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) – re ply
Lilianna CHOMIAK1, Piotr MACIASZEK1, Rob ert WACHOCKI2, Marek WIDERA1, * and Tomasz ZIELIÑSKI1
1 Adam Mickiewicz Uni ver sity, In sti tute of Ge ol ogy, Krygowski 12, 61-680 Poznañ, Po land
2 Konin Lig nite Mine, 9 600-lecia Av e nue, 62-540 Kleczew, Po land
Chomiak, L., Maciaszek, P., Wachocki, R., Widera, M., Zieliñski, 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) – re ply. Geo log i cal Quar terly, 63 (2): 429–433, doi:
10.7306/gq.1477
As so ci ate ed i tor: Anna Wysocka
We ex pected that our pa per on the cre vasse-splay microdelta (Chomiak et al., 2019) would arouse the in ter est of other re - search ers for at least two rea sons. First, this is the first such palaeoform dis cov ered and de scribed within the Mid-Mio cene lig nite seam in Po land. Sec ond, the microdelta siliciclastic de pos its are strongly de formed both duc tile and brit tle. There fore, we would like to thank Tom van Loon for his ef fort to com ment on our ar ti cle, in clud ing his words of ap pre ci a tion, and above all, for point ing out some of the ter mi no log i cal and in ter pre ta tive short com ings. Our re ply will be in line with the is sues dis - cussed in his com ment.
Key words: tec tonic graben, lig nite seam, SSDS, dom ino struc tures, tec ton ics, seismics.
INTRODUCTION
Cre vasse-splay se quences are known from an cient, but mainly from mod ern sed i men tary en vi ron ments. In the lat ter case, they can be ex am ined in great de tail both when they were form ing (ini tial phase of flood ing) and when the flood waters drained back into the river chan nels (e.g., Smith et al., 1989;
Zwoliñski, 1992; Pérez-Arlucea and Smith, 1999; Farrell, 2001;
Gêbica and Soko³owski, 2001; Gouw and Autin, 2008; Van Toorenenburg et al., 2018 and ref er ences therein). Some cre - vasse splays are ac cu mu lated pre dom i nantly in stag nant wa ter (ponds, lakes), form ing so-called cre vasse-splay microdeltas (e.g., Teisseyre, 1985; Bristow et al., 1999; Cahoon et al., 2011;
Ciarcia and Vitale, 2013; Zieliñski, 2014 and ref er ences therein). The sit u a tion is com pletely dif fer ent in the case of a rock re cord in which siliciclastics are rarely in ter preted as rep re - sent ing cre vasse splays (e.g., Horne et al., 1978; Guion, 1984;
Field ing, 1986; Kasiñski, 1986; Kirschbaum and McCabe, 1992; Flores, 1993; Davies-Vollum and Kraus, 2001; Burns et al., 2017) or cre vasse-splay microdeltas (e.g., Michaelsen et al., 2000) based on lim ited data ob tained mainly from bore hole
pro files. Most of the stud ies come from re search on coal-bear - ing for ma tions con tain ing hard coal or lig nite. Thus, we ex - pected to find (un der fa vour able min ing con di tions) the oc cur - rence of cre vasse-splay de pos its in Pol ish lig nite mines as well.
First, we dis cov ered undeformed siliciclastics with sed i men tary brec cia rest ing on the top of a cre vasse-splay body in the Tomis³awice opencast – Konin Lig nite Mine in cen tral Po land (Widera, 2016a, 2017; Widera et al., 2017). Sim i lar find ings were only a mat ter of time. In 2018, we dis cov ered a rel a tively wide spread layer of sands, with a small amount of or ganic mat - ter, split ting the lig nite seam in the nearby JóŸwin IIB opencast, which also be longs to the Konin Lig nite Mine. Sur pris ingly, these siliciclastic de pos its were typ i cal of a cre vasse-splay microdelta, and were strongly de formed both plas tic and brit tle.
The co-oc cur rence of such de pos its and de for ma tions is un - known from the Pol ish lig nite-bear ing ar eas and prob a bly from other coal/lig nite-bear ing re gions in the world. There fore, we con cluded that they were worth de scrib ing and in ter pret ing (Chomiak et al., 2019). Opin ion con vinced us of the need of our in ter loc u tor, van Loon, who claimed that these sed i ments are
“…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,…” (Van Loon, 2019).
The cre vasse-splay sed i ments dis cussed are sit u ated within the first Mid-Pol ish (first Lusatian) lig nite seam, which be longs to the lower part of the Poznañ For ma tion – the youn gest Neo gene lithostratigraphic unit in cen tral Po land (Widera, 2007). This lig nite ac cu mu lated in the backswamp area of a Mid dle Mio cene low-ly -
* Cor re spond ing au thor, e-mail: widera@amu.edu.pl
Re ceived: May 21, 2019; ac cepted: May 25, 2019; first pub lished on line: June 5, 2019
ing mire, as ev i denced by, inter alia, the oc cur rence of de pos its of a cre vasse-splay microdelta (Chomiak et al., 2019). In gen eral, the study area cov ers the shal low fault-bounded Kleczew Graben, where the Me so zoic bed rock, built of car bon ate rocks (lime stone and marl), is lo cated sev eral tens of metres deep in the ax ial zone of the graben. It seems im por tant, in the con text of the sed i men - tary and deformational or i gin un der dis cus sion, that the ac cu mu la - tion of the lig nite/peat in the study area was co in ci dent with tec - tonic sub si dence (synsedimentary tec ton ics) of up to 20 m (Widera, 2007; Chomiak et al., 2019). Ob vi ously, this is small com pared to the Kleszczów Graben men tioned by the in ter loc u - tor, where the depth of the tec tonic de pres sion reaches 550 m, and the con tin u ous thick ness of the main lig nite seam is over 250 m (Piwocki, 1992; Widera, 2013). Hence, we fully agree with the opin ion that “…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…” (see Van Loon, 2019). Sim i larly, his state ment 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…” is un doubt edly true. How ever, it is con fus ing for us that at the time of tec tonic ac tiv ity “…they were con sol i dated but not yet lithified.” In this case, we can say with cer tainty that the cre vasse-splay microdelta de pos its un der dis cus sion were not con sol i dated in the Mid dle Mio cene nor are they cur rently con sol i - dated. In fact, they are still loose and only slightly com pacted (Chomiak et al., 2019).
DOMINO STRUCTURES
The terms “dom ino-type de for ma tions”, “dom ino-style ge - om e try” and “dom ino-type brecciation” was used in our pa per in a purely de scrip tive sense. It meant that the sandy clasts in the brecciated lay ers looked like dom ino cubes or blocks (see Chomiak et al., 2019, their fig. 9C and the text). They are, in fact, known as ‘dom ino boudins’ in struc tural ge ol ogy and are a widely ac cepted ki ne matic clas si fi ca tion of Goscombe et al.
(2004). How ever, other names to de scribe such deformational struc tures were, and still are in use, e.g., “dom ino-style faults”
(e.g., McClay and Ellis, 1987), “fault blocks” or “ro tated fault blocks” (Vendeville, 1991), “dom ino fault blocks” (Stew art and Ar gent, 2000), “dom ino-type struc tures” (e.g., Markiewicz and Becker, 2009; Cymerman, 2014), “tilt blocks” (Jack son and Hudec, 2017), etc. In our opin ion, most of these names are un - der stand able to the reader who is fa mil iar with all kinds of de for - ma tion. How ever, we do not see any in di ca tions to the con trary for one of the de for ma tions de scribed by us to be named “dom - ino boudins”, as pro posed by van Loon (2019).
Apart from ter mi no log i cal is sues, the pres ence of dom ino boudins (dom ino-type de for ma tions) in the cre vasse-splay microdelta de pos its is a more im por tant re sult. In our pa per (Chomiak et al., 2019), we treated these de for ma tions as a part of the brec cia, and not sep a rate from it. There fore, this dis cus - sion pro vides a good op por tu nity to make up for our pre vi ous short com ing be cause “these struc tures con se quently de serve some more at ten tion” (Van Loon, 2019). It should be noted that dom ino boudins are known from geodynamic set tings of var i - ous ages and dif fer ent scales, i.e. from min eral-grain to con ti - nen tal-crustal scales. They are pres ent in all types of rocks and could form in both brit tle and duc tile-brit tle con di tions (e.g., McClay and Ellis, 1987; Vendeville, 1991; Goscombe et al., 2004; D¹browski and Grasemann, 2014; Jack son and Hudec, 2017 and ref er ences therein). More over, as our in ter loc u tor writes “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)” (see Van Loon, 2019). These beau ti ful high-qual ity pho to graphs, adapted from Yang and van Loon (2016), do not fully cor re spond to what we have shown in fig - ure 9C (Chomiak et al., 2019). Most of our dom ino boudins (as orig i nally cre ated, as they are now) are a steeply slop ing (up to 25°) layer of sand. On the con trary, the dom ino boudins pre - sented in pho to graphs at tached by van Loon (2019: figs. 1 and 2) lie hor i zon tally.
The most im por tant thing in the two afore men tioned ex am - ples of dom ino boudins is that they can be used in the ki ne matic anal y sis. In other words, such asym met ric boudins may be help ful as shear sense in di ca tors (e.g., Goscombe et al., 2004;
D¹browski and Grasemann, 2014). There fore, we will now ad - dress it in more de tail than was done in our pre vi ous pa per. The in ter pre ta tion of the or i gin of dom ino boudins shown in fig ure 9C (Chomiak et al., 2019) is sup ported by a graph i cal ex pla na tion of this pro cess (Figs. 1 and 2). There are two pos si bil i ties for the for ma tion of dom i nos in an extensional set ting (Stew art and Ar - gent, 2000). In both cases, the sed i men tary multilayers are tilted, char ac ter ized by vary ing rhe ol ogy (sands vs coaly
Fig. 1. Dom ino boudins de fined by Chomiak et al. (2019) as the dom ino-type de for ma tions
A – the same pho to graph that Chomiak et al. (2019) pre sented in their figure 9C; B – in ter pre ta tive sketch of a frag ment of the
pho to graph marked in Figure 1A
sands), and their ex ten sion is in duced by grav ity. As the trig ger mech a nism, we ob vi ously rec og nize that seis mic shocks were caused by Mid-Mio cene tec tonic ac tiv ity in the study area (Chomiak et al., 2019). The first case is when there is no dis - place ment be tween the dom ino layer and the un der ly ing layer.
In this case, the dom i noes were tilted (ro tated) syn thet i cally ac - cord ing to the in cli na tion of the beds (Fig. 2A). A dif fer ent sit u a - tion oc curs if a rel a tively weak layer was pres ent at the base of the dom ino boudins (e.g., coaly sand), along which there was a de tach ment. In this case, the dom i nos were set an ti thet i cally (Fig. 2B, C). Thus, un der the same over all shear sense the dom ino boudins can be tilted in the op po site di rec tion de pend - ing on the pres ence or ab sence of a basal de tach ment. The sec ond ex am ple dis cussed above and shown in Fig ure 2C closely cor re sponds to what is shown in Fig ure 1 and de picted by Chomiak et al. (2019) in their fig ure 9C. It must be added that such dom i nos (an ti thet i cally ro tated) are known from nu mer ous geo log i cal ex am ples (e.g., Stew art and Ar gent, 2000;
Goscombe et al., 2004; Jack son and Hudec, 2017) and sandbox mod els (e.g., McClay and Ellis, 1987; Vendeville, 1991). Sum ma riz ing this sec tion, we dis agree with the state - ment that “…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” (Van Loon, 2019). The above ex pla na tions show that struc tural ge ol o gists reach the same con clu sions by an a lyz ing struc tures sim i lar (in the sense of the ge om e try and rhe ol ogy of the sed i ments) to those we stud ied (cf. Figs. 1 and 2).
TECTONICS VERSUS SEISMICS
In deed, in our orig i nal pa per (Chomiak et al., 2019) we com - bined all of the stud ied soft-sed i ment de for ma tion struc tures (SSDS) with seismites. How ever, van Loon (2019) states that
“…the SSDS in these lay ers should there fore not be called seismites…”. We still be lieve that these de for ma tions arose as a re sult of seis mic ac tiv ity caused by tec tonic move ments in the Kleczew Graben area. The term ‘seismites’ has been used by us in a gen eral sense for all SSDS stud ied. Fur ther more, we did not find any ev i dence that these SSDS were “…de formed again later by other pro cesses” (Van Loon, 2019).
Read ing the com men tary by van Loon (2019), we came to the con clu sion that the is sue of a sud den in crease in pore-wa ter pres sure in the backswamp area was ex plained am big u ously.
Hence, we want to ex plain that the ground wa ter ta ble rise, fol - low ing a strong earth quake, was rel a tive. It means that the sed i - ments of the cre vasse-splay microdelta were sub jected to tec - tonic low er ing (i.e., col lapse reach ing the Me so zoic bed rock, and maybe even the Perm ian salts) by a few decimetres (see Chomiak et al., 2019, their fig ure 10D). In our opin ion, this could cause wa ter to flow to wards the top of the sed i ments, lead ing to their de for ma tion. A sim i lar sit u a tion may have oc curred dur ing the flood, when ris ing wa ter lev els in the river chan nel caused the ground wa ter ta ble to rise in the overbank (backswamp) area. Due to the slow ness of this pro cess in re la tion to the sud - den tec tonic col lapse, we have not dis cussed this pos si bil ity in our pa per (Chomiak et al., 2019). Un for tu nately, we have found no re cord of sand blows in the rock, as shown by van Loon (2019: fig. 4), or re lated sand dykes. They would be very strong ev i dence of the seis mic or i gin of de for ma tions caused by tec - tonic move ments such as those pro duced by earth quakes (Sims and Garvin, 1995). On the other hand, the pres ence of these forms and struc tures ex cludes the ar eal and uni form (not point or lin ear) ris ing and out flow of wa ter nec es sary for the for - ma tion of wide spread hy drau lic frac tur ing (e.g., Obermeier et al., 2005) in the case of the brec cia.
We con sider the afore men tioned tec tonic col lapse the re - sult of karstic pro cesses as sug gested by one of the re view ers.
This is jus ti fied by the fact that such large col lapses of ten cause SSDS (e.g., Moretti et al., 2011). Maybe we did not ex press it clearly and con fused our in ter loc u tor who asks ‘’…why would karst-like dis so lu tion oc cur in an or ganic-rich sandy layer?”
(Van Loon, 2019). Ob vi ously, the karstification pro cess would only cover the Me so zoic top, which is com posed of car bon ates, not or ganic-rich sands. In fact, only a few lay ers con tain
>1 wt.% of or ganic mat ter (Fig. 1). We have not found the ef - fects of karst pro cesses in the re search area and, there fore, we re jected its in flu ence on the for ma tion of the SSDS in ves ti - gated. How ever, ef fects of the karstification pro cess in the form of karst cav erns are known from other Pol ish lig nite-bear ing ar - eas, for ex am ple from the Z³oczew Graben (Kasiñski, 2015) and the Kleszczów Graben men tioned in the in tro duc tory sec - tion (Widera, 2016b, and ref er ences therein).
FINAL REMARKS
Our pa per (Chomiak et al., 2019), at least partly, fills a gap in the geo log i cal lit er a ture on cre vasse-splay microdeltas and their de for ma tion in the rock re cord. How ever, we re al ize that not ev ery thing was con vinc ingly ex plained, and some is sues were de scribed in an am big u ous way. That is why we thank the com men tary by van Loon (2019) for the op por tu nity to clar ify a few of the most de bat able is sues once again. Some of them will Fig. 2. Mod els de pict ing syn thetic and an ti thetic ro ta tion
of dom ino boudins de vel op ing in the same over all shear sense (mod i fied from Stew art and Ar gent, 2000) A – syn thetic ro ta tion of dom i nos and an ti thetic shear be tween them (with out basal de tach ment); B – an ti thetic ro ta tion of dom i nos and syn thetic shear be tween them (with basal de tach ment); C – ex pla - na tion for the for ma tion of the dom ino boudins shown in Fig ure 1 and by Chomiak et al. (2019) in their fig ure 9C; note that the sed i ment lay ers are tilted and a rel a tively weak (coaly sand) bed in re la tion to the sandy lay ers is pres ent at the base of dom i nos
re main the sub ject of dis cus sion for the fu ture. Thus, the best sum mary of this re ply seems to be the words of our in ter loc u tor who states 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” (Van Loon, 2019).
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