Ha et al. Progress in Earth and Planetary Science
https://doi.org/10.1186/s40645-019-0293-y(2019) 6:47

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Progress in Earth andPlanetary Science

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Litho-, bio-, and chemostratigraphy of the
Middle Triassic carbonate succession in the
North-Central Coast Region of Vietnam
Thuy Thi Nhu Ha1, Hideko Takayanagi1, Katsumi Ueno2, Yoshihiro Asahara3, Koshi Yamamoto3 andYasufumi Iryu1*

Abstract
Middle Triassic carbonates extend from the North-Central Coast region of Vietnam to northern Laos. We conducted
sedimentological, paleontological, and geochemical analyses on the carbonates of the Hoang Mai Formation in
Nghe An province, Vietnam. The carbonates consist of the following six lithostratigraphic units (from bottom totop): sandy limestone (unit 1), peloidal packstone with a microbialite interlayer (unit 2), peloidal packstone andoncoidal floatstone (unit 3), peloidal–bioclastic packstone (unit 4), peloidal packstone with variable quantities of
bioclasts and microbialite (unit 5), and peloidal packstone with variable quantities of bioclasts (unit 6). The sandy
nature of unit 1 and of three of the interlayers in unit 2 indicates a supply of terrigenous material during the earlystages of formation of the Hoang Mai carbonate platform. The dominance of carbonates with a fine-grained matrix
throughout the overlying succession implies a low-energy depositional environment. Common occurrences ofdasycladalean algae in units 4–6 indicate a back-reef lagoon environment. A total of 24 foraminiferal taxa, including
Citaella dinarica and C. deformata, were identified. These two age-diagnostic species occur in units 3–6, suggestingthat this interval is correlated with the Pelsonian and that the lower units (units 1–2) are potentially correlated with
the Bithynian. Thus, the entire core section can be assigned to the middle Anisian (lower Middle Triassic). An
assessment of the diagenetic alteration using geochemical parameters indicates that the carbonates of the studied
succession have not retained their initial strontium (87Sr/86Sr) and oxygen (δ18O) isotope compositions. In contrast,
the carbonates in the lower (units 1–2) and middle (units 2–5) intervals have retained their initial carbon isotope
compositions (δ13C), making them suitable for C isotopic analyses. In the lower interval, we observe a slight δ13Cenrichment, followed by a gradual δ13C-depletion upwards into the middle interval. The observed trend in δ13C
values from the Hoang Mai Formation can be correlated with similar trends reported from South China andRomania. Samples within the upper interval (units 5–6) of the Vietnamese δ13C profile are relatively δ13C-depleted,which is likely the result of diagenetic overprinting.
Keywords: Chemostratigraphy, Foraminiferal biostratigraphy, Carbon isotope, Carbonate, Middle Triassic, Hoang Mai
Formation, Vietnam* Correspondence: yasufumi.iryu.d8@tohoku.ac.jp
1
Department of Earth Science, Graduate School of Science, Tohoku
University, Sendai 980-8578, Japan
Full list of author information is available at the end of the article© The Author(s). 2019 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0
International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, andreproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to
the Creative Commons license, and indicate if changes were made.

Ha et al. Progress in Earth and Planetary Science(2019) 6:47

Page 2 of 21abFig. 1 Location of the study site. a Maps showing the study site. b Paleogeographic map of the Late Triassic (modified from Martindale etal. 2015)

Ha et al. Progress in Earth and Planetary Science(2019) 6:47

Introduction
Middle Triassic carbonate platforms extend westwards fromthe North-Central Coast region of Vietnam (Fig. 1a) tonorthern Laos (Dang 2006). The Trang An Landscape Complex carbonates, located near the southern margin of the
Red River Delta, are designated a UNESCO World Natural
Heritage site for their spectacular limestone karst peaks surrounded by steep, almost vertical cliffs. Large volumes of thelimestone platforms have been mined for raw materials for
cement. Several geological and paleontological studies havebeen conducted on the Middle Triassic carbonates in thisregion because of their importance as a tourist attraction
and a mineral resource as well as for their scientific interest(Liem 1966; Geological Survey of Vietnam 1995; Shigeta etal. 2010; Thanh and Khuc 2011). However, many of thosestudies are written in Vietnamese (e.g., Vu and Trinh 1969;
Vu et al. 1972; Liem 1973; Do 1983) and cannot easily beunderstood by international readers. The lithostratigraphyof the Triassic succession in the region is well established(e.g., Dang 2006). The Middle Triassic strata (including thecarbonates) are widely distributed in Thanh Hoa and Nghe
An provinces of the North-Central region of Vietnam aswell as in northern Laos (e.g., Miyahigashi et al. 2017).
Paleontological studies have been conducted on Middle
Triassic ammonoids (Vu 1984, 1991; Shigeta et al. 2010),
bivalves (Vu and Trinh 1969; Vu 1991), and foraminifera(Liem 1966; Ueno et al. 2019), including species that are
age-diagnostic.
The Middle Triassic carbonates in this region are key
to furthering our understanding of the geological development of the Indochina and South China blocks (e.g.,
Cai and Zhang 2009; Metcalfe 2011; Faure et al. 2014)
and to reconstructing the paleobiogeography of the
Paleo-Tethys (Miyahigashi et al. 2017). Here, we report
the litho-, bio-, and chemostratigraphy of the Middle
Triassic carbonate succession in Nghe An Province in
the North-Central Coast region of Vietnam (Fig. 1a).
This paper provides the first chemostratigraphic data for
the Middle Triassic carbonates in this region.
Carbon isotope stratigraphy of marine carbonates provides records of past secular changes in the global carbon cycle and serves as a useful tool for stratigraphic
correlation (Saltzman and Thomas 2012). However, theMiddle Triassic carbon isotope stratigraphy is not wellestablished compared with other geological periods, andreported δ13C profiles are not necessarily consistent witheach other (Additional file 3: Figure S1). Consequently,
here, we provide a new high-resolution Middle Triassic
carbon isotope stratigraphy that is constrained by foraminiferal biostratigraphy.
Geological setting
The territory of Vietnam spans the South China and Indochina blocks. The boundary between them is considered

Page 3 of 21

to be the Red River Fault Zone (Tapponnier et al. 1990;
Osanai et al. 2008) or the Song Ma Suture (Lepvrier et al.
2004, 2008; Metcalfe 2011, 2012; Udchachon et al. 2017),
both of which strike NW–SE. The Indochina Block occupies the majority of Vietnam, with the South China Blockbeing limited to the country’s northern corner. Structurally, Vietnam is divided into five blocks: the Northeastern,
Northwestern, Truongson, Kontum, and Nambo blocks(Nam 1995), with the last three blocks belonging to the
Indochina Plate. The study site is near the northernborder of the Truongson Block.
Triassic marine siliciclastics and associated carbonatesin the Sam Nua Basin (Khuc 2000) extend along thenortheastern margin of the Indochina Block from theThanh Hoa and Nghe An provinces of Central Vietnam
northwestward to northern Laos. The deposits are mainly
Middle Triassic in age and are divided into three formations, which (from bottom to top) are the Dong Trau,
Hoang Mai, and Quy Lang formations (Dang 2006). According to Thanh and Khuc (2011), the Dong Trau Formation is 1000–1500 m thick and consists mainly of
conglomerate, sandstone, felsic tuff, and tuffaceous sandstone, interbedded with cherty shale, siltstone, sandy shale,marl, rhyolite, and dacite. The Hoang Mai Formation,
resting conformably on the Dong Trau Formation, is ~500 m thick and consists of basal marl and overlying bedded limestone (Khuc 2000). The limestone yields marine
fossils such as bivalves (Costatoria curvirostris and C. proharpa; Thanh and Khuc 2011) and foraminifers (Liem
1966). However, the stratigraphic distribution of the foraminifers is not well defined. The Quy Lang Formation
conformably overlies the Hoang Mai Formation and consists of 550–650 m-thick siliciclastics interbedded withcarbonate (Thanh and Khuc 2011). This formation yields
Balatonites sp., an age-diagnostic (middle Anisian) ammonoid, and abundant shallow-marine bivalves such as
Costatoria goldfussi mansuyi and Trigonodus tonkinensis(Shigeta et al. 2010).
During the Anisian, the Paleo-Tethyan oceanic lithosphere was subducted beneath the Indochina Block, anda restricted (narrow) oceanic basin existed between the
Indochina and South China blocks (Cai and Zhang2009). The Anisian limestones extending along thenorthern margin of the Indochina Block in Vietnam andLaos formed in this restricted ocean basin (Fig. 1b).

Methods/Experimental
Drill cores (cores no. 1 and no. 4) were obtained from carbonate rocks of the Hoang Mai Formation in the Nghe
An province, North-Central Coast region of Vietnam(Fig. 1). The cores were 130.0 and 160.0 m long (Figs. 2
and 3), and recovery was 100% and 99.3%, respectively.
The carbonates have a monoclinal structure, striking NE–
SW to (less commonly) NNE–SSW and dipping 50°–60°

Ha et al. Progress in Earth and Planetary Science(2019) 6:47

Page 4 of 21

Fig. 2 Lithostratigraphy and chemostratigraphy of carbonates from core no. 1. Vertical profiles of δ13C and δ18O values, Sr, Mn, and Fe concentrations,and Mn/Sr and 87Sr/86Sr values of bulk carbonate samples from core no. 1. Note that the pink areas denote the threshold for samples determined tobe diagenetically altered, based on criteria defined by Denison et al. (1994) and Jacobsen and Kaufman (1999). The blue area denotes 87Sr/86Sr of
Anisian seawater (McArthur et al. 2001). BG, bioclastic grainstone; BP, bioclastic packstone; IG, intraclastic grainstone; MB, microbialite; NR, no recovery;OF, oncoidal floatstone; PBP, peloid bioclast packstone; PBP/G, peloid bioclast packstone/grainstone; PG, peloidal grainstone; PP, peloidal packstone;
PP/G, peloidal packstone/grainstone; PPMB: peloidal packstone with/interlayers of microbialite; SL, sandy limestone

SE to ESE around the drill sites, and therefore, the core
lengths were 1.5–2.0 times greater than the actual thickness of the beds (~ 150–190 m thick).
A total of 290 bulk carbonate samples were collectedfrom the two cores for geochemical analyses (Additional file 1: Table S1 and Additional file 2: Table S2).
Efforts were made to avoid large bioclasts and microbial

products (microbialites and oncoids), large cemented
crystals, veins, stylolites, and pressure-solution seams.
Additional samples were taken from veins and cements
to compare their chemical and isotopic compositions
with those of the bulk carbonate samples.
To identify and exclude diagenetically altered samples,mineral abundance and minor element (strontium (Sr),Ha et al. Progress in Earth and Planetary Science(2019) 6:47

Page 5 of 21

Fig. 3 Lithostratigraphy and chemostratigraphy of carbonates from core no. 4. Vertical profiles of δ13C and δ18O values, Sr, Mn, and Fe
concentrations, and Mn/Sr and 87Sr/86Sr values of bulk carbonate samples from core no. 4. For the definition of the pink and blue areas andabbreviations, see the Fig. 2 caption

Ha et al. Progress in Earth and Planetary Science(2019) 6:47

manganese (Mn), and iron (Fe)) concentrations were determined. Mineral abundances were determined following
Suzuki et al. (2006) with a Phillips X’pert-MPD PW3050
system at the Institute of Geology and Paleontology, Tohoku University, Japan (IGPS). Based on the results of the
X-ray diffraction (XRD) analysis, 42 samples were excluded from the subsequent minor element and isotope
analyses because of their relatively high contents of dolomite (10–84 wt%). Powdered samples (0.3 mg) were dissolved in ~ 4.5–10.0 mL of 2% nitric acid. The solutions
were analyzed for concentrations of three minor elements(Sr, Mn, and Fe) using an Agilent 7700x inductively
coupled plasma mass spectrometer at the Department ofEarth and Environmental Sciences, Nagoya University,
Japan. Minor element concentrations are expressed asppm (= μg/g). The external precision determined by replicate analyses of the laboratory standard was approximately
4.9%, 4.6%, and 4.7% for the Mn, Fe, and Sr concentrations, respectively.
To provide chronological constraints on the studied carbonate succession, strontium isotope ratios (87Sr/86Sr) andforaminiferal assemblages were analyzed for 53 and 68
samples, respectively. The 87Sr/86Sr was measured with a
VG Sector 54-30 thermal ionization mass spectrometer at
the Department of Earth and Environmental Sciences, Nagoya University, following protocols by Asahara et al.
(1999, 2006) and Suzuki et al. (2012). Replicate analyses of
the National Institute of Standards and Technology(NIST) Standard Reference Material 987 during this study
gave a value of 0.710254 ± 0.000008 (1σ, n = 9). All measurements were normalized to 0.710248 (McArthur et al.
2001).
To establish chemostratigraphy, the carbon (δ13C)
and oxygen (δ18O) isotope ratios of 248 samples were
measured with a Thermo Fisher Delta V isotope ratio
mass spectrometer coupled to a ThermoQuest Kiel-III
automated carbonate device, at IGPS. The samples (~0.1 mg) were reacted with 100% phosphoric acid at ~72 °C. The isotope ratios were expressed in conventional (δ‰) notation and calibrated to the NBS-19
international standard, relative to Vienna Pee Dee
Belemnite (VPDB). The external precisions (1σ) forthe carbon and oxygen isotope analyses based on replicate measurements (n = 71) of the laboratory reference materials JCp-1 were 0.02‰ and 0.03‰,
respectively.
The δ18O values for Mg-calcite are higher thanthose for pure calcite (e.g., Tarutani et al. 1969). If a
fractionation correction for Mg-calcite relative to purecalcite was applied (0.06‰ per mol% MgCO3, according to Tarutani et al. 1969; 0.17‰ per mol% MgCO3,
according to Jiménez-López et al. 2004), the Mg effecton the δ18O values is negligible because the Mg contents of the analyzed samples were < 0.1 mol% MgCO.

Page 6 of 21

Results
Core no. 1
Lithostratigraphy

The studied carbonate succession (Fig. 2) represents acontinuous depositional record, with no observed erosional surfaces that indicate a hiatus. Dissolution cavities
are common in the upper half of the core and are filledwith various-sized breccias derived from the surrounding limestone and/or laminated lime mud. Dissolution
vugs are common and are filled with sparry calcite.
The interval from the base of the core (130.0 mdfrh, meters depth from the reference horizon (= the ground surface when the drilling was conducted)) to 117.0 mdfrh
consists mostly of sandy limestone with peloids (Fig. 4a).
The limestone is characteristically grayish-black to black incolor, likely due to small (mud-sized) grains of quartz, as
indicated by XRD analyses and thin-section observations.
Peloids are irregularly shaped with obscure outlines andare coarse sand sized or smaller. Bioclasts are rare but include ostracods, foraminifers, and echinoids. Bioturbationis common. Intergranular pore spaces are filled with micrite, which is partly replaced with sparry calcite. Dolomite
content is high, reaching 70 wt% in several horizons in thislower interval of the core (130.0–117.0 mdfrh).
The core interval 117.0–63.5 mdfrh, which is the middle
interval of the core, is dominated by peloidal packstone(Fig. 4c). Peloids are irregular or (rarely) subrounded andmostly in the medium to very coarse sand size fractions.
Bioclasts, including foraminifers, ostracods, bivalves(dominated by thin-shelled bivalves), gastropods, andechinoids, are rare and possess micrite envelopes. Intergranular pore space is filled with micrite and some sparrycalcite. Bioturbation is common. Borings are filled withpeloids, with a grainstone texture. A layer of microbialite
in a matrix of peloidal packstone, at 108.0–97.2 mdfrh, iscomposed of loosely overlapped micritic laminae, binding
peloids derived from the peloidal packstone matrix(Fig. 4c). The middle interval contains three interlayers ofsandy limestone that are lithologically similar to those in
the lower interval (130.0–117.0 mdfrh).
The interval from 63.5 mdfrh to the top of the core
consists of peloidal packstone and oncoidal floatstone.
The peloidal packstone is similar to that in the interval
below (117.0–63.5 mdfrh). Oncoids are rounded to subrounded, pebble sized or smaller, and composed ofloosely overlapping, irregularly undulated micritic envelopes (Fig. 4d). The nuclei of some oncoids are bioclasts.
Small oncoids are commonly aggregated to form larger
oncoids. The matrix of the oncoidal floatstone is a peloidal packstone to grainstone. Bioclasts have micrite envelopes regardless of their lithology. Intergranular porespace in the oncoidal floatstone and the peloidal packstone is filled with micrite and some sparry calcite.
Dolomite is common in the lower half of this intervalHa et al. Progress in Earth and Planetary Science(2019) 6:47

Page 7 of 21

Fig. 4 Microfacies of the studied carbonates. a Sandy limestone with peloids, unit 1, 117.0 mdfrh in core no. 1, sample 1-140. Note bioturbation (arrows).
b Microbialites (m) with a matrix of peloidal packstone (pp), unit 2, 105.5 mdfrh in core no. 1, sample 1-128. Note poorly laminated micritic mass binding
peloids (arrows) derived from the peloidal packstone matrix. c Peloidal packstone, unit 2, 67.4 mdfrh in core no. 1, sample 1-86. d Dolomitized oncoidal
floatstone, unit 3, 51.2 mdfrh in core no. 1, sample 1-67. Note oncoids (arrows) and dolomite rhombs (do). e Peloid bioclast packstone, unit 4, 156.5
mdfrh in core no. 4, sample 4-139. Note bioclasts of dasycladalean algae (arrows) and echinoid (double arrowhead) and thick micrite envelopes (whitearrowheads). f Peloid bioclast packstone rich in dasycladalean algae (arrows), unit 5, 140.8 mdfrh in core no. 4, sample 4-123. g Microbialite (m) with amatrix of peloidal grainstone (PG), unit 5, 98.3 mdfrh in core no. 4, sample 4-86. Note a gastropod (g) with a thick micrite envelope (arrowhead) h Bioclastic
grainstone, unit 6, 38.5 mdfrh, sample 4-35, in core no. 4. Note bioclasts of dasycladalean algae (arrows) and a thin-shelled bivalve (arrowhead). Scalebar, 1 mm

Ha et al. Progress in Earth and Planetary Science(2019) 6:47

and exceeds 10 wt% in nine horizons, reaching a maximum content of 59 wt% (Fig. 4d).
Minor element concentrations

The Sr concentrations in the core samples range between 50 and 200 ppm, with seven high-concentration
outliers (Fig. 2; Additional file 1: Table S1). Sr concentrations gradually decrease upwards from the baseto 53.2 mdfrh, with the high-concentration outliers
being found at 98.5–76.5 mdfrh. From 53.2 mdfrh to
the top of the core, Sr concentrations fluctuate at ~100 ppm (mean = 98 ppm).
Mn concentrations are less than 250 ppm, except forthree high-concentration outliers. Near the base of thecore (128.8–120.6 mdfrh), Mn concentrations decrease
upwards from 83 to 15 ppm, followed by an interval of
low Mn concentrations (10–60 ppm) at 120.6–63.4 mdfrh.
Above this interval, Mn concentrations fluctuate, showing
a slight increase upwards between ~ 50 and ~ 100 ppm at
63.4–27.4 mdfrh. Mn concentrations then increase to 210
ppm at 26.5 mdfrh. From here to the top of the core, concentrations show relatively large amplitude variability but
broadly decrease upwards to 71 ppm, with three highconcentration outliers at 22.3–19.5 mdfrh.
Fe concentrations vary mostly between 70 and 400 ppm.
Fe concentrations show a broad decrease upwards from
327 to 74 ppm (with one high-concentration outlier) in
the lowermost interval from the base to 98.5 mdfrh andthen increase to ~ 200 ppm (with two high-concentration
outliers) in the overlying interval up to 64.9 mdfrh. From
this horizon to the top of the core, Fe concentrations
range from 200 to 400 ppm, except for five highconcentration (> 600 ppm) outliers.
Carbon, oxygen, and strontium isotope stratigraphies

The δ13C values in the core range from 0.60 to2.28‰ (Fig. 2; Additional file 1: Table S1). In the lowest interval of the core (from the base to 115.4 mdfrh),
the δ13C values vary quite widely, from 0.91 to
2.19‰. Above this (from 115.4 to 76.5 mdfrh), theδ13C values increase upwards from 1.25 to 2.28‰.
The δ13C values in the uppermost part of the core
range between 0.60 and 2.28‰ and decrease upwards
to reach core-top values of 1.13–1.33‰.
The δ18O values range from − 9.36 to − 5.65‰. The18
δ O values increase from − 9.36 to − 7.59‰ in the lowest interval of the core (from the base to 120.6 mdfrh).
Above this interval, the δ18O values range mostly between − 8.5 and − 6.0‰ and gradually increase to the
top of the core.
The 87Sr/86Sr values ranges from 0.70853 to 0.70981(Fig. 2; Additional file 1: Table S1). Ratios vary from0.70872 to 0.70938 in the interval of 127.7–64.9 mdfrh andfrom 0.70853 to 0.70981 in the interval of 59.7–2.4 mdfrh.

Page 8 of 21

Biostratigraphy

Foraminifers are rare in the lower interval of core no. 1(from the base up to ~ 64 mdfrh), but numerous in theupper interval of the core. Twelve taxa are identified in 25
samples (Table 1, Fig. 5). In the lower part, foraminiferal
fauna is simple in composition and entirely represented by
the genera Hemigordiellina and Endotriadella, indeterminable duostominids (probably of the genera Duostomina
and/or Diplotremina) and nodosarrids (Fig. 5a, e, x, and y).
These are all long-ranging taxa and are therefore not very
useful for detailed biochronologic assessment. The scarcity
of foraminifers in this interval is probably due to the dominance of sandy (detrital) limestone facies with some finegrained siliciclastic material, which is not conducive for
hosting shallow-water dwelling benthic forms. Foraminiferal recovery is poor also at 55–32 mdfrh in the upperinterval, which may be related to the development of common dolomite levels in this interval (Fig. 2).
Endotriadella wirzi (Fig. 5s), whose first appearance
datum is recorded within the Pelsonian (late middle Anisian) according to Salaj et al. (1983), first occurs at
63.51 mdfrh. This species has a longer range in the Nanpanjiang Basin of South China, from the latest Olenekian to theearly Carnian (Lehrmann et al. 2015), but is usually morecharacteristic of the middle to late Anisian and Ladinian(Salaj et al. 1983; Oraveczné Scheffer 1987). Endotriadellapentacamerata (Fig. 5r), whose first appearance datum is recorded within the Pelsonian (late middle Anisian) in thetype area of this species (Salaj et al. 1983), first occurs in thiscore at 25.60 mdfrh. These two Endotriadella species arealso found in core no. 4. In the uppermost part of core no. 1(at 9.28 and 7.10 mdfrh), three age-diagnostic species,
namely, Citaella dinarica (Fig. 5l–p), C.? deformata (Fig. 5t–v), and Endotriada tyrrhenica (Fig. 5k), appear at virtually
the same level. Of these, C. dinarica is well known as a typical Anisian index species (e.g., Salaj et al. 1983; Rettori
1995) with a range in the early to middle Anisian (Ueno etal. 2018). Citaella? deformata, which has rather diverse
morphology, is now considered a reliable indicator of thePelsonian (Ueno et al. 2018). Endotriada tyrrhenica is generally reported from the upper part of the Anisian (broadly
corresponding to the Pelsonian and Illyrian) (Lehrmann etal. 2015). Although identification was qualified, Pilammina
cf. densa (Fig. 5c) is found in sample 1-3 (at 2.43 mdfrh)
from the topmost part of core no. 1. Pilammina densa characterizes the Pelsonian and Illyrian in large areas of Eurasia(e.g., Salaj et al. 1983; Velledits et al. 2011; Miyahigashi et al.
2017).
Based on the foraminiferal biostratigraphy, Ueno et al.
(2019) conclude that the uppermost part of core no. 1(above 9.28 mdfrh) is Pelsonian (late middle Anisian), as isprobably the entire upper interval of the core (above
63.54 mdfrh). This further suggests that the lower half ofcore no. 1, where foraminiferal recovery is poor, isHa et al. Progress in Earth and Planetary Science(2019) 6:47

cPage 9 of 21

d

f

e

g

b

h

a
l

m

j
n

o
q

i

s

p

k

t

r

u

v

w

x

y

Fig. 5 Photomicrographs of important foraminifers obtained from the studied cores. a Indeterminable nodosariid, sample 4-83; b Pilamminella grandis(Salaj in Salaj et al. 1967), sample 4-34; c Pilammina cf. densa Pantić 1964, sample 1-3; d Austrocolomia sp., sample 4-136; e Hemigordiellina sp., sample 4-83;
f Valvulina cf. azzouzi Salaj 1977, sample 4-79; g Gaudryina sp., sample 4-102; h Indeterminable involutinid (Aulotortus?), sample 4-139; i Triadodiscus cf.
praecursor (Gaździcki in Gaździcki et al. 1975), sample 4-129; j Endoteba sp. A., sample 4-117; k Endotriada tyrrhenica Vachard et al. 1994, sample 1-14; l–p
Citaella dinarica (Kochansky-Devidé and Pantić 1966), l sample 4-29, m–p sample 4-55; q Earlandinita sp., sample 4-16; r Endotriadella pentacamerata (Salajin Salaj et al. 1967), sample 4-83; s Endotriadella wirzi (Koehn-Zaninetti 1969), sample 1-82; t–v Citaella? deformata (Salaj in Salaj et al. 1967), t sample 4-55, u
sample 4-29, v sample 4-139; w Ammobaculites? sp., sample 4-9; x and y indeterminable duostominids (probably Duostomina or Diplotremina), x sample 114, y sample 1-7. Scale bar, 200 μm (applicable to all specimens)

potentially pre-Pelsonian in age and may possibly be Bithynian (early middle Anisian). This assessment also concurs with stratigraphic evidence that the conformably
underlying Dong Trau Formation is correlated to the early
to middle Anisian (Thanh and Khuc 2011). Thus, the
Hoang Mai Formation in core no. 1 is correlated to the
middle Anisian (early Middle Triassic).

Core no. 4
Lithostratigraphy

The studied carbonate succession has no observed hiatuses
and represents a continuous depositional record (Fig. 3).
Dissolution cavities filled with various-sized breccias and/or
laminated lime mud are common in the upper core down
to 110 mdfrh. Dissolution vugs are common and are filledwith sparry calcite.

The lowest interval of the core, from the base(160.0 mdfrh) to 141.9 mdfrh, is composed mainly of peloid
bioclast packstone (Fig. 4e). The packstone has abundant irregularly shaped peloids of up to coarse sand size. Bioclasts,
with micrite envelopes, are common, including dasycladalean algae and, less commonly, foraminifers, ostracods, gastropods, bivalves, and brachiopods. Many of the bioclasts,
however, are poorly preserved, and could not be taxonomically identified. Intergranular pore space is filled with micrite and some sparry cement.
The core interval from 141.9 to 40.4 mdfrh consists ofpeloidal packstone with variable amounts of bioclasts(Fig. 4f) and microbialite (Fig. 4g) and is associated withoncoidal floatstone. The peloidal packstone and related
types (e.g., peloid bioclast packstone, peloidal grainstone)
have similar components to those of the peloid bioclast
packstone in the lowest core interval (160.0–135.0 mdfrh).

113

114

Pilammina spp.

x

x

x

Indet. nodosariids

x

x

x

Indet. duostominids(Diplotremina/Duostomina)xx
x

Endotriadella spp.

Earlandinita spp.

x

x

Endotriadella pentacamerata

Endotriadella wirzi

Endotriada tyrrhenica

x

x

x

x

x

Citaella? deformataxCitaella dinarica

Pilammina cf. densa

x
x

x
x

Hemigordiellina spp.

110

115

1-21

1-29

1-34

1-36

1-40

1-43

1-53

1-74

1-75

1-77

1-80

1-81

1-82

x

x

x

?

x

?

x

?

x

x

x

x

x

x

x

x

x

x

x

?

x

x

x

x

x

x

x

x

1-90

1112

1-136

x

x

?

x

1-145

1-154

x

x

2.43 4.50 5.00 7.10 8.97 9.28 9.48 14.62 20.38 24.22 25.60 29.76 31.68 40.08 56.34 57.34 59.66 62.55 63.34 63.51 71.20 90.53 113.51 120.62 128.82

Taxon

107

Depth in core (mdfrh)

106

103

SampleUnit 1

Unit 3

Lithostratigraphic unit

Unit 2

No. 1

Core

Table 1 List of foraminifers from core no. 1

Ha et al. Progress in Earth and Planetary Science(2019) 6:47
Page 10 of 21

Ha et al. Progress in Earth and Planetary Science(2019) 6:47

The microbialite is composed of an irregular frameworkof laminated and clotted micrite, in which irregularly
shaped peloids and bioclasts are loosely bound. Microbial
filaments are commonly preserved and measure 40–
90 μm in diameter. The bioclasts all have micrite envelopes and include dasycladalean algae, small foraminifers,
and, to a lesser extent, gastropods, bivalves, and echinoderms. Intergranular pore space is filled with micrite andsome sparry cement. Dolomite contents exceed 10 wt% in
18 horizons and reach a maximum value of 84 wt%.
The interval from 40.4 mdfrh to the core top is composed mainly of peloidal packstone associated with grainstone, both with variable amounts of bioclasts. The peloids
are irregularly shaped and coarse sand-sized or smaller.
Bioclasts are common to abundant and are derived fromdasycladalean algae, foraminifers, gastropods, and bivalves(Fig. 4h). Many bioclasts, however, could not be taxonomically identified because of poor preservation. The bioclasts
all have micrite envelopes of varying thickness (the bioclasts
with thick micrite envelopes can be classed as cortoids). Intergranular pore space is filled with micrite and somesparry cement.
Minor element concentrations

The Sr concentrations in the core samples lie mainly between 100 to 170 ppm, with four higher outliers and one
lower outlier (Fig. 3; Additional file 2: Table S2). The Srconcentrations vary irregularly and have no systematic
trend with depth.
The Mn concentrations in the lowest interval of the core(from the base to 88.5 mdfrh) range from 70 to 190 ppm,
with one outlier, and increase slightly upwards. In the interval above (87.3–79.4 mdfrh), Mn concentrations fall in anarrow range from 100 to 122 ppm. Mn concentrations are
more variable in the interval above (76.4–20.2 mdfrh) andrange between 101 and 226 ppm. Mn concentrations are
the most variable in the uppermost interval, from
19.5 mdfrh to the core top (116 to 370 ppm).
The Fe concentrations range mostly from 100 to 250
ppm, with a random distribution of higher concentration
outliers. The Fe concentration profile, excluding the outliers, decreases slightly upwards.
Carbon, oxygen, and strontium isotope stratigraphies

The δ13C values range from − 0.89 to 1.48‰ (Fig. 3;
Additional file 2: Table S2). From the base of the core
to 52.9 mdfrh, the δ13C values fall in a narrow rangeof 0.54–1.48‰. In the upper core interval (41.8–
0 mdfrh), δ13C values broadly decrease upwards to −0.78‰.
The δ18O values range from − 9.27 to − 4.42‰. δ18O
values in the lowest interval of the core, from the base to147.7 mdfrh, are highly variable, ranging between − 7.82
and − 4.65‰. From the top of this interval to 101.5 mdfrh,

Page 11 of 21

the δ18O values decrease to − 9.27‰ and then increase to− 4.77‰ from 101.5 to 80.4 mdfrh. The δ18O profile thenincreases upwards from − 8.00 to − 6.01‰ from 76.4 to
8.5 mdfrh and abruptly decreases in the uppermost 8.5 m
to reach a value of − 7.32‰ at the top of the core.
The 87Sr/86Sr values ranges between 0.70833 and0.70906, showing no systematic trend (Fig. 3; Additional file 2: Table S2).

Biostratigraphy

Foraminifers are more abundant and diverse in core no.
4 than in core no. 1. A total of 23 taxa of foraminifers
were identified from 43 samples (Table 2; Fig. 5). The
higher foraminiferal abundance compared with core no.
1 corresponds to the more common shallow-marine
peloidal–bioclastic microfacies in core no. 4, which
would have been a more suitable habitat for benthic foraminifers. Most foraminifers found in core no. 1, especially those from the uppermost interval (except
Pilammina cf. densa), are also present in core no. 4.
Citaella dinarica and C.? deformata are diagnostic
throughout core no. 4, as in the uppermost 10 m in coreno. 1. Endotriadella wirzi and E. pentacamerata, which
also occur in the upper interval of core no. 1, are foundfrom near the base up to ~ 65 mdfrh. Endotriada tyrrhenica is common in the middle part of core no. 4, with thesame upper limit as Endotriadella wirzi at 64.67 mdfrh.
There are also single occurrences of biostratigraphicallyuseful foraminifers, including Pilammina grandis (Fig. 5b)
in sample 4-34 (at 37.66 mdfrh), which has been reported
from various areas in Eurasia and is restricted to the Anisian (Gaździcki et al. 1975; Salaj et al. 1983; Velledits et al.
2011; Lehrmann et al. 2015; Miyahigashi et al. 2017). Triadodiscus cf. praecursor (Fig. 5i) is putatively identified insample 4-129 (at 146.67 mdfrh). This species was originally described from the Pelsonian of the Polish Muschelkalk (Gaździcki et al. 1975). Valvulina azzouzi, which isalso putatively identified in the middle of core no. 4 (sample 4-79 at 91.68 mdfrh), is known to appear first in the
middle Anisian in the West Carpathian Mountains, where
extensive biostratigraphic research on Triassic foraminifers has been conducted (Salaj et al. 1983).
Ueno et al. (2019) classified core no. 4 as Pelsonian (latemiddle Anisian), based mainly on the co-occurrence of C.
dinarica and C.? deformata. This core has a similar faunalcomposition to that of the upper part of core no. 1, implying stratigraphic continuity between these successions in
the two cores. In core no. 4, the age-diagnostic C. dinarica
is present only up to 19.49 mdfrh. In view of the chemostratigraphic uniformity in the uppermost part of the core(Fig. 3), and the observation that there are no erosionalsurfaces, it is highly likely that the entire core is Pelsonianin age.

4-19

4-21

4-29

4-34

Unit 5
4-43

4-46

4-50

4-53

?

x

x

?

x

x

x

x

x

?

x

x

x

?

x

x

x

x

x

x

x

x

x

x

Indet. involutinids

Indet. nodosariids

Indet. duostominids (Diplotremina/Duostomina) x

Triadodiscus cf. praecursor

Ammobaculites? spp

Austrocolomia sp.

Earlandinita spp.

Endotriadella spp.

Endotriadella pentacamerata

x

x
x

x

x

x

x

x

Endotriadella wirzi

x

x

?

x

Endotriada tyrrhenica

Endoteba sp. B

Endoteba sp. A

Pilamminella grandis

Pilammina spp.

Citaella? deformata

Citaella cf. cheni

Citaella dinarica“Calcitornella” spp.

Hemigordiellina spp.

Trochammina jaunensis

Valvulina cf. azzouzi

Gaudryina? sp.

Taxon

4-40

4-55

4-59

4-64

4-68

4-70

4-73

4-77

4-79

4-80

x

x

x

?

x

x

x

x

x

x

x

?

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

x

12.00 13.73 19.49 22.17 24.12 32.41 37.66 43.04 54.52 57.24 61.29 64.67 69.32 73.39 77.21 81.60 83.27 86.61 89.70 91.68 92.72

4-16

Depth in core (mdfrh)

4-10

Unit 6
4-09

SampleNo. 4

Lithostratigraphic unit

Core

Table 2 List of foraminifers from core no. 4

Ha et al. Progress in Earth and Planetary Science(2019) 6:47
Page 12 of 21

x

x

x
x

x

x

x

x

x

x

x

x

x

x

x

x

4-121

x

x

?

x

4-122

x

?

x

x

x

x

4-123

x

x

4-124

x

x

4-126

x

x

x

x

4-129

x

Indet. nodosariids

Indet. involutinidsx

Indet. duostominids(Diplotremina/
Duostomina)

x

x

?

x

x

x

x

4-117

x

x

?

x

x

x

x

4-115

Triadodiscus cf.
praecursor

x

x

x

x

4-112

Ammobaculites? spp

Austrocolomia sp.

Earlandinita spp.

Endotriadella spp.

x

x

Endotriadella wirzi

Endotriadellapentacamerataxx

Endotriada tyrrhenica

Endoteba sp. B

Endoteba sp. A

Pilamminella grandis

Pilammina spp.

Citaella? deformata

x

x

?

x

Citaella cf. cheni

Citaella dinarica

4-110

x

x

4-106

x
x

x

4-102

x

x

4-99

“Calcitornella” spp.
x

4-93

Hemigordiellina spp.

x

4-91

x

?

4-89

Trochammina jaunensis

Valvulina cf. azzouzi

Gaudryina? sp.

Taxon

4-86

x

x

x

x

x

4-136

x

4-139

93.75 95.68 97.35 98.25 101.44 106.03 110.15 116.44 119.56 123.69 126.96 129.64 132.73 134.36 138.06 139.48 140.83 141.44 143.35 146.67 153.36 156.52

Depth in core (mdfrh)

4-85

4-81

Sample4-83

Unit 5

Lithostratigraphic unit

Unit 4

No. 4

Core

Ha et al. Progress in Earth and Planetary Science(2019) 6:47
Page 13 of 21

Ha et al. Progress in Earth and Planetary Science(2019) 6:47

Page 14 of 21

Fig. 6 Composite lithostratigraphy and carbon isotope stratigraphyof bulk carbonate samples from this study (cores no. 1 and 4). The
δ13C values presented are those of samples that are notdiagenetically altered, as inferred from trace element (Mn and Sr)criteria as defined in the subsection “Isotope and minor element
profiles” of the main text. Using these criteria, 67 samples were
removed from consideration (Additional file 1: Table S1 andAdditional file 2: Table S2). The marked decrease observed in theδ13C profile in the upper interval (uppermost unit 5 and unit 6;
shown in gray) is likely due to diagenetic overprinting (meteoric
diagenesis). For the definition of the pink and blue areas andabbreviations, see the Fig. 2 caption

Discussion
Carbonate succession in the study area and its
sedimentary environment

The rock units surrounding the drill sites have a monoclinal structure, striking NE–SW (in some cases up to
NNE–SSW) and dipping 50°–60° SE to ESE. Core no. 1
was drilled ~ 800 m north of core no. 4. Normal faults
occur between the two drill sites. Based on the spatialconfiguration of the sandy limestone (unit 1) around thedrill sites, it is expected that unit 1 occurs ~ 115 m below
the base of core no. 4. As the uppermost horizon of unit 1
in core no. 1 is located at 117.0 mdfrh, the base of coreno. 4 is at a similar stratigraphic level to the top of coreno. 1. This interpretation is supported by the δ13C data,
where δ13C values in the lowest interval of core no. 4(160–155 mdfrh, 1.0–1.4‰; Fig. 3, Additional file 2: Table
S2) predominately fall into the same narrow range as
those in the uppermost part of core no. 1 (0–5 mdfrh,
1.0–1.3‰; Fig. 2, Additional file 1: Table S1). Foraminiferal biostratigraphy provides further support for the correlation of the two cores. Nevertheless, the lithologicaldifferences between core no. 1 and core no. 4 (i.e., peloid
bioclast packstone vs. oncoidal floatstone, respectively) indicate that they do not overlap in time. Consequently, weinterpret the base of core no. 4 as stratigraphically just a
few meters above the top of core no. 1 (Fig. 6). The total
thickness of the studied succession is thus estimated to be~ 150–190 m.
The carbonate succession is composed of six lithostratigraphic units that are numbered sequentially from the base(unit 1) to the top (unit 6). Unit 1 (130.0–117.0 mdfrh incore 1) is composed mainly of sandy limestone; unit 2(117.0–63.5 mdfrh in core 1) consists chiefly of peloidal
packstone with a microbialite interlayer; unit 3 (63.5–
0 mdfrh in core 1) is composed mainly of peloidal packstone and oncoidal floatstone; unit 4 (160.0–141.9 mdfrh incore 4) is mainly peloid bioclast packstone; unit 5 (141.9–
40.4 mdfrh in core 4) is a peloidal packstone with a variable
quantity of bioclasts and microbialite; and unit 6 (40.4–
0 mdfrh in core 4) is dominated by peloidal packstone witha variable abundance of bioclasts. The sandy nature of unit
1 and of three interlayers in unit 2, along with dark rockHa et al. Progress in Earth and Planetary Science(2019) 6:47

color of these units, indicates a supply of terrigenous material in the early stages of deposition on the platform where
the Hoang Mai Formation carbonates accumulated. The
dominance of carbonates with a fine-grained matrix
throughout the remaining units suggests deposition in alow-energy subaqueous environment.
The occurrence of oncoids in the peloidal packstonematrix from units 3 and 5 does not necessarily indicate ahigh-energy depositional environment in which frequent
overturning occurred, as the fine carbonate fractions insuch settings are swept away. It is known that the overturning of algal nodules is not limited to water motion but
can also be driven by biotic activity on the seafloor (Prager
and Ginsburg 1989; Matsuda and Iryu 2011). At present,
the overturning mechanism of oncoids in the Hoang Mai
Formation is not clear without further taphonomical
study. Present-day dasycladalean algae are typical of warm,
shallow bays and back-reef lagoons (Berger and Kaever
1992; Ohba et al. 2017). Consequently, it is thought that
units 4–6, in which dasycladalean algae are common, were
deposited in the back-reef lagoon of the Hoang Mai carbonate platform. This interpretation does not conflict withthe common occurrence of a microbial framework in thiscarbonate succession.

Isotope and minor element profiles
To identify carbonate samples that likely retain the original
seawater 87Sr/86Sr, Denison et al. (1994) proposed criteriaof Sr/Mn > 2 or Mn < 300 ppm. The Mn/Sr ratio has also
been proposed as a method to distinguish between diagenetically altered (Mn/Sr > 2) and unaltered (Mn/Sr < 2) carbonate samples for 87Sr/86Sr analysis (Jacobsen and
Kaufman 1999). These criteria have been used to identify
diagenetically altered carbonates, not only for 87Sr/86Sr analysis but also for δ13C and δ18O measurements. We define
diagenetically altered samples as those with high Mn concentrations (> 300 ppm) or with high Mn/Sr values (> 2).
By applying these definitions, 67 samples were identified as
not having retained their initial δ13C and δ18O values.
The 87Sr/86Sr values of the carbonates studied here aremuch higher than that of global seawater during theAnisian (McArthur et al. 2001; Korte et al. 2003), irrespective of these criteria. Consequently, in this study, we
estimate geologic ages from foraminiferal biostratigraphyrather than 87Sr/86Sr data. Possible explanations for thehigh 87Sr/86Sr values include diagenetic alteration by
87
Sr-enriched waters. Such diagenesis has been reported
for Cretaceous carbonates in the Middle East (e.g., Paganoni et al. 2016).
δ18O values in carbonates reflect the temperature of precipitation and the δ18O composition of the water fromwhich they precipitated (e.g., Tucker and Wright 1990).
Based on the equation relating the temperature, and theδ18O values of carbonates and the water (Friedman andPage 15 of 21

O’Neil 1977), and on conversion between the VPDB and
VSMOW scales (Coplen et al. 1983), we can calculate theδ18O values of calcite precipitated in oxygen isotope equilibrium with ambient seawater (equilibrium calcite). Assuming that modern tropical open-ocean sea-surface
temperatures (20 to 35 °C) and δ18O values (− 1.5 to 2.0‰)
apply to Anisian seawater, the δ18O values of the equilibrium calcite are estimated to be − 5.4 to − 1.1‰, which arehigher than the values measured in the studied carbonates(− 9.3 to − 4.4‰). This indicates that the initial oxygen isotope composition of the studied carbonates has been resetby diagenesis. Alternatively, it is possible that the δ18O
composition of Anisian seawater was much lower thanmodern-day values. The former interpretation is supportedby the relatively low δ18O values measured in the diagenetic
products, cement, and vein calcites found in the cores compared with those of the studied carbonates (Fig. 7).
The composite δ13C profile of the carbonate succession
initially increases upwards from the base (lower interval,
130.0–76.5 mdfrh in core no. 1), followed by a prolonged
gradual decrease upwards (middle interval, 76.5–0 mdfrh
in core no. 1 and 160.0–52.9 mdfrh in core no. 4), with theupper interval characterized by a marked upwards decrease(41.8–0 mdfrh in core no. 4) (Fig. 6). The δ13C values ofthe succession fall in the δ13C range (− 1.1 to 2.1‰) of Anisian whole-rock samples and brachiopod shells (Korte et al.
2005). The δ13C profiles of the lower and middle intervals
are characterized by a smooth, systematic increase and decrease, respectively, with no distinctly anomalous values,
and are correlated with coeval δ13C profiles in other areas
as discussed below (Fig. 8). The δ13C and δ18O values fromthese intervals do not show a significant positive correlation(Fig. 7). Consequently, we consider that the carbonates in
the lower and middle intervals have retained their initial
δ13C values. Although the δ18O and 87Sr/86Sr values of carbonates are reset or modified by diagenesis, their δ13C
values are commonly retained. As a result of the extreme
differences in the concentrations of oxygen and carbon indiagenetic fluids, carbonate minerals equilibrate with fluid
δ18O values at water:rock ratios (< 10) that are three ordersof magnitude lower than those (103) at which they equilibrate with fluid δ13C values (Banner and Hanson 1990). It
is also shown that Sr–Ca-rich brines (similar to interstitialwater in limestones) can also reset the 87Sr/86Sr values of alimestone at low water:rock ratios similar to those calculated for the equilibration of the δ18O values (Banner andHanson 1990).
In contrast to the lower and middle intervals, the δ13C
profile of the upper interval shows a marked decrease
upwards (Fig. 3). This trend remains evident after removing the δ13C values of the 20 samples that wereidentified as being altered by diagenesis, based on Mn
and Sr data (Fig. 7). Dissolution cavities filled by breccias
with various clast sizes and laminated lime mud areHa et al. Progress in Earth and Planetary Science(2019) 6:47

Page 16 of 21

Fig. 7 Cross-plots displaying the relationship between δ13C and δ18O values of the studied bulk carbonate samples. Vein samples are from 1.14–1.20,
3.29–3.33, 120.39–120.43, and 148.68–148.72 mdfrh in core no. 4. Cement samples are from 82.28–82.32 and 136.33–136.39 mdfrh in core no. 4. *Samples
identified as diagenetically altered based on trace element (Mn and Sr) criteria. **Samples that passed the geochemical screening for diagenetic alteration
but were identified as altered based on petrographic observations (shown in gray in Fig. 6)

dominant in and immediately below the upper interval(upper 52.8 m of core 4; Fig. 3). Therefore, it is likelythat the carbonates in the upper interval have been diagenetically altered and that the marked decrease in δ13C
values was caused by diagenetic overprinting (i.e., meteoric diagenesis).
The δ13C profile of the studied carbonate succession is
not controlled by facies changes altering the δ13C valuesof whole-rock samples because of the following two reasons. (1) The δ13C values do not show any distinct
changes at lithological boundaries (unit boundaries); and(2) it is reported that microbially induced micrite is usually depleted in 12C, relative to marine carbonates (Wuand Chafets 2002). There is, however, no increase inδ13C values related to sedimentary facies dominated by
microbial products such as oncoids and microbialite(units 3 and 5).
Correlation of the δ13C profiles with other areas

To understand if the Vietnamese δ13C profile representschanges in the δ13C of dissolved inorganic carbon(δ13CDIC) in global seawater during the Bithynian–Pelsonian (Anisian), we compare our profile with regional studies that are considered the same interval.
Several studies have shown temporal variations in theδ13C values of carbonates through the Anisian (Fig. 8,
Additional file 3: Figure S1). Examples include carbonatesfrom Kamura, Japan (Zhang et al. 2017; Additional file 3:
Figure S1k); the Eastern Sichuan Basin, China (Huang etal. 2012); Nanpanjiang, South China (Payne et al. 2004;
Sun et al. 2012; Lehrmann et al. 2015; Additional file 3:
Figure S1 h, i, j); Thongde in the Zanskar Himalayas, India(Baud et al. 1989; Additional file 3: Figure S1f); Himachal

Pradesh, India (Galfetti et al. 2007); Sal Formation, WNWof the village of Sal, Oman (Hauser et al. 2001); the Taşkent section in the Aladag Nappe, Turkey (Lau et al. 2016);
North Dobrogea, Romania (Atudorei et al. 1997; Additional file 3: Figure S1e); the Tatra Mountains, Poland(Jaglarz and Szulc 2003; Additional file 3: Figure S1d); theGermanic Basin, Germany (Szulc 2000; Additional file 3:
Figure S1c); the Belanské Tatry Mountains, Slovakia(Rychliński and Szulc 2005; Additional file 3: Figure S1b);
North Switzerland (Feist-Burkhardt et al. 2008; Additional file 3: Figure S1a); and the Dolomites and Lagonegro Basin in northern and southern Italy, respectively(Preto et al. 2009). To correlate these δ13C profiles andprovide chronological constraints, we used the reported
foraminiferal, conodont, and ammonoid biostratigraphies
along with known regional stratigraphic frameworks(Additional file 3: Figure S1). Based on the chronologies
used, the δ13C profiles are not necessarily coincident witheach other. This result contrasts with the widely accepted
chemostratigraphic frameworks for the upper Barremianto the upper Aptian (e.g., Menegatti et al. 1998; Yamamoto
et al. 2013) and the Cenomanian to the Turonian (e.g.,
Herrle et al. 2015; Gyawali et al. 2017).
The higher-resolution δ13C sampling of the profiles ofNorth Dobrogea (Fig. 8a) and the three sections in the
Nanpanjiang Basin (Bianyang (Fig. 8c), Guandao II(Fig. 8d), and Guandao (Fig. 8e)) makes these sections
relatively reliable in terms of comparing with the Vietnamese δ13C profile (Fig. 8b). The other sections, mentioned
above, have a low-resolution carbon isotope stratigraphy(Baud et al. 1989; Szulc 2000; Hauser et al. 2001; Jaglarz
and Szulc 2003; Feist-Burkhardt et al. 2008; Lau et al.
2016; Zhang et al. 2017), poor age control (Rychliński andHa et al. Progress in Earth and Planetary Science(2019) 6:47

Page 17 of 21

e

a
b

c

d

Fig. 8 Anisian δ13C profiles of carbonates from Romania, Vietnam, and South China. a North Dobrogea, Romania (Atudorei et al. 1997); b North-Central
Coast region, Vietnam (this study); c Bianyang section, Nanpanjiang Basin, South China (Sun et al. 2012); d Guandao II section, Nanpanjiang Basin, South
China (Sun et al. 2012); e Guandao section, Nanpanjiang Basin, South China (Payne et al. 2004). The vertical red double arrows indicate the interval
represented by the studied carbonate succession in Vietnam. The small horizontal arrows indicate the basal horizon of the Bithynian δ13C decrease
followed by the interval of prolonged (Bithynian–Pelsonian) stability or gradual decrease in values, as reported from South China and Romania.
Abbreviations: Aeg, Aegean; Bith, Bithynian; Illy, Illyrian; Lad, Ladinian; Olen, Olenekian; Pels, Pelsonian

Szulc 2005; Feist-Burkhardt et al. 2008), and have been influenced by local changes in water chemistry such as
freshwater influx and high organic carbon burial flux inthe basin (Szulc 2000; Huang et al. 2012).
The Vietnamese δ13C profile is characterized by aslight increase upwards in the lower interval, followed
by a gradual decrease in the middle interval, which,based on foraminiferal biostratigraphy, started during
the Bithynian and continued into the Pelsonian (Fig. 6). Similar trends are recognized in the Romanian andSouth China δ13C profiles (Fig. 8). The Bithynian–Pelsonian carbonate δ13C profile from North Dobrogea(Fig. 8a; Atudorei et al. 1997) shows an interval withhigh δ13C values (~ 4‰), followed by a prolonged
stable interval with lower values of ~ 2‰. A distinct
negative isotope excursion of ~ 2‰ occurs in the Bithynian, although this excursion might reflect thecontrol of lithological facies on the carbon isotope
composition. The δ13C profile from North Dobrogea

is based on data from the Desli Caira and Agighiol
sections, and the isotope excursion corresponds tothe boundary between the two sections. The Bithynian–Pelsonian carbonate δ13C profile of the Guandao
II section (Fig. 8d; Sun et al. 2012) shows an intervalof high values (~ 4‰) followed by an interval oflower values (~ 2‰), with the negative isotope excursion between the two intervals occurring in the Bithynian. The Bithynian–Pelsonian carbonate δ13C
profile of the Bianyang section also shows a decreasing upwards trend, starting in the Bithynian, from ~ 4
to ~ 2‰; however, this trend is gradual rather thanabrupt (Fig. 8c; Sun et al. 2012). The carbonate δ13C
profile of the Guandao section exhibits positive andnegative excursions (~ 1‰ each) in the Bithynian
followed by a prolonged stable interval with values of~ 2‰ in the Bithynian and Pelsonian (Fig. 8e; Payne
et al. 2004). Although these δ13C profiles do not correlate precisely, they all display a Bithynian δ13C

Ha et al. Progress in Earth and Planetary Science(2019) 6:47

decrease followed by an interval of prolonged (Bithynian–Pelsonian) stability or gradual δ13C decrease.
A possible explanation for the differences observed in theabovementioned δ13C profiles include local, basin-wide,
and global heterogeneities in the δ13CDIC in seawater, whichmust be taken into account when interpreting geochemicalrecords of ancient seas (Immenhauser et al. 2003, 2008).
Carbonates formed on platforms with enhanced biologicalcarbon uptake are depleted in 12C compared with shelf andbasin carbonates. The δ13C values of the carbonates in thisstudy, however, are not significantly higher than those ofother areas. Szulc (2000) relates Middle Triassic variationsin δ13C (and δ18O) values to changes in water chemistry(e.g., freshwater influx) caused by sea-level changes and meteoric diagenesis. Therefore, some of the δ13C profiles
might record local phenomena, which is likely to at least
partially contribute to the discrepancies among the studies.
The Vietnamese δ13C profile probably represents globalchanges in seawater δ13CDIC values, except for the upperinterval, as the studied succession was formed on a carbonate platform and represents a continuous depositional record without erosional surfaces. The middle to late Anisian(Bithynian to Pelsonian) interval of relatively stable δ13C
values suggests that no global carbon-cycle perturbations
occurred during this time.

Conclusions
We conducted an integrated study on the lithostratigraphy,
biostratigraphy, and chemostratigraphy of carbonates in
two drill cores from the Middle Triassic Hoang Mai Formation in Nghe An Province in the North-Central Coast region of Vietnam. The cores cover a ~ 150–190 m-thickcarbonate succession. The main conclusions are as follows.
1. The carbonates consist of the following six
lithostratigraphic units (from bottom to top):
sandy limestone (unit 1), peloidal packstone with amicrobialite interlayer (unit 2), peloidal packstone
and oncoidal floatstone (unit 3), peloid bioclast
packstone (unit 4), peloidal packstone with variable
quantities of bioclasts and microbialite (unit 5), andpeloidal packstone with variable amounts of bioclasts(unit 6). The sandy nature of unit 1 and of threeinterlayers in unit 2, along with dark rock color ofthese units, indicates a supply of terrigenous material
in the early stage of the Hoang Mai carbonate
platform. The predominance of carbonate with a finegrained matrix throughout the overlying units (2–6)
suggests a low-energy depositional environment. We
infer that unit 3, which contains abundant oncoids,
was formed and deposited in such a low-energy
environment. It is likely that units 4–6, with abundant
dasycladalean algae, were deposited in a back-reeflagoon environment.

Page 18 of 21

2. A total of 24 foraminiferal taxa were identified from68 stratigraphic levels. The middle–upper parts of theexamined core succession (units 3–6) are inferred to
be Pelsonian in age, based principally on theoccurrence of Citaella dinarica and C.? deformata,and secondarily on the presence of Endotriadellawirzi, E. pentacamerata, and Endotriada tyrrhenica.
Units 1 and 2 are probably attributed to the Bithynian.
Thus, the entire core succession is considered to be
middle Anisian (early Middle Triassic).
3. The 87Sr/86Sr values measured in the carbonates
are higher than that of global seawater during the
Middle Triassic. The low δ18O values of the
carbonates indicate that their initial oxygen isotope
composition has been reset. The δ13C profile of thelower and middle intervals displays a slight increase
upwards, followed by a prolonged gradual decrease.
The δ13C values fall within a narrow range of 1.0–
2.0‰ and do not show positive or negative isotope
excursions. This δ13C trend can be correlated with the
Bithynian decrease followed by a prolonged interval(Bithynian–Pelsonian) of stability or gradual decreasein δ13C values, as reported from sections in South
China and Romania. The upper interval of theVietnamese δ13C profile is characterized by a marked
decrease to the core top that likely reflects diagenetic
overprinting.

Additional files
Additional file 1: Table S1. δ13C and δ18O values, minor element(Sr, Mn, and Fe) concentrations, and 87Sr/86Sr values of bulk carbonate
samples from Core No. 1. Diagenetically altered samples are those withhigh Mn concentrations (> 300 ppm) or with high Mn/Sr values (> 2).
(PDF 66 kb)
Additional file 2: Table S2. δ13C and δ18O values, minor element (Sr, Mn,
and Fe) concentrations, and 87Sr/86Sr values of bulk carbonate samples
from Core No. 4. Diagenetically altered samples are those with high Mnconcentrations (> 300 ppm) or with high Mn/Sr values (> 2). (PDF 65 kb)
Additional file 3: Figure S1. Anisian δ13C profiles of carbonates fromvarious sites/sections. a, North Switzerland (Feist-Burkhardt et al. 2008); b,
Belanské Tatry Mountains, Slovakia (Rychliński and Szulc 2005); c, Germanic
Basin, Germany (Szulc 2000); d, Polish Tatra Mountains (Jaglarz and Szulc
2003); e, North Dobrogea, Romania (Atudorei et al. 1997); f, Thongde in the
Zanskar Himalayas, India (Baud et al. 1989); g, North-Central Coast region,
Vietnam (this study); h, Bianyang, Nanpanjiang, South China (Sun et al.
2012); i, Guandao II, Nanpanjiang, South China (Sun et al. 2012), j, Guandao,
Nanpanjiang, South China (Payne et al. 2004); k, Kamura, Japan (Zhang et al.
2017). The vertical red double arrows indicate the interval represented by
the studied carbonate succession in Vietnam. The small horizontal arrows
indicate the basal horizon of the Bithynian δ13C decrease followed by theinterval of prolonged (Bithynian–Pelsonian) stability or gradual decrease invalues, as reported from South China and Romania. Abbreviations: Aeg,
Aegean; Bith, Bithynian; Illy, Illyrian; Lad, Ladinian; Olen, Olenekian; Pels,
Pelsonian. (PDF 661 kb)

Abbreviations
mdfrh: Meters depth from the reference horizon; VPDB: Vienna Pee Dee Belemnite;
VSMOW: Vienna Standard Mean Ocean WaterHa et al. Progress in Earth and Planetary Science(2019) 6:47

Acknowledgments
The manuscript was significantly improved by the comments andsuggestions of Prof. Tada (Editor) and two anonymous reviewers.
Authors’ contributions
TTNH and YI conceptualized and designed this study. TTNH and YI conducted
the fieldwork and sampling. TTNH, HT, YA, and KY carried out the chemical
and isotopic analysis. KU performed biostratigraphic analysis. TTNH, HT, and KU
collaborated with the corresponding author in the interpretation of the data
and the construction of the manuscript. All authors read and approved the final
manuscript.
Funding
This work was supported by Grant-in-Aid for Scientific Research from the Japan
Society for the Promotion of Science to Y.I. (26302008) and K.U. (18 K03829).
Availability of data and materials
Please contact the corresponding author regarding data requests.
Competing interests
The authors declare that they have no competing interests.
Author details
1
Department of Earth Science, Graduate School of Science, Tohoku
University, Sendai 980-8578, Japan. 2Department of Earth System Science,
Fukuoka University, Fukuoka 814-0180, Japan. 3Department of Earth andEnvironmental Sciences, Nagoya University, Nagoya 464-8601, Japan.
Received: 26 November 2018 Accepted: 7 June 2019

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