Original

Geological Implication of a Turreted Gastropod and Astartid Pelecypod Bearing

Horizon in the Nodular Limestone of Sukar Nala Section near Zirabad of Bagh,

Dhar District, M.P., India

Tapas Kumar Gangopadhyay1*, Manisha Maiti (Talukdar)2

Received : 2 Febuary 2011 ; Accepted : 13 September 2011

AbstractCretaceous beach environments have been studied in many parts of the world. The present study area probably

adds one such area from India. The area bears signature of beach environment in the form bidirectional

orientation of turreted gastropods of the Families Terebriedae and Pyramidellidae. Besides cause of convex

downward position of associated astartid pelecypod shells has been unraveled from sedimentologic point of view.

Keywords: Nan, Wiang Sa, Pha Dang Kwai, Phrae, Doi Tham Pha Klong, Colaniella, Palaeofusulina, late

Changhsingian, the end of the Permian

1,2 Department of geology, Bengal Engineering & Science University, Shibpur, Howrah-711103, India.

* Corresponding author’s email: tapasgeolbe@yahoo.com

IntroductionCretaceous Period in the Mesozoic Era spans

from 135 – 65.5 Ma. It has witnessed many a

unique phenomenon like splitting of continents, the

most severe marine transgression (Cenomonian -

Turonian), widening of oceans (The Atlantic), rifting

of continents,widespread chalk deposition, Mesozoic

marine revolution and ended with a massive extinction.

In such a time India was having its solitary excursion

towards north to collide finally with the Asia during the

Early Eocene. At that time Madagascar and Seychelles

were attached with the western coast of India. In the

Lower Upper Cretaceous Madagascar separated

from India. Sea water of Turonian - Coniacian marine

transgression found its route to penetrate western

and central India along the newly rejuvenated E – W

trending Narmada graben and as a result Bagh Group

of carbonates were deposited therein. The present work

has been done on a carbonate mud horizon of Bagh

beds at Sukar Nala section near Zirabad (Figure 1)

being infested with essentially turreted tiny gastropod

shells with bipolar distribution (Figure 5) of their azimuths

and few disarticulated astartid bivalve shells in concave

up positions. Attempt has been made to give a deep

insight to 1) perfectly identify the gastropods (family

level) 2) their special orientation in this fashion and

3) finally arrived at a conclusion which unraveled a

hitherto unattempted story concealed in this horizon.

46 J Sci Technol MSUGangopadhyay T.

Figure 1 Locations map.

StratigraphyThe Bagh Group is constituted by two Formations

(Figure 2) viz. the older Nodular Limestone and the

younger Coralline Limestone1. Corals are, however,

lacking and bryozoans, instead, dominated in the

Younger Formation, hence hereafter the formation

will be named as Bryozoan Limestone Formation2.

The basal contact of the Nodular Limestone with a

siliciclastic unit, the Nimar Group is gradational at

many places, although sharp contact is also not rare.

Nodular Limestone Formation is divisible into three

sub-units vertically. The contact of the lowermost and

middle sub-unit is demarcated by a thick ( 3-5 cm.)

purple clay horizon.The contact between the middle

and upper sub-unit is demarcated by a regionally

persistent, heavily bored hardground (H1) (Figure

3) except in one side of Sukar Nala section where

instead of hardground ,the present horizon is exposed.

The upper contact of Nodular Limestone with the

Bryozoan Limestone is, on the other hand, sharp

everywhere. The Bryozoan Limestone is overlain, at

places by the Lameta Group. The nature of contact

is, however, yet to be recognized. The Decan Trap

unconformably overlies the Lameta Group, the Bagh

Group as well as the Nimar Group mentioned above

in different sections. Manifold increase in mining

activity for limeston

Vol 31, No 1, Jun-Feb 2012 Geological Implication of a Turreted Gastropod and Astartid Pelecypod Bearing Horizon in the Nodular Limestone of Sukar Nala Section near Zirabad of Bagh, Dhar District,M.P., India

47

Figure 2 Stratigraphic succession of the Bagh Group

(after Bardhan2).

Figure 3 Heavily bored hardground (H1).

in the study area has exposed many good sections which

reveal vital sedimentological informations hitherto unknown.

Statement of the problemIt has already been discussed that the present study area

comprises a shallow water marine carbonate sequence—

divisible into two already mentioned formations, with

abundant macro and micro invertebrate fossils, But the

first Hard ground (H1) is almost present in all the sections.

But surprisingly, in the western side of the Sukarnala

section at Zirabad the hardground (H1) is not recognisable.

But on closer observation it has been found that there

is a horizon at the level of the hardground (H1) of the

other areas, strewn with numerous turreted gastropod

shells intact as well as fragmentary monogeneric

disarticulate bivalve shells essentially in concave-up

position. On closer investigation it has been found

that the turreted gastropod shells are distributed with

azimuths pointing in opposite directions. Although of

these two opposite directions number of fossils are

greater in number in one direction than in the other.

However, about 2 cm. below this horizon there is a level

which is infested with Thallassinoid burrows (Figure 4). Now

summing up the observations the problems that crop out are –

1. Why hardground is not recognizable in the

present area?

2. Why gastropod shells show bipolar orientation?

3. Why bivalve shells are majorly disarticulated

and show a concave up position?

Figure 4 Thalassinoid burrow bellow hardground H1.

48 J Sci Technol MSUGangopadhyay T.

The orientation concave down can be produced

by many processes but can be preserved only in

the absence of significant horizontal force.

Discussion and concomitant conclisionWhy hardground is not recognizable?

It is interesting to note that in this section of the locality

Zirabad the regionally persisting hardground is not

recognisable. Hardgrounds are generally planner,

bored, synsedimentary cemented surface. The borings

are made on this thin horizon by polychaete annelids

and sponges. The turreted minute gastropods of the

present area belong to the family Terebreidae &

Pyramidellidae. Terebried gastropods are predators of

annelids and pyramidelids gastropods are predators

of sponges. Hence absence of annelids & sponge

is the main reason for absence of borings, presence

of which certainly proves existence of hardground.

Again it has been established in this work that

bidirectional orientation of the fossil terebrid and

pyramidlid gastropods shells are probably indicative

of nearshore beach environment. According to

Bathurst ‘Recognition of fossils in carbonate beach

rock is likely to be difficult but it is worth bearing in

minds that they should differ in several ways from

carbonate sediments that are cemented in other

environments. First, they have or, had an aragonite

or, high magnesian calcite cement. If the aragonite

has been dissolved and replaced by the calcite then

it’s one time existence may be impossible to discover.’

So another reason may be that it was

palaeobeach so thin aragonitic hardground

might had developed but later it was dissolved.

Why gastropod shells show bipolar

orientation? (Figure 5 & 6)

Turreted gastropod shells are conical objects. Its

orientations are guided by (1) slope (2) nature of current

in the medium. Obviously, slope was towards the sea

and in the present case majority of the gastropod

shells, though there is bipolar distribution (700 and

2500) (Figure 6), show azimuth of the shell pointing

towards the prevailing current direction (Probably

perpendicular to Longshore current) and the length of

the shell is perpendicular to the shoreline. This can be

explained in the way that these were representative

of downslope recessive current and this type of

phenomenon has already been established in beach

environment – from both present and past. Bipolar

orientation of the gastropods most probably represent

prevalence of to and fro motion of the waves and

dominant current direction was near perpendicular

to longshore current.

Figure 5 Bipolar oriented gastropods shells.

Figure 6 Rose diagram depicting the azimuths of

the gastropod shells point two opposite

directions 700 and 2500.

Vol 31, No 1, Jun-Feb 2012 Geological Implication of a Turreted Gastropod and Astartid Pelecypod Bearing Horizon in the Nodular Limestone of Sukar Nala Section near Zirabad of Bagh, Dhar District,M.P., India

49

Why bivalve shells are essentially convex

down? (Figure 7)

From pattern of orientation of gastropod shells It

has been established that this horizon might be the

product of beach environment, although convex up

is the most stable orientation of single valve of the

pelecypode shell. However, convex down orientations

are also not uncommon like in the present case. These

are decoupled valves of biconvex pelecypode shells

and were affected by nearshore agitated water. Once

they are taken into suspension, while falling, convex

down is the most common phenomenon. According to

Toots3, the orientation convex down can be observed

on beaches. It seems that the predominating orientation

happened at the extreme upper limit of wave action.

Fossil examples, interpreted as beach accumulation

of this type have been found the upper Cretaceous

rocks of Wyoming. However, once they have been

deposited as a clast in the mud, they are quickly

buried in the cohesive mud and further wave action

had not been able to reorient them. And this might

had happened in the present case also.

AcknowledgementMr. Rajesh Kundu helped in formatting the manuscript

References1. Bose PN. Geology of Lower Narbada Valley

between Nimawar and Kawant. Memoir

geological survey of India. 1884; V.21, Pt-1.

2. Bardhan S, Gangopadhyay TK, Mondal U.: How

far did India Drift during the. Late Cretaceous?

Placenticeras kaffrarium Etheridge, 1904

(Ammonoidea) used as a measuring tape.

Sedimentary geology 2002; V.147: pp-193-217.

3. Toots H. Orientation and distribution of fossils

as environmental indicators. Wyoming

geogical association guide book, 19 th

American Field Conference 1965; pp-219-229

Figure 7 Convex down orientation of the bivalve shell.

50 J Sci Technol MSUGangopadhyay T.

Measurements of azimuths of gastropods shells:

Range (Degree) Count Azimuth (Degree)

00 - 100 0  

110 - 200 0  

210 - 300 0  

310 - 400 0  

41 - 50 0  

51 - 60 0  

61 - 70 14 65; 68; 65; 70; 67; 62; 63; 65; 63; 62; 64; 61; 70; 68

71 - 80 21 72; 73; 75; 77; 72; 77; 77; 78; 71; 74; 76; 79; 75; 75; 78; 75; 79; 72; 74; 76; 80

81 - 90 7 85; 82; 87; 83; 83; 84; 83

91 - 100 6 95; 91; 92; 91; 95; 99

101 - 110 8 102; 104; 110; 107; 109; 110; 108; 110

111 - 120 0  

121 - 130 0  

131 - 140 0  

141 - 150 0  

151 - 160 0  

161 - 170 0  

171 - 180 0  

181 - 190 0  

191 - 200 1 200

201 - 210 3 207; 210; 209

211 - 220 7 215; 211; 214; 218; 214; 219; 217

221 - 230 3 225; 227; 223

231 - 240 8 235; 238; 235; 240; 233; 234; 237; 240

241 - 250 6 250; 250; 242; 246; 250; 250

251 - 260 19 252; 252; 254; 256; 258; 259; 254; 258; 255; 257; 256; 258; 260; 251; 253; 252;

260; 260; 260

261 - 270 6 268; 270; 262; 265; 264; 262

271 - 280 2 273; 275

281 - 290 0  

291 - 300 0  

301 - 310 0  

311 - 320 0  

321 - 330 0  

331 - 340 0  

341 - 350 0  

351 - 360 0