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)
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, al. 600-lecia 9, 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). Geo log i cal Quar terly, 63 (1): 162–177, doi:
10.7306/gq.1456
As so ci ate ed i tor: Anna Wysocka
Cre vasse-splay microdelta de pos its and their soft-sed i ment de for ma tion struc tures (SSDS) are de scribed from a tec toni cally ac tive lig nite-bear ing area. These strongly de formed siliciclastic de pos its, sit u ated be tween two lig nite benches, are typ i cal of a cre vasse-splay microdelta. They ac cu mu lated in the overbank zone of a Mid dle Mio cene river sys tem (backswamp area) where shal low ponds or lakes oc ca sion ally ex isted. The de for ma tion takes the form of de formed lam i na tion and load (load casts and flame struc tures) struc tures as well as seis mic brec cias within the first Mid-Pol ish lig nite seam. Duc tile de for ma tion struc tures were gen er ated first by liq ue fac tion 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. 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. Hence, we pro vide strong ev i dence for ac cu mu la tion of cre vasse-splay sed i ments in the stand ing wa ter of a backswamp area, and for tec tonic ac tiv ity in cen tral Po land as the Mid dle Mio cene lig - nite ac cu mu lated.
Key words: backswamp area, SSDS, duc tile and brit tle de for ma tions, earth quakes, seismites.
INTRODUCTION
Siliciclastic in ter ca la tions, or in gen eral terms, interbeds of min eral de pos its (sands, clays, lac us trine lime stones, tonsteins, etc.) within peat-to-coal seams are very com mon in both mod ern and an cient sed i men tary suc ces sions (e.g., Horne et al., 1978; Kasiñski, 1989; Schäfer et al., 1995; S³omka et al., 2000; Ulièný et al., 2000; Holdgate, 2005; Opluštil, 2005;
Rajchl and Ulièný, 2005; Rajchl et al., 2008; Flores, 2013; Mach et al., 2013; Mastej et al., 2015 and ref er ences therein). In most cases, they form sheet-like sandy bod ies that are in ter preted as cre vasse splays (e.g., Guion, 1984; McCabe, 1984; Field ing, 1986; Kasiñski, 1986; Kirschbaum and McCabe, 1992; Flores, 1993; Smith and Pérez-Arlucea, 1994; Pérez-Arlucea and Smith, 1999; Diessel et al., 2000; Davies-Vollum and Kraus, 2001; Farrell, 2001; Gouw and Autin, 2008; Van Asselen et al., 2009; Cahoon et al., 2011; Widera, 2016a; Widera et al.,
2017a). The cre vasse splays are cre ated when nat u ral lev ees are breached dur ing the ini tial phase of overbank flood ing (e.g., Smith et al., 1989; Farrell, 2001; Bridge, 2003; Zieliñski, 2014;
Toonen et al., 2016). Thus, their sed i ments mainly ac cu mu lated in subaerial en vi ron ments and less fre quently in sub aque ous con di tions, that is, in stand ing wa ter (ponds or lakes), that ex ist on a floodplain (backswamp) in close prox im ity to an ac tive (me an der ing or anastomosing) river chan nel (Horne et al., 1978; Tye and Coleman, 1989; Michaelsen et al., 2000; Gêbica and Soko³owski, 2001; Bos et al., 2009). In the lat ter case, this is shown by the pres ence of steeply dip ping lay ers typ i cal of
“prograding splay de pos its” (sensu Bristow et al., 1999), rep re - sent ing a cre vasse-splay microdelta (Michaelsen et al., 2000;
Zieliñski, 2014). Such de pos its, with well-de vel oped foreset laminae, are well ex posed in the JóŸwin IIB lig nite opencast mine in cen tral Po land (Fig. 1).
Both cre vasse-splay sed i ments (but not a cre vasse-splay microdelta) and brec cia (but not of tec tonic or i gin) are known from the nearby Tomis³awice lig nite opencast mine (Fig. 1A;
Widera, 2016a, 2017; Widera et al., 2017a). Seis mi cally-in - duced brec cias and plas tic de for ma tion struc tures not re lated to cre vasse splays are also known from other Pol ish lig nite-bear - ing ar eas (e.g., van Loon et al., 1995; Gruszka and Zieliñski, 1996; Gruszka and van Loon, 2007; Widera and Ha³uszczak,
* Cor re spond ing au thor, e-mail: widera@amu.edu.pl
Re ceived: October 10, 2018; ac cepted: January 28, 2019; first pub lished on line: March 14, 2019
2011). More over, plas tic soft-sed i ment de for ma tion struc tures have been de scribed from other delta and cre vasse-splay suc - ces sions (e.g., Ulièný et al., 2000; Mach et al, 2013; Burns et al., 2017). Wholly de formed de pos its of a cre vasse-splay microdelta seem not have been de scribed un til now.
This pa per there fore: (1) de scribes and in ter prets the clastic fa cies of the cre vasse-splay suc ces sion; (2) iden ti fies all soft- sed i ment de for ma tion struc tures pres ent and char ac ter ises the mech a nism of their or i gin; and (3) pro poses a con cep tual model for the cre vasse-splay and de for ma tion struc tures formed dur ing peat ac cu mu la tion in Mid dle Mio cene mires, which trans formed into the first Mid-Pol ish lig nite seam in cen tral Po land.
GEOLOGICAL SETTING
The study area, that is, the JóŸwin IIB lig nite opencast mine, where the strongly dis turbed siliciclastic de pos its are ex posed, com prises the north ern most part of the Kleczew Graben. This fault-bounded shal low tec tonic de pres sion is lo cated sev eral kilo metres north of Konin in cen tral Po land (Fig. 1). The Me so - zoic bed rock of the graben, which oc curs at a depth of up to a
few tens of metres, is built of limy sand stones of Late Cre ta - ceous age (Widera, 2007, 2014). The en tire Paleogene is rep - re sented by a strati graphic gap; so far, de pos its of this age have not been found in this ter ri tory. Around the Paleogene/Neo gene bound ary, the Ce no zoic evo lu tion of the Kleczew Graben be - gan (Widera, 2007). Thus, in this area only Neo gene and Qua - ter nary suc ces sions are pres ent (Fig. 2; Widera, 2014; Widera et al., 2017b).
The Neo gene con sists of two lithostratigraphic for ma tions:
the KoŸmin For ma tion over lain by the Poznañ For ma tion. The KoŸmin For ma tion of ear li est to Mid dle Mio cene age is com - posed of fluvio-lac us trine sand and silt de pos its with lig nite interbeds (Widera, 2007). It is capped by the Poznañ For ma tion of Mid dle Mio cene to Early Plio cene age. This for ma tion is di - vided into two lithostratigraphic mem bers from its lithological and ge netic di ver sity; the lower Grey Clays Mem ber and the up - per Wielkopolska Mem ber (Fig 2; Piwocki and Ziembiñska- Tworzyd³o, 1997; Widera 2013a).
The Grey Clays Mem ber, called also the Mid-Pol ish Mem - ber, con tains the first Mid-Pol ish lig nite seam (MPLS-1), which is 3–13 m thick, av er ag ing 6.6 m, in the re search area. The max i mum thick ness of the lig nite seam (13 m) cor re sponds to Fig. 1. Lo ca tion of the study area
A – lo ca tion on a map of Po land; B – out line of the JóŸwin IIB lig nite de pos its (52°40’11”N, 18°17’44”E); C – mapped lig nite walls and dewatering chan nels at the low er most ex ploi ta tion level in May and June 2018; D – ap prox i mate ex tent of the siliciclastic de pos its ex am ined within the first Mid-Pol ish lig nite seam (MPLS-1); in ter sec tion point marks the mar gin of siliciclastic de pos its within the lig nite seam (MPLS-1) mapped in the field; note lo ca tion of the key sec tion (bore hole no. 88/48) used to show the Ce no zoic stra tig ra phy of the study area shown in Fig ure 2
approx. 26 m of the orig i nal thick ness of the peat (Widera, 2015). It was formed in the mid dle part of the Mid dle Mio cene, at ~15 Ma (Kasiñski and S³odkowska, 2016; S³odkowska and Kasiñski, 2016). MPLS-1, in clud ing the siliciclastic interbeds stud ied, is be ing ex ploited in the JóŸwin IIB lig nite opencast mine, which be longs to the Konin Lig nite Mine. It is cur rently be - lieved that the ac cu mu la tion of peat, from which MPLS-1 formed, took place as low-ly ing mires in the overbank zone of the Mid dle Mio cene flu vial sys tem (Widera, 2016a; Widera et al., 2017a). Clays, char ac ter ized by a grey col our, with wood re - mains (xylites) that be long (to gether with MPLS-1) to the Grey Clays Mem ber, rest in a few places at the top of MPLS-1 (Fig. 2;
Piwocki and Ziembiñska-Tworzyd³o, 1997; Widera, 2007, 2013a; Widera et al., 2017b).
The Neo gene ends with the muddy Wielkopolska Mem ber, which over lies the lig nite-bear ing Grey Clays Mem ber in cen tral Po land (Fig. 2). These pre dom i nantly muddy de pos its, with sandy chan nel-fills, are at trib uted to a Late Neo gene anastomosing river sys tem (Widera, 2013a; Widera et al., 2017b). The Wielkopolska Mem ber is up to 20 m thick in the JóŸwin IIB opencast area (Fig. 2).
The Qua ter nary cover, mainly con sist ing of slightly con sol i - dated glaciogenic de pos its (till, gravel, sand and mud), caps this geo log i cal suc ces sion (Fig. 2). It forms a rel a tively thick and con tin u ous sed i men tary body, an av er age of 40–50 m thick, which is lo cally glaciotectonically de formed (Widera, 2018).
More over, the Qua ter nary de pos its have been par tially re - moved by ero sive pro cesses of the Pleis to cene Scan di na vian ice-sheets and their meltwaters (Widera, 2013b, 2014; Widera et al., 2017b).
DATA AND METHODS
The de formed siliciclastic de pos its, split ting MPLS-1, were well ex posed be tween May and Sep tem ber 2018 in the JóŸwin IIB lig nite opencast mine (Figs. 1 and 3). At that time, at the low - er most ex ploi ta tion level, the sedimentological ob ser va tions pre sented herein were car ried out and 3 rep re sen ta tive sam - ples of the de pos its in ves ti gated were col lected for lab o ra tory
anal y sis. The mine walls and dewatering chan nels were also mapped in places where the siliciclastic de pos its were ex posed (Fig. 1C, D), and 42 palaeocurrent di rec tions were mea sured.
Fur ther more, the out line of the Ce no zoic stra tig ra phy for the study ter ri tory was pre sented on a ba sis of a key sec tion – bore - hole no. 88/48 (Fig. 2).
The sec tion ex am ined con sists of lower and up per benches of the lig nite seam (MPLS-1) that are up to 1 and 7–8 m thick, re spec tively. These lig nites are split by clastic fa cies, the sub - ject of this re search. Dur ing field work they formed a lens-shaped sandy body, which was more than 550 m long and
~1–2 m thick in a N–S di rec tion, while in the E–W di rec tion it was wedge-shaped with a thick ness de creas ing from ~1 to 0 m to the west (Fig. 3).
Based on de tailed field stud ies, partly sup ported by re sults of lab o ra tory anal y ses, nine clastic fa cies and two lig nite lithotypes were dis tin guished. For de scrip tion of tex tural and struc tural fea tures of these clastic fa cies the let ter code of Miall (1977; Ta ble 1) is ap plied, while for the un der- and over ly ing lig - nites the cod i fi ca tion of lithotypes of Widera (2012, 2016b; Ta - ble 1) is used. Sed i men tary struc tures are termed af ter Collinson and Thomp son (1982) and Allen (1982), while ter mi - nol ogy of soft-sed i ment de for ma tion struc tures is af ter D¿u³yñski and Walton (1965) and Allen (1982). Cri te ria for dis - tin guish ing soft-sed i ment de for ma tion struc tures caused by earth quakes, are used here as pro posed by var i ous re search - ers (e.g., Sims, 1975; Obermeier and Pond, 1998; Rossetti and Santos, 2003; Obermeier et al., 2005; Moretti and Sabato, 2007; van Loon, 2009; Owen and Moretti, 2011; Owen et al., 2011; Moretti and van Loon, 2014; Moretti et al., 2016).
Rep re sen ta tive sam ples of three mac ro scop i cally dis tin - guish able beds were se lected for lab o ra tory test ing to de ter - mine the grain size and or ganic mat ter con tent. These sam ples were from: the up per most, non-de formed layer of mas sive sand – sam ple A; the brecciated, pla nar cross-strat i fied lay ers of both sands (with out or ganic mat ter) – sam ple B; and or ganic sands – sam ple C, re spec tively (Fig. 4 and Ta ble 2). In the last case, the organics were burned in an oven at 550°C. To de ter mine the grain size, the stan dard siev ing method was used. Sta tis tics of the grain-size dis tri bu tion were com puter cal cu lated with a Fig. 2. Key sec tion of the Ce no zoic suc ces sion in the study area
Note po si tion of the de formed clastic interbeds within the first Mid-Pol ish lig nite seam (MPLS-1), in clud ing both duc tile and brit tle de for ma tion struc tures; note also that the up per most layer of the siliciclastic de pos its is non-de formed
SIEWCA grain-size pro gram at the In sti tute of Ge ol ogy in Poznañ, Po land. The re sults of these cal cu la tions, in clud ing grain-size sta tis ti cal pa ram e ters and their de scrip tions, are given in Ta ble 2. Over all, the data ob tained were help ful in de - scrib ing the rhe o log i cal prop er ties of the de pos its ex am ined in terms of their sus cep ti bil ity to duc tile and brit tle de for ma tion.
RESULTS
PETROGRAPHY AND SEDIMENTOLOGY OF THE LIGNITE SEAM
The first Mid-Pol ish lig nite seam (MPLS-1) con sists of two benches that are split by siliciclastic de pos its in the study area (Figs. 2–5). These lig nite benches com prise two lithotypes with a detritic and xylodetritic tex ture. They are both char ac ter ized by a mas sive and frac tured struc ture. Be cause the clastic fa - cies are sit u ated be tween them, these lithotypes re quire brief de scrip tion and in ter pre ta tion.
De scrip tion of lig nite lithotypes. The lower bench of MPLS-1 is built of a detritic lig nite with a mas sive and frac tured struc ture – DLm(fr), while the up per bench is made of a xylodetritic lig nite with a mas sive and frac tured struc ture – XDLm(fr) (Figs. 4 and 5; Ta ble 1). These two lithotypes in clude a greater per cent age of fine-graded plant de tri tus (<1 cm) than do xylites, that is, fos sil ised wood re mains (>1 cm). The lower lig nite di vi sion in cludes >90%, and the up per di vi sion >50%, of
Code Fa cies
Sm sand with a mas sive struc ture
SCm(d) coaly sand with a mas sive and de formed struc ture Sh sand with a hor i zon tal strat i fi ca tion Sh(d) sand with a hor i zon tal and de formed struc ture SCh(d) coaly sand with a hor i zon tal and de formed struc ture St sand with a trough cross-strat i fied struc ture Sr sand with a rip ple cross-strat i fied lam i na tion Sp(d) sand with a pla nar cross-strat i fied and de formed
struc ture
SCp(d) coaly sand with a pla nar cross-strat i fied and de formed struc ture
Code Lithotypes
DLm(fr) detritic lig nite with a mas sive and frac tured struc ture XDLm(fr) xylodetritic lig nite with a mas sive and frac tured
struc ture
T a b l e 1 Cod i fi ca tion of fine-grained siliciclastic fa cies
(af ter Miall, 1977) and lig nite lithotypes (af ter Widera, 2012, 2016b) used in this pa per
Fig. 3. Gen eral view of the ex am ined siliciclastic suc ces sion ex am ined, rep re sent ing the Poznañ For ma tion (Grey Clays Mem ber) of Middle Mio cene age, sit u ated be tween two benches of the lig nite seam (MPLS-1)
A – cre vasse-splay de pos its of al most equal thick ness seen in a south–north di rec tion;
B – cre vasse-splay de pos its that show thick ness re duc tion to the west
fine-de tri tal ma trix (Kwieciñska and Wag ner, 1997; Widera, 2012, 2016b).
The ex po sure stud ied in the JóŸwin IIB lig nite opencast mine also in cludes a frac ture sys tem, com pris ing two or thogo - nal sets of open frac tures (cleats) that are also per pen dic u lar to the bed ding (see Fig. 5A), that are the best formed among all Pol ish lig nite seams. These frac ture sets are ad di tion ally NW–SE- and NE–SW-strik ing, that is, par al lel and per pen dic u - lar to the elon ga tion of the main tec tonic struc tures in cen tral Po land (Widera, 2014).
In ter pre ta tion of lig nite lithotypes. The or i gin of the detritic lig nite is as so ci ated with a fen or open-wa ter en vi ron - ment, while the xylodetritic lig nite was formed when bushy veg - e ta tion dom i nated in the mire area (Teichmüller, 1989; Markiè and Sachsenhofer, 1997; Ticleanu et al., 1999; Widera, 2012, 2016b). The pres ence of the clastic fa cies within MPLS-1, rep - re sent ing cre vasse-splay de pos its, pro vides ad di tional ev i - dence for the ex is tence of a low-ly ing mire. Be cause this mire was lo cated near the river chan nel it can there fore be in ter - preted as a backswamp (e.g., McCabe, 1984; Kirschbaum and McCabe, 1992; Flores, 1993; Diessel et al., 2000;
Davies-Vollum and Kraus, 2001; Toonen et al., 2016).
The NW–SE-strik ing frac tures are closely as so ci ated with pre-Ce no zoic re gional tec tonic trends. There fore, the for ma tion of the frac tures (cleats) is at least par tially linked with tec tonic pro cesses that af fected MPLS-1 syn- and postdepositionally (Widera, 2014).
SEDIMENTOLOGY OF THE SILICICLASTIC DEPOSITS
The siliciclastic de pos its oc cur ring within MPLS-1 con sist pre dom i nantly of fine-grained sands that are char ac ter ized by a unimodal grain-size dis tri bu tion. They are interbedded with a slightly coarser frac tion (me dium sand), which is en riched with or ganic mat ter. More over, these sands are well to very well sorted, slightly coarse-skewed as well as mesokurtic and leptokurtic (Ta ble 2). In gen eral, they may be di vided into two fa cies as so ci a tions. The first as so ci a tion en com passes the lower and mid dle parts, while the sec ond as so ci a tion con sti - tutes the up per most part of the sec tion ex am ined (Figs. 4 and 5). Within both as so ci a tions, nine fa cies have been dis tin - guished (Ta ble 3) rep re sent ing dif fer ent en vi ron ments of the cre vasse- splay de po si tion.
De scrip tion of fa cies as so ci a tion 1 (FA1). Fa cies as so ci - a tion 1 (FA1) rep re sents five fa cies that are per va sively de - formed in plas tic and brit tle modes. FA1 may be di vided into three seg ments (Fig. 4A). The lower seg ment is 20–40 cm thick and com prises sand and coaly sand that were orig i nally subhorizontally strat i fied. They were postdepositionally dis - turbed, hence the beds are cur rently un du lated and bro ken – fa - cies Sh(d) and SCh(d) (Fig. 4B). The mid dle seg ment of FA1 is the thick est, up to 50–90 cm, and its in ter nal struc ture is best vis i ble. The pla nar cross-strat i fied beds dif fer greatly due to the col our con trast be tween sands and coaly sands with or ganic con tent just ex ceed ing 1 wt.% (Figs. 4A and 5A). The de pos its de scribed there fore in clude only two fa cies that are also strongly de formed – Sp(d) and SCh(d). The lighter beds are in the range of 0.6–6 cm thick, while the darker ones are 0.1–4 cm thick. They are all in ter nally lam i nated. The palaeoflow di rec - tions have been mea sured from these beds. The mea sured palaeocurrent di rec tions vary from 215 to 285°, with an av er age value of 253°. The up per seg ment of FA1 com prises de formed de pos its that con tain the fol low ing 3 fa cies: SCm(d), Sh(d) and SCh(d) (Fig. 4A). Their summed thick ness is in the range of 10–40 cm. These de pos its are mainly mas sive, though rem - nants of hor i zon tal strat i fi ca tion may be also ob served (Fig. 5A, B).
In ter pre ta tion of fa cies as so ci a tion 1 (FA1). FA1 cor re - sponds to a sub aque ous cre vasse splay that ac cu mu lated in stand ing wa ter. Near-hor i zon tally ly ing de pos its of the lower and up per seg ments, and steeply dip ping (12–25°) sed i ments of the mid dle seg ment are char ac ter is tic of bottomset, topset and foreset beds, re spec tively (Fig. 4A and Ta ble 3). Es pe cially well formed are the so-called ‘prograding splay de pos its’, al low - ing this form to be called a cre vasse-splay microdelta (Teisseyre, 1985; Bristow et al., 1999; Michaelsen et al., 2000;
Spicer et al., 2002; Ciarcia and Vitale, 2013; Zieliñski, 2014). It is worth not ing here that fluvio-deltaic de po si tion in the overbank zone, in clud ing the backswamp ar eas, is quite com - mon and well doc u mented from both mod ern and an cient sed i - men tary suc ces sions (e.g., Smith et al., 1989; Tye and Coleman, 1989; Pérez-Arlucea and Smith, 1999; Opluštil, 2005; Rajchl and Ulièný, 2005; Rajchl et al., 2008; Bos et al., 2009; Cahoon et al., 2011; Mach et al., 2013; Toonen et al., 2016). Fi nally, it must be stated that the large-scale foreset beds mea sured in di cate a gen er ally unimodal WSW palaeotransport di rec tion in the SW part of the area un der study (see Figs. 1C, D and 3).
Sam ple
A B C
Me dian grain size
2.330 phi
fine sand 2.416 phi
fine sand 1.930 phi
me dium sand
0.199 mm 0.187 mm 0.263 mm
Mean grain size
2.364 phi
fine sand 2.413 phi
fine sand 1.922 phi
me dium sand
0.194 mm 0.188 mm 0.264 mm
Stan dard de vi a tion
0.466 phi
well-sorted 0.328 phi
very well-sorted 0.486 phi
well-sorted
0.724 mm 0.796 mm 0.714 mm
Skew ness –0.125 coarse-skewed –0.017 nearly sym met ri cal –0.030 nearly sym met ri cal
Kurtosis 1.190 leptokurtic 1.056 mesokurtic 1.061 mesokurtic
For lo ca tion of sam pling sites see Fig ure 4
T a b l e 2 Sta tis ti cal cal cu la tions and de scrip tions of rep re sen ta tive sam ples taken from siliciclastic de pos its within MPLS-1,
JóŸwin lig nite opencast mine in cen tral Po land
Fig. 4. Rep re sen ta tive sec tion of the siliciclastic de pos its stud ied with the lower and up per benches of the lig nite seam (MPLS-1) at the base and top, re spec tively
A – gen eral view of the de pos its rep re sent ing the cre vasse-splay microdelta; note po si tion of sam pling and fa cies from which they were col - lected; B – close-up view of the tran si tion be tween the foreset and bottomset laminae; note clearly in clined lay ers in the foreset and al most hor i zon tally ly ing, slightly dis turbed lay ers in the bottomset; C – close-up view of the up per most undeformed de pos its with re mains of trough and rip ple cross-lam i na tion; note the wide spread mas sive struc ture in the non-de formed sed i ments; for ex pla na tions of the fa cies and lithotype codes see Ta ble 1
Fig. 5. Seis mi cally-in duced de for ma tion struc tures in the lower and mid dle parts of the siliciclastic de pos its
A – gen eral view of the sec tion; B – com monly brecciated de pos its with ad di tional plas tic de for ma tion struc tures only in their low er most part; C – close-up view of the de formed lam i na tion in form of asym met ri cal folds; note that the plas tic ally de formed de pos its are also
brecciated; the pen is 14 cm long
De scrip tion of fa cies as so ci a tion 2 (FA2). Fa cies as so ci - a tion 2 (FA2) is up per most in the siliciclastic suc ces sion ex am - ined. FA2 is pre dom i nantly mas sive and 10–30 cm thick (Figs. 4A and 5A), al though rem nants of hor i zon tal and most of - ten trough and rip ple cross-strat i fi ca tion lo cally oc cur (Fig. 4C).
Thus, this as so ci a tion con sists of 4 fa cies: Sm, Sh, St and Sr.
Fur ther more, FA2 is char ac ter ized by a nearly white col our and the pres ence of small, fos sil ised rootlets (Fig. 4A, C).
In ter pre ta tion of fa cies as so ci a tion 2 (FA2). In con trast to FA1, non-de formed de pos its of FA2 are as signed in this study to a subaerial cre vasse splay due to the lack of ‘prograding splay de pos its’ that are typ i cal of ac cu mu la tion in stand ing wa - ter (e.g., Horne et al., 1978; Teisseyre, 1985; Bristow et al., 1999; Michaelsen et al., 2000; Gêbica and Soko³owski, 2001;
Zieliñski, 2014). In the case of these poorly strat i fied de pos its, it can be pre sumed that ini tially they were strat i fied (sub)hor i zon - tally in the lower part with lo cally oc cur ring lay ers of rip ple and trough cross-strat i fi ca tion in the up per part of FA2. Thus, the barely vis i ble fa cies Sh shows that sed i men ta tion took place over the en tire depositional sur face as un con fined sheet flow (e.g., Field ing, 1986; Farrell, 2001). By con trast, the mi nor fa - cies St and Sr in di cate trac tional de po si tion, re lated to the move ment of rip ples and small dunes, on the top sur face of the cre vasse splay (e.g., Collinson and Thomp son, 1982; Bristow et al., 1999; Zieliñski, 2014; Widera et al., 2016a). How ever, the com mon mas sive struc ture of FA2 (fa cies Sm) may be caused by both good sort ing of sed i ments (Ta ble 2) and the de struc tive role of the rootlets of the veg e ta tion from which the up per lig nite bench of MPLS-1 was formed. In the lat ter case, bioturbation may even lead to com plete de struc tion of the orig i nal strat i fi ca - tion (van Loon, 2009; Widera et al., 2017a).
SOFT-SEDIMENT DEFORMATION STRUCTURES
Soft-sed i ment de for ma tion struc tures (SSDS) in volve the cre vasse-splay microdelta unit. How ever, plas tic ally dis turbed
struc tures are pres ent only in the low est por tion, while brit tle de - for ma tion seems to be ubiq ui tous, in volv ing the en tire sandy body. The first group of duc tile de for ma tions con sists of de - formed lami na tions, load casts and flame struc tures (Figs. 5 and 6). Con se quently, the sec ond group of brit tle de for ma tions com prises only the brec cia (Figs. 4–9).
De scrip tion of de formed lam i na tion. De formed lam i na tion oc curs in beds rep re sent ing the bottomset and the low est 20–30 cm of the foreset. They are best seen among the foreset laminae where they are formed as straight and asym met ri cal folds (Figs. 5B, C and 6C). These folds range in height from 3–7 cm and are be tween 5–10 cm in width. In gen eral, the de - formed sets of beds are bounded by non-de formed ones that are also made up of interfingering sands (pale beds) and or ganic sands (dark beds). The folded sed i ments are ad di tion ally bro ken, that is, brecciated. In this case, it is sig nif i cant that the fail ures (frac tures) are ar ranged subvertically (Figs. 5C and 6C).
In ter pre ta tion of de formed lam i na tion. The for ma tion of the de formed lam i na tion in volves a plas tic state of wa ter logged sed i ments. This is re lated to par tial liq ue fac tion of dis turbed lay - ers with dif fer ent com pe tences for con tin u ous de for ma tion struc tures (Allen, 1982). In the case of the de pos its ex am ined, the sandy beds are com pe tent, while the coaly sand (approx.
1 wt.% of organics) is in com pe tent as re gards fold ing. Ob vi - ously, the pro cess of inter-layer fold ing in di cates compressional con di tions that can be caused by ground wa ter flow pres sure (from the river chan nel to the backswamp area) or by grav - ity-driven slumps (e.g., Alsop et al., 2017). At least a par tial im - pact of the hor i zon tal com po nent of down ward move ment, con - sis tent with the slope of the lay ers, is clearly ob serv able in the basal foreset seg ment of the cre vasse-splay microdelta. This is best seen in the asym me try of some folds cre at ing the de - formed lam i na tion (see Fig. 5B, C).
De scrip tion of load struc tures. Load casts oc cur in the siliciclastic de pos its stud ied sep a rately, or are as so ci ated with flame struc tures (Fig. 6A, B). They are lo cated only in de pos its
Fa cies as so ci a tions Fa cies De scrip tion In ter pre ta tion Depositional
en vi ron ment
De formed sands and coaly sands (FA1)
Sh(d), SCh(d)
Fine-grained sand and coaly (car bo na ceous) sand strat i fied hor i zon tally, de formed plas tic - ally and brecciated; up to 40 cm thick; un der - lain by lower bench of lig nite seam (MPLS-1)
Bottomset beds de pos ited along the bot tom of stand ing wa ter, i.e.
pond or lake, ex ist ing in the backswamp area; lower seg ment of the cre vasse-splay microdelta;
over lain by foreset beds
Sub aque ous cre vasse splay,
i.e. cre - vasse-splay
microdelta Sp(d),
SCp(d)
Fine- to me dium-grained sand and coaly (car bo na ceous) sand with a pla nar cross-strat i fi ca tion; de formed plas tic ally and brecciated in the low er most part; com pletely brecciated in the mid dle and up per part; up
to 50–90 cm thick; beds are in clined at an an gle of 12–25°
Foreset beds ac cu mu lated on a slop ing sur face along the delta front; mid dle seg ment of the cre - vasse-splay microdelta front; un - der lain by bottomset beds and
over lain by topset beds SCm(d),
Sh(d), SCh(d)
Sand and coaly (car bo na ceous) sand with a mas sive struc ture or poorly vis i ble hor i zon tal
strat i fi ca tion; all lithofacies fully brecciated;
up to 50 cm thick
Topset beds de pos ited above wa - ter level in pond or lake ex isted in the backswamp area; up per seg -
ment of the cre vasse-splay microdelta
Undeformed sands and coaly sands (FA2)
Sm, Sh, St, Sr
Fine-grained sand pre dom i nantly with a mas - sive struc ture, some times with re mains of trough and rip ple cross-strat i fi ca tion or hor i -
zon tal bed ding; no traces of de for ma tions;
only up to 30 cm thick; over lain by up per bench of lig nite seam (MPLS-1)
Al lu vial fen ac cu mu lated at the top of the older cre vasse-splay
microdelta de pos its in the overbank (backswamp) area; no ponds or lakes in the backswamp
area; FA2 cov ers FA1
Subaerial cre - vasse splay
For ex pla na tions of fa cies cod i fi ca tion see Ta ble 1
T a b l e 3 Gen eral sedimentological char ac ter is tics of the siliciclastic fa cies iden ti fied within MPLS-1, JóŸwin lig nite opencast mine
in cen tral Po land
rep re sent ing the bottomset and low er most foreset beds. In the case of the in di vid ual load casts, their ob served width is in the range of 8–20 cm, while their height is up to 12 cm. Fur ther - more, the un der- and over ly ing lay ers are slightly un du lated.
This type of duc tile de for ma tion is char ac ter ized by par tial ho - mogeni sa tion, that is, the in ter nal lam i na tion is in vis i ble and is sur rounded by ir reg u larly dis turbed laminae (Fig. 6A). In con - trast, the load casts ac com pa ny ing the flame struc tures are smaller and they have orig i nal lam i na tion pre served, al beit dis - turbed. These flame struc tures look like pointed tongues of coaly sand pierc ing the over ly ing lay ers of sand. Some of these struc tures are in clined (Fig. 6B).
In ter pre ta tion of load struc tures. Both load casts and flame struc tures are the re sult of rhe o log i cal dif fer ences be - tween ad ja cent, su per im posed beds as seen from their liq ue - fac tion fea tures (D¿u³yñski and Walton, 1965; Collinson and Thomp son, 1982). They are gen er ally formed due to un sta ble den sity gra di ents (An ke tell et al., 1970) or un equal load ing (Allen, 1982). The liq ue fac tion ef fects of interbedded sed i ments
Fig. 6. Ex am ples of duc tile de for ma tion struc tures in the low er most parts of the cre vasse-splay microdelta de pos its A – load casts with ho mog e nized in ter nal struc ture in the bottomset laminae; B – flame and ac com pa ny ing load casts in the bottomset laminae; C – de formed lam i na tion in the low er most part of the foreset laminae; note al most plane-par al lel lam i nated de pos its that un der- and over lie the de for ma tion struc tures; the pen is 14 cm long
Fig. 7. Brit tle de for ma tion struc tures in the in the mid dle and up per parts of the cre vasse-splay microdelta de pos its A – brecciated de pos its capped by non-de formed ones; B – close-up view of the per va sively brecciated sed i ments; note cha ot i - cally ar ranged clasts in the up per part of the pho to graph and clasts ar ranged in an or derly man ner in the lower part of the pho to graph;
the pen is 14 cm long
with di verse rhe o log i cal prop er ties, re sult ing from the shak ing of the de pos its, have been re pro duced ex per i men tally (e.g., An ke - tell et al., 1970; Owen, 1996; Moretti et al., 1999). In the case of the de pos its ex am ined, the coaly sands, which are en riched in or ganic mat ter, are more (hy dro)plas tic. There fore, sandy beds are im mersed in more plas tic coaly-sandy beds, or these coaly sands in ject into the over ly ing sands (Fig. 6A, B).
De scrip tion of brec cia. The brecciation of the de pos its stud ied, rep re sent ing a cre vasse-splay microdelta, is the most char ac ter is tic and com mon fea ture (Figs. 4–9). Both de pos its were pre vi ously plas tic ally de formed (Figs. 5 and 6) and those that pre served their orig i nal strat i fi ca tion are clearly brecciated (Figs. 7–9). This brec cia is ma trix-sup ported in the lower and up per parts (bottomset and topset beds) and clast-sup ported in the mid dle part (foreset beds) of the suc ces sion ex am ined. In the for mer case, the brec cia is ar ranged cha ot i cally, while in the lat ter case it is or derly. This is es pe cially well vis i ble in plan view (Fig. 8A, B, D). Based on lab o ra tory anal y ses, the brecciated de pos its con sist of white in or ganic sandy clasts, while a grey to black ma trix of coaly sands con tains slightly more than 1 wt.%
or ganic mat ter. The size of the in di vid ual sandy clasts is
0.5–6 cm (av er ag ing 3 cm) long and 0.2–5 cm (av er ag ing 1.5 cm) thick (Figs. 8 and 9).
The cha ot i cally ar ranged brec cia, cor re spond ing to the bottomset and topset beds, is char ac ter ized by the pres ence of steeply dip ping frac tures and microfaults with ver ti cal dis place - ments of up to 1 cm (Fig. 9A, B). By con trast, the or derly ar - ranged brec cia in the foreset beds is slightly ro tated along the in clined strat i fi ca tion. This is the case when the layer of in or - ganic sand is un der lain by a rel a tively thick, or ganic-en riched, layer of car bo na ceous sand. The sandy clasts here form ‘dom - ino-type’ de for ma tion struc tures (Fig. 9C). Fi nally, the ini tial fail - ures of the de pos its ex am ined can also be ob served. These fail ures (cracks, frac tures) are set subvertically and open both up wards and down wards (Fig. 9D).
In ter pre ta tion of brec cia. The brec cia de scribed above was orig i nally com posed of interbedded heterolithic de pos its that dif fered sig nif i cantly in terms of their rhe o log i cal prop er ties.
How ever, their rhe o log i cal ani so tropy and sus cep ti bil ity to the brecciation did not play a ma jor role, as both lay ers of or ganic and in or ganic sands are strongly and equally brecciated (Figs. 4–9). The only ex cep tion here is where the de formed clasts of in or ganic sands dis play dom ino-style ge om e try Fig. 8. Brec cia stud ied seen in cross-sec tion and in plan view
A, B – the cha otic brec cia in plan view and its in ter pre tive sketch; C–E – the or derly ar ranged brec cia in plan view (D) and in cross-sec tion (E), re spec tively; the pen cap is 5 cm long, while the pen is 14 cm long
(Fig. 9C). In this case, the layer of un der ly ing or ganic-rich sands of more plas tic be hav iour, con sti tutes the de tach ment sur face along which the slight downslope move ment and ro ta tion (in the op po site di rec tion to the dip of the beds) of more com pe tent sandy clasts has oc curred (e.g., Basilone et al., 2016; Törõ and Pratt, 2016).
The brit tle for ma tion, that is, brecciation, took place af ter these duc tile struc tures were formed. This is clear, as the brit tle struc tures are su per im posed on plas tic ones (see Figs. 5 and 6). Thus, at least two de for ma tion events in volved partly the same de pos its, that is, the low er most por tion of the microdelta.
Fur ther more, brecciation of the de pos its rep re sent ing the cre - vasse-splay microdelta – FA1 – oc curred prior to de po si tion of the over ly ing sed i ments re lated herein to the subaerial cre - vasse splay – FA2 – which were not de formed (see Figs. 4–6).
The for ma tion of the brec cia is at trib uted to hy drau lic frac tur ing driven by high pore-wa ter pres sure in the liq ue fied de pos its.
Brit tle de for ma tion of non-lithified sed i ments re quires much stron ger seis mic shak ing than is needed for the for ma tion of plas tic de for ma tion struc tures (Obermeier and Pond, 1998;
Obermeier et al., 2005). There fore, the ques tion arises of whether there has been a sud den in crease in pore-wa ter pres - sure, re sult ing in frac tures and microfaults (Fig. 9A, B).
In this pa per, the or i gin of the brec cia linked with synsedimentary tec ton ics (sensu lato) that af fected the study area in the Mid dle Mio cene. Other geo log i cal facts that sup port this point of view (dis cussed be low) are taken into ac count in this in ter pre ta tion. More over, the lack of non-seis mic trig ger mech a nisms that can pro duce sim i lar brec cias, such as over - load ing, storm waves, per ma frost, and so on, may be in di rect ev i dence of a tec tonic or i gin (Rossetti and Santos, 2003;
Moretti and Sabato, 2007; Owen and Moretti, 2011; Owen et al., 2011; Moretti and van Loon, 2014; Moretti et al., 2016). Thus, one pos si ble ex pla na tion for the dra matic growth in pore-wa ter 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. As a re sult, the ini tial fail ures were first cre ated, which are di rected both up wards and down wards (see Fig. 9D).
Fi nally, all the cre vasse-splay microdelta sed i ments were layer-by-layer de formed in the form of the in ter preted seis mic brec cia.
DISCUSSION
Is sues re lated to the de po si tion and de for ma tion of the siliciclastic sed i ments, which split the first Mid-Pol ish lig nite seam (MPLS-1) in cen tral Po land, are first ex plained and dis - cussed in terms of the con cep tual model of de po si tion and de - for ma tion for the for ma tion of the two cre vasse splays, fol lowed by dis cus sion of the cri te ria, by means of which the de for ma tion struc tures stud ied may be linked with tec ton ics.
MODEL OF DEPOSITION AND DEFORMATION
We pro pose a con cep tual model that graph i cally il lus trates the for ma tion of both as so ci a tions of siliciclastic fa cies and their de for ma tion struc tures (Fig. 10). This is shown in four stages, the first and fourth of which re late to the de po si tion of the strongly de formed sed i ments of the cre vasse-splay microdelta – FA1 and non-de formed de pos its of the cre vasse-splay – FA2 (Fig. 10A, F). How ever, the other two stages re fer to the en vi - ron men tal con di tions nec es sary for the for ma tion of duc tile and brit tle de for ma tion struc tures, re spec tively (Fig. 10B–E).
At the first stage, when the lower bench of MPLS-1 ac cu mu - lated, shal low lakes and ponds were pres ent in the mire (backswamp) area (Fig. 10A). These were lo cated close to the Fig. 9. Ex am ples of brit tle de for ma tion struc tures in the
cre vasse-splay microdelta de pos its
A – sharp-edged and dis placed sandy clasts sur rounded by a coaly-sandy ma trix; B – subvertically ar ranged frac tures and microfaults with ver ti cal off set of up to 1 cm; C – dom ino-type brecciation of in clined sandy bed rest ing above the or ganic-rich sandy layer; D – ini tial fail ures of brecciated de pos its in di cat ing their crack ing both up wards and down wards
Fig. 10. Con cep tual model show ing de po si tion and soft-sed i ment de for ma tion of the siliciclastic de pos its interbedded with the first Middle Mio cene lig nite seam (MPLS-1) in the JóŸwin IIB opencast mine in cen tral Po land
A – ini tial stage, the cre vasse-splay microdelta for ma tion in the backswamp area; B – duc tile de for ma tion struc tures of wa ter-sat u rated de - pos its caused by weak seis mic shak ing; C – close-up view of the plas tic ally de formed de pos its shown in Fig ure 10B; com pare with Fig ures 5 and 6; D – brit tle de for ma tion struc tures in con di tions of in creas ing wa ter pres sure caused by strong seis mic shocks and small-scale col - lapse; E – close-up view of the brit tle-de formed de pos its shown in Fig ure 10D; com pare with Fig ures 5, 6 and 9; F – ac cu mu la tion of non-de - formed cre vasse-splay de pos its in a subaerial backswamp en vi ron ment
river chan nel, which wa ter sourced the wa ter that breached the nat u ral levee and flooded the backswamp. At that time the cre - vasse splay was formed that ex tended to this res er voir of stand - ing wa ter. This is how the so-called ‘prograding splay de pos its’
were formed that are char ac ter is tic of a cre vasse-splay microdelta (Fig. 10A; Teisseyre, 1985; Bristow et al., 1999;
Michaelsen et al., 2000; Spicer et al., 2002; Ciarcia and Vitale, 2013; Zieliñski, 2014).
Af ter wards, the ground wa ter ta ble was low ered to cover only only the bottomset and low er most 20–30 cm of the foreset beds (Fig. 10B). In such con di tions liq ue fac tion of the sandy de - pos its be low the ground wa ter ta ble took place. As a re sult, plas - tic de for ma tion struc tures (de formed lam i na tion, load and flame struc tures) arose in the lower parts of the cre vasse-splay microdelta (Fig. 10B, C).
Brit tle de for ma tion struc tures prob a bly could have arisen dur ing a mod est, but rel a tively wide spread tec tonic col lapse of the microdelta suc ces sion. Shortly af ter that, the ground wa ter ta ble was lower, while the potentiometric sur face was lo cated higher (Fig. 10D). This sit u a tion could cause a sud den rise in pore-wa ter pres sure and force wa ter up wards. The change of grain pack ing could ad di tion ally in crease the pore pres sure and con se quently the ex tent of the hy drau lic frac tur ing (Obermeier and Pond, 1998; Obermeier et al., 2005). Thus, the en tire suc - ces sion of the cre vasse-splay microdelta is per va sively brecciated, es pe cially de pos its rep re sent ing the foreset beds (Fig. 10D, E).
The last stage dis tin guished is ba si cally a rep e ti tion of the first stage. How ever, in this case, the cre vasse splay was de - pos ited subaerially as seen from the lack of “prograding splay de pos its”. Among these de pos its plas tic and brit tle de for ma - tions do not oc cur, and so they were not dis turbed tec toni cally (Fig. 10F).
IDENTIFICATION OF SEISMIC DEFORMATION STRUCTURES
As rec om mended by Owen et al. (2011), cor rect rec og ni tion of seis mi cally-in duced de for ma tion struc tures (seismites) re - quires a three-step ap proach. First, a fa cies anal y sis must be car ried out, as com pleted above. Then the mech a nisms of de - for ma tion should be pro vided, which is also pre sented above.
Fi nally, the cri te ria for iden ti fy ing these de for ma tion struc tures should be dis cussed as be low (Owen et al., 2011). Thus, over the past few de cades, many re search ers have pro posed di ag - nos tic cri te ria for the dif fer en ti a tion be tween seis mic and non-seis mic de for ma tions of un con sol i dated sed i ments (e.g., Sims, 1975; Rossetti and Santos 2003; Obermeier et al., 2005;
Moretti and Sabato, 2007; van Loon, 2009; Owen and Moretti, 2011; Owen et al., 2011; Moretti and van Loon, 2014; Moretti et al., 2016 and ref er ences therein). The most im por tant of these cri te ria are as fol lows: (1) large ar eal ex tent and lat eral con ti nu - ity; (2) oc cur rence in a tec toni cally ac tive area; (3) sim i lar ity to other earth quake-in duced de for ma tions; and (4) lack of non- seis mic mech a nisms of de for ma tion for ma tion.
The first cri te rion has been met in this case. The area stud - ied is more than 0.1 km2, while the to tal length of mine walls (in - clud ing dewatering chan nels sub jected to de tailed ob ser va tions and map ping) ex ceeded 1 km (see Fig. 1C, D). Thus, the en tire suc ces sion of the cre vasse-splay microdelta is strongly de - formed. For the sec ond cri te rion, it is pos si ble to in di cate some sup port ing data. Thus, there are faults oc cur ring in strata un - der ly ing MPLS-1, within the lig nite seam as well as cross ing MPLS-1 (Widera, 2007, 2013b, 2016c). The frac ture sets (cleats) within MPLS-1, doc u mented a few hun dred metres west of the siliciclastic de pos its stud ied, in part in di cate tec tonic ac tiv ity (Widera, 2014). Hence, the area of the JóŸwin IIB lig nite
opencast mine was ac tive syn- and postdepositionally. In other words, the area un der study was strongly af fected tec toni cally when MPLS-1, in clud ing the siliciclastic interbeds, ac cu mu lated in the Mid dle Mio cene. The soft-sed i ment de for ma tion struc - tures iden ti fied in the field and char ac ter ized in this pa per are mor pho log i cally sim i lar to struc tures from ar eas that were un - doubt edly sub ject to seis mic shocks, that is, syndepositional tec ton ics (e.g., D¿u³yñski and Walton, 1965; van Loon et al., 1995; Gruszka and Zieliñski, 1996; Rossetti and Santos 2003;
Gruszka and van Loon, 2007; Moretti and Sabato, 2007;
Widera and Ha³uszczak, 2011; Basilone et al., 2016; Törõ and Pratt, 2016; etc.). Some of these deformational struc tures in - duced by seis mi cally trig gered liq ue fac tion (or fluidisation) have been mod elled in the lab o ra tory us ing, for ex am ple, a shak ing ta ble (e.g., Owen, 1996; Moretti et al., 1999).
Nev er the less, other pos si ble non-seis mic trig ger mech a - nisms lead ing to liq ue fac tion (and/or fluidisation) must also be con sid ered. Firstly, sud den changes in the wa ter-ta ble level can pro duce such SSDSs as sand boils and dikes (e.g., Holzer and Clark, 1993). How ever, these are mor pho log i cally dif fer ent from the load casts and flame struc tures de scribed in this pa per (see Figs. 5 and 6). Duc tile de for ma tion struc tures within in the foreset laminae, as in the case of mi grat ing tidal dunes (Chiarella et al., 2016), can be formed by the sud den de po si tion or over load ing. Such an in ter pre ta tion is pos si ble, al though it only partiy ex plains the or i gin of the de for ma tion struc tures stud ied. It is worth not ing that plas tic ally dis turbed lay ers of the cre vasse-splay microdelta in volve the bottomsets and lower por tion of the foresets (see Fig. 5). In con trast, load struc tures are pres ent along the en tire foreset units (and dom i nate the up - per parts) of tidal dunes (cf. Chiarella et al., 2016: fig. 12). Due to the rel a tively shal low oc cur rence of the Cre ta ceous bed rock, mainly built of limy sand stones and marls, the pro cess of karstification must also be con sid ered in the cre ation of SSDSs.
In this case, de for ma tion struc tures would fol low the col lapse with its char ac ter is tic fea tures, that is, a con i cal shape, and large-scale and ver ti cally elon gated deformational struc tures (e.g., Moretti et al., 2011). So far, no cav erns have been found in the JóŸwin IIB opencast area, that may be ev i dence of dis so - lu tion of car bon ate rocks in the bed rock, and the mor phol ogy of the SSDSs in ves ti gated is dras ti cally dif fer ent from those typ i cal of karst-in duced col lapse.
Hence, all the de for ma tion struc tures ex am ined in this pa - per, both duc tile and brit tle, may be in ter preted as seismites (Seilacher, 1969), in clud ing the seis mic brec cia (Shukla and Sharma, 2018). Cur rent knowl edge of the geo log i cal set ting to - gether with field ob ser va tions pre clude other fac tors (changes in the height of the wa ter ta ble, sud den de po si tion, over load ing, karst, etc.).
CONCLUSIONS
This pa per de scribes de pos its of cre vasse splays, in clud ing of a cre vasse-splay microdelta, which split the first Mid dle Mio - cene lig nite seam (MPLS-1) in cen tral Po land. The stud ies con - sider both the sedimentological char ac ter is tics of the siliciclastic fa cies and the struc tural char ac ter is tics of the de for ma tion struc tures. The ma jor con clu sions of the re search can be sum - ma rized as fol lows:
1. Cur rently ex ploited in the JóŸwin IIB opencast (Konin Lig - nite Mine, cen tral Po land), the lig nite seam, MPLS-1, of Mid dle Mio cene age, is interbedded with weakly com pacted siliciclastic de pos its. These were in ter preted as two su per im posed cre - vasse splays and two fa cies as so ci a tions FA1 and FA2, cor re - spond ing to these splays, were dis tin guished. In ad di tion, FA1
is strongly de formed, while FA2 shows no traces of de for ma - tion.
2. The lower FA1 ac cu mu lated in stand ing wa ter (a lake or pond) in a backswamp area dur ing the for ma tion of MPLS-1.
The pres ence of three seg ments, rep re sent ing bottomset, foreset and topset, is typ i cal of a delta. Be cause it is rel a tively small and is ge net i cally as so ci ated with a cre vasse splay, it is in ter preted as a cre vasse-splay microdelta. In con trast, the up - per FA2 was de pos ited subaerially as a thin ner, one-seg ment body, a “prograding splay de posit”.
3. Only the cre vasse-splay microdelta de pos its are strongly dis turbed, with both plas tic and brit tle de for ma tion. Plas tic de - for ma tion struc tures (de formed lam i na tion, load casts and flame struc tures) were formed first, while brit tle de for ma tion (brec cia) took place later be ing im posed on pre vi ous struc tures.
Both types of de for ma tion were most likely gen er ated by seis - mic shocks (earth quakes) that caused liq ue fac tion of the de - pos its, re sult ing in (hy dro)plas tic dis tur bances or hy drau lic frac - tur ing, that is, brecciation. The or ganic mat ter con tent (more than 1 wt.%) likely played a sig nif i cant role in the rhe o log i cal prop er ties of sed i ments of very sim i lar grain size. Thus, the coaly sands could have de formed more plas tic ally than the in or -
ganic sands that are much more com pe tent and could have re - sulted in brit tle de for ma tion.
4. The de for ma tion struc tures in ves ti gated meet the most im por tant cri te ria for earth quake-in duced struc tures as sug - gested by other re search ers, and other mech a nisms of their for ma tion can not be con vinc ingly pointed to. There fore, they can be con sid ered as seismites. Fi nally, the cre vasse-splay micro delta from the JóŸwin IIB opencast in cen tral Po land, sed i - ments of which are partly plas tic ally de formed and per va sively brecciated, is the first doc u mented such de posit in all the Pol ish lig nite-bear ing ar eas.
Ac knowl edge ments. We would like to thank M. Dziamara from the Geo log i cal De part ment of the Konin Lig nite Mine for his lo gis ti cal sup port. The au thors are deeply grate ful to J.R. Kasiñski (Pol ish Geo log i cal In sti tute – Na tional Re search In sti tute, War saw, Po land) and M. Moretti (Uni ver sity of Bari, It - aly) for their eval u a tion of the type script. In ad di tion, many thanks are ex tended to A. Wysocka (Uni ver sity of War saw, Po - land), the Co-Ed i tor of the Geo log i cal Quar terly, for her ed i to rial han dling. This pa per is sup ported by the Na tional Sci ence Cen - tre, Po land, through re search pro ject no.
2017/27/B/ST10/00001 (to MW).
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