapse, or for an up ward wa ter move - ment!
More over, the au thors men tion “karst-in duced col lapse”.
This com pli cates the ex pla na tion even more: why would karst-like dis so lu tion oc cur in an or ganic-rich sandy layer? And if karst oc curred, why did the pro cesses in volved not af fect other lay ers, and why just this level, and only this level? If the dom ino-type brec cia re sulted from col lapse by dis so lu tion of the un der ly ing sed i ments (which is hardly imag in able, if only be - cause they still form a straight band), they cer tainly should not be con sid ered as seis mi cally in duced! In this re spect, the au - thors are in con sis tent in their in ter pre ta tion. The au thors com - pli cated their ex pla na tion be cause they did ap par ently not re al - ize that a dom ino-type con fig u ra tion could be achieved only if some pro cess re sults in lat eral ad di tional space (which is ex - plained above as a re sult of ten sional tec ton ics that must have been pres ent in an ac tive graben).
Fig. 3. Weath er ing of a diagenetically formed struc ture, re sem bling nod ules
Close anal y sis in di cates that the lay ers form loops (cf. Su and Sun, 2011), which must have formed when the sed i ment was still unlithified; Mesoproterozoic Wumishan Fm., near the rail way sta tion of Zhuwo (China)
Fig. 4. Sand blows, a char ac ter is tic fea ture formed dur ing earth quakes, formed af ter the 2012 Emilia earth quake in It aly, in di cat ing pore-wa ter es cape and liq ue fac tion/fluidization, which must have re sulted
in the ex pul sion of a wa ter/sed i ment mix ture from a bur ied layer (from Emergeo Work ing Group, 2012)
FINAL REMARKS
Soft-sed i ment de for ma tion struc tures have re ceived much in ter est in the past few de cades. They posed prob lems for a long time (see Van Loon and Brodzikowski, 1987), and many of them still do. One of the main prob lems is that SSDS are pres - ent in rocks from Archean to mod ern, and in all en vi ron ments (Van Loon, 2009). More over, their or i gin can be endogenically in duced (e.g., tec ton ics), exogenically in duced (e.g., sud den
over load ing, for in stance by an event de posit) or be in duced by at mo spheric pro cesses rang ing from fall ing rain drops to the im - pact of me te or ites. The un for tu nate con se quence is that the de - scrip tions and anal y ses of SSDS are scat tered over al most all types of earth-sci ence lit er a ture, which makes it al most im pos - si ble to keep up with new in sights. The most ef fec tive way is prob a bly writ ing some com ments that may be found by read ers who are in ter ested in the ma te rial that is com mented upon. Let this con tri bu tion be of such help.
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